1719 lines
66 KiB
XML
1719 lines
66 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook" version="5.0"
|
|
xml:id="manual.ext.profile_mode" xreflabel="Profile Mode">
|
|
<?dbhtml filename="profile_mode.html"?>
|
|
|
|
<info><title>Profile Mode</title>
|
|
<keywordset>
|
|
<keyword>C++</keyword>
|
|
<keyword>library</keyword>
|
|
<keyword>profile</keyword>
|
|
</keywordset>
|
|
</info>
|
|
|
|
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.intro" xreflabel="Intro"><info><title>Intro</title></info>
|
|
|
|
<para>
|
|
<emphasis>Goal: </emphasis>Give performance improvement advice based on
|
|
recognition of suboptimal usage patterns of the standard library.
|
|
</para>
|
|
|
|
<para>
|
|
<emphasis>Method: </emphasis>Wrap the standard library code. Insert
|
|
calls to an instrumentation library to record the internal state of
|
|
various components at interesting entry/exit points to/from the standard
|
|
library. Process trace, recognize suboptimal patterns, give advice.
|
|
For details, see the
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://ieeexplore.ieee.org/document/4907670/">Perflint
|
|
paper presented at CGO 2009</link>.
|
|
</para>
|
|
<para>
|
|
<emphasis>Strengths: </emphasis>
|
|
<itemizedlist>
|
|
<listitem><para>
|
|
Unintrusive solution. The application code does not require any
|
|
modification.
|
|
</para></listitem>
|
|
<listitem><para> The advice is call context sensitive, thus capable of
|
|
identifying precisely interesting dynamic performance behavior.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
The overhead model is pay-per-view. When you turn off a diagnostic class
|
|
at compile time, its overhead disappears.
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
<para>
|
|
<emphasis>Drawbacks: </emphasis>
|
|
<itemizedlist>
|
|
<listitem><para>
|
|
You must recompile the application code with custom options.
|
|
</para></listitem>
|
|
<listitem><para>You must run the application on representative input.
|
|
The advice is input dependent.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
The execution time will increase, in some cases by factors.
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.using" xreflabel="Using"><info><title>Using the Profile Mode</title></info>
|
|
|
|
|
|
<para>
|
|
This is the anticipated common workflow for program <code>foo.cc</code>:
|
|
<programlisting>
|
|
$ cat foo.cc
|
|
#include <vector>
|
|
int main() {
|
|
vector<int> v;
|
|
for (int k = 0; k < 1024; ++k) v.insert(v.begin(), k);
|
|
}
|
|
|
|
$ g++ -D_GLIBCXX_PROFILE foo.cc
|
|
$ ./a.out
|
|
$ cat libstdcxx-profile.txt
|
|
vector-to-list: improvement = 5: call stack = 0x804842c ...
|
|
: advice = change std::vector to std::list
|
|
vector-size: improvement = 3: call stack = 0x804842c ...
|
|
: advice = change initial container size from 0 to 1024
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Anatomy of a warning:
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Warning id. This is a short descriptive string for the class
|
|
that this warning belongs to. E.g., "vector-to-list".
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Estimated improvement. This is an approximation of the benefit expected
|
|
from implementing the change suggested by the warning. It is given on
|
|
a log10 scale. Negative values mean that the alternative would actually
|
|
do worse than the current choice.
|
|
In the example above, 5 comes from the fact that the overhead of
|
|
inserting at the beginning of a vector vs. a list is around 1024 * 1024 / 2,
|
|
which is around 10e5. The improvement from setting the initial size to
|
|
1024 is in the range of 10e3, since the overhead of dynamic resizing is
|
|
linear in this case.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Call stack. Currently, the addresses are printed without
|
|
symbol name or code location attribution.
|
|
Users are expected to postprocess the output using, for instance, addr2line.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The warning message. For some warnings, this is static text, e.g.,
|
|
"change vector to list". For other warnings, such as the one above,
|
|
the message contains numeric advice, e.g., the suggested initial size
|
|
of the vector.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>Three files are generated. <code>libstdcxx-profile.txt</code>
|
|
contains human readable advice. <code>libstdcxx-profile.raw</code>
|
|
contains implementation specific data about each diagnostic.
|
|
Their format is not documented. They are sufficient to generate
|
|
all the advice given in <code>libstdcxx-profile.txt</code>. The advantage
|
|
of keeping this raw format is that traces from multiple executions can
|
|
be aggregated simply by concatenating the raw traces. We intend to
|
|
offer an external utility program that can issue advice from a trace.
|
|
<code>libstdcxx-profile.conf.out</code> lists the actual diagnostic
|
|
parameters used. To alter parameters, edit this file and rename it to
|
|
<code>libstdcxx-profile.conf</code>.
|
|
</para>
|
|
|
|
<para>Advice is given regardless whether the transformation is valid.
|
|
For instance, we advise changing a map to an unordered_map even if the
|
|
application semantics require that data be ordered.
|
|
We believe such warnings can help users understand the performance
|
|
behavior of their application better, which can lead to changes
|
|
at a higher abstraction level.
|
|
</para>
|
|
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.tuning" xreflabel="Tuning"><info><title>Tuning the Profile Mode</title></info>
|
|
|
|
|
|
<para>Compile time switches and environment variables (see also file
|
|
profiler.h). Unless specified otherwise, they can be set at compile time
|
|
using -D_<name> or by setting variable <name>
|
|
in the environment where the program is run, before starting execution.
|
|
<itemizedlist>
|
|
<listitem><para>
|
|
<code>_GLIBCXX_PROFILE_NO_<diagnostic></code>:
|
|
disable specific diagnostics.
|
|
See section Diagnostics for possible values.
|
|
(Environment variables not supported.)
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<code>_GLIBCXX_PROFILE_TRACE_PATH_ROOT</code>: set an alternative root
|
|
path for the output files.
|
|
</para></listitem>
|
|
<listitem><para>_GLIBCXX_PROFILE_MAX_WARN_COUNT: set it to the maximum
|
|
number of warnings desired. The default value is 10.</para></listitem>
|
|
<listitem><para>
|
|
<code>_GLIBCXX_PROFILE_MAX_STACK_DEPTH</code>: if set to 0,
|
|
the advice will
|
|
be collected and reported for the program as a whole, and not for each
|
|
call context.
|
|
This could also be used in continuous regression tests, where you
|
|
just need to know whether there is a regression or not.
|
|
The default value is 32.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<code>_GLIBCXX_PROFILE_MEM_PER_DIAGNOSTIC</code>:
|
|
set a limit on how much memory to use for the accounting tables for each
|
|
diagnostic type. When this limit is reached, new events are ignored
|
|
until the memory usage decreases under the limit. Generally, this means
|
|
that newly created containers will not be instrumented until some
|
|
live containers are deleted. The default is 128 MB.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<code>_GLIBCXX_PROFILE_NO_THREADS</code>:
|
|
Make the library not use threads. If thread local storage (TLS) is not
|
|
available, you will get a preprocessor error asking you to set
|
|
-D_GLIBCXX_PROFILE_NO_THREADS if your program is single-threaded.
|
|
Multithreaded execution without TLS is not supported.
|
|
(Environment variable not supported.)
|
|
</para></listitem>
|
|
<listitem><para>
|
|
<code>_GLIBCXX_HAVE_EXECINFO_H</code>:
|
|
This name should be defined automatically at library configuration time.
|
|
If your library was configured without <code>execinfo.h</code>, but
|
|
you have it in your include path, you can define it explicitly. Without
|
|
it, advice is collected for the program as a whole, and not for each
|
|
call context.
|
|
(Environment variable not supported.)
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
</section>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.design" xreflabel="Design"><info><title>Design</title></info>
|
|
<?dbhtml filename="profile_mode_design.html"?>
|
|
|
|
|
|
<para>
|
|
</para>
|
|
<table frame="all" xml:id="table.profile_code_loc">
|
|
<title>Profile Code Location</title>
|
|
|
|
<tgroup cols="2" align="left" colsep="1" rowsep="1">
|
|
<colspec colname="c1"/>
|
|
<colspec colname="c2"/>
|
|
|
|
<thead>
|
|
<row>
|
|
<entry>Code Location</entry>
|
|
<entry>Use</entry>
|
|
</row>
|
|
</thead>
|
|
<tbody>
|
|
<row>
|
|
<entry><code>libstdc++-v3/include/std/*</code></entry>
|
|
<entry>Preprocessor code to redirect to profile extension headers.</entry>
|
|
</row>
|
|
<row>
|
|
<entry><code>libstdc++-v3/include/profile/*</code></entry>
|
|
<entry>Profile extension public headers (map, vector, ...).</entry>
|
|
</row>
|
|
<row>
|
|
<entry><code>libstdc++-v3/include/profile/impl/*</code></entry>
|
|
<entry>Profile extension internals. Implementation files are
|
|
only included from <code>impl/profiler.h</code>, which is the only
|
|
file included from the public headers.</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<para>
|
|
</para>
|
|
|
|
<section xml:id="manual.ext.profile_mode.design.wrapper" xreflabel="Wrapper"><info><title>Wrapper Model</title></info>
|
|
|
|
<para>
|
|
In order to get our instrumented library version included instead of the
|
|
release one,
|
|
we use the same wrapper model as the debug mode.
|
|
We subclass entities from the release version. Wherever
|
|
<code>_GLIBCXX_PROFILE</code> is defined, the release namespace is
|
|
<code>std::__norm</code>, whereas the profile namespace is
|
|
<code>std::__profile</code>. Using plain <code>std</code> translates
|
|
into <code>std::__profile</code>.
|
|
</para>
|
|
<para>
|
|
Whenever possible, we try to wrap at the public interface level, e.g.,
|
|
in <code>unordered_set</code> rather than in <code>hashtable</code>,
|
|
in order not to depend on implementation.
|
|
</para>
|
|
<para>
|
|
Mixing object files built with and without the profile mode must
|
|
not affect the program execution. However, there are no guarantees to
|
|
the accuracy of diagnostics when using even a single object not built with
|
|
<code>-D_GLIBCXX_PROFILE</code>.
|
|
Currently, mixing the profile mode with debug and parallel extensions is
|
|
not allowed. Mixing them at compile time will result in preprocessor errors.
|
|
Mixing them at link time is undefined.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.design.instrumentation" xreflabel="Instrumentation"><info><title>Instrumentation</title></info>
|
|
|
|
<para>
|
|
Instead of instrumenting every public entry and exit point,
|
|
we chose to add instrumentation on demand, as needed
|
|
by individual diagnostics.
|
|
The main reason is that some diagnostics require us to extract bits of
|
|
internal state that are particular only to that diagnostic.
|
|
We plan to formalize this later, after we learn more about the requirements
|
|
of several diagnostics.
|
|
</para>
|
|
<para>
|
|
All the instrumentation points can be switched on and off using
|
|
<code>-D[_NO]_GLIBCXX_PROFILE_<diagnostic></code> options.
|
|
With all the instrumentation calls off, there should be negligible
|
|
overhead over the release version. This property is needed to support
|
|
diagnostics based on timing of internal operations. For such diagnostics,
|
|
we anticipate turning most of the instrumentation off in order to prevent
|
|
profiling overhead from polluting time measurements, and thus diagnostics.
|
|
</para>
|
|
<para>
|
|
All the instrumentation on/off compile time switches live in
|
|
<code>include/profile/profiler.h</code>.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.design.rtlib" xreflabel="Run Time Behavior"><info><title>Run Time Behavior</title></info>
|
|
|
|
<para>
|
|
For practical reasons, the instrumentation library processes the trace
|
|
partially
|
|
rather than dumping it to disk in raw form. Each event is processed when
|
|
it occurs. It is usually attached a cost and it is aggregated into
|
|
the database of a specific diagnostic class. The cost model
|
|
is based largely on the standard performance guarantees, but in some
|
|
cases we use knowledge about GCC's standard library implementation.
|
|
</para>
|
|
<para>
|
|
Information is indexed by (1) call stack and (2) instance id or address
|
|
to be able to understand and summarize precise creation-use-destruction
|
|
dynamic chains. Although the analysis is sensitive to dynamic instances,
|
|
the reports are only sensitive to call context. Whenever a dynamic instance
|
|
is destroyed, we accumulate its effect to the corresponding entry for the
|
|
call stack of its constructor location.
|
|
</para>
|
|
|
|
<para>
|
|
For details, see
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://ieeexplore.ieee.org/document/4907670/">paper presented at
|
|
CGO 2009</link>.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.design.analysis" xreflabel="Analysis and Diagnostics"><info><title>Analysis and Diagnostics</title></info>
|
|
|
|
<para>
|
|
Final analysis takes place offline, and it is based entirely on the
|
|
generated trace and debugging info in the application binary.
|
|
See section Diagnostics for a list of analysis types that we plan to support.
|
|
</para>
|
|
<para>
|
|
The input to the analysis is a table indexed by profile type and call stack.
|
|
The data type for each entry depends on the profile type.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.design.cost-model" xreflabel="Cost Model"><info><title>Cost Model</title></info>
|
|
|
|
<para>
|
|
While it is likely that cost models become complex as we get into
|
|
more sophisticated analysis, we will try to follow a simple set of rules
|
|
at the beginning.
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Relative benefit estimation:</emphasis>
|
|
The idea is to estimate or measure the cost of all operations
|
|
in the original scenario versus the scenario we advise to switch to.
|
|
For instance, when advising to change a vector to a list, an occurrence
|
|
of the <code>insert</code> method will generally count as a benefit.
|
|
Its magnitude depends on (1) the number of elements that get shifted
|
|
and (2) whether it triggers a reallocation.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Synthetic measurements:</emphasis>
|
|
We will measure the relative difference between similar operations on
|
|
different containers. We plan to write a battery of small tests that
|
|
compare the times of the executions of similar methods on different
|
|
containers. The idea is to run these tests on the target machine.
|
|
If this training phase is very quick, we may decide to perform it at
|
|
library initialization time. The results can be cached on disk and reused
|
|
across runs.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Timers:</emphasis>
|
|
We plan to use timers for operations of larger granularity, such as sort.
|
|
For instance, we can switch between different sort methods on the fly
|
|
and report the one that performs best for each call context.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Show stoppers:</emphasis>
|
|
We may decide that the presence of an operation nullifies the advice.
|
|
For instance, when considering switching from <code>set</code> to
|
|
<code>unordered_set</code>, if we detect use of operator <code>++</code>,
|
|
we will simply not issue the advice, since this could signal that the use
|
|
care require a sorted container.</para></listitem>
|
|
</itemizedlist>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.design.reports" xreflabel="Reports"><info><title>Reports</title></info>
|
|
|
|
<para>
|
|
There are two types of reports. First, if we recognize a pattern for which
|
|
we have a substitute that is likely to give better performance, we print
|
|
the advice and estimated performance gain. The advice is usually associated
|
|
to a code position and possibly a call stack.
|
|
</para>
|
|
<para>
|
|
Second, we report performance characteristics for which we do not have
|
|
a clear solution for improvement. For instance, we can point to the user
|
|
the top 10 <code>multimap</code> locations
|
|
which have the worst data locality in actual traversals.
|
|
Although this does not offer a solution,
|
|
it helps the user focus on the key problems and ignore the uninteresting ones.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.design.testing" xreflabel="Testing"><info><title>Testing</title></info>
|
|
|
|
<para>
|
|
First, we want to make sure we preserve the behavior of the release mode.
|
|
You can just type <code>"make check-profile"</code>, which
|
|
builds and runs the whole test suite in profile mode.
|
|
</para>
|
|
<para>
|
|
Second, we want to test the correctness of each diagnostic.
|
|
We created a <code>profile</code> directory in the test suite.
|
|
Each diagnostic must come with at least two tests, one for false positives
|
|
and one for false negatives.
|
|
</para>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.api" xreflabel="API"><info><title>Extensions for Custom Containers</title></info>
|
|
<?dbhtml filename="profile_mode_api.html"?>
|
|
|
|
|
|
<para>
|
|
Many large projects use their own data structures instead of the ones in the
|
|
standard library. If these data structures are similar in functionality
|
|
to the standard library, they can be instrumented with the same hooks
|
|
that are used to instrument the standard library.
|
|
The instrumentation API is exposed in file
|
|
<code>profiler.h</code> (look for "Instrumentation hooks").
|
|
</para>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.cost_model" xreflabel="Cost Model"><info><title>Empirical Cost Model</title></info>
|
|
<?dbhtml filename="profile_mode_cost_model.html"?>
|
|
|
|
|
|
<para>
|
|
Currently, the cost model uses formulas with predefined relative weights
|
|
for alternative containers or container implementations. For instance,
|
|
iterating through a vector is X times faster than iterating through a list.
|
|
</para>
|
|
<para>
|
|
(Under development.)
|
|
We are working on customizing this to a particular machine by providing
|
|
an automated way to compute the actual relative weights for operations
|
|
on the given machine.
|
|
</para>
|
|
<para>
|
|
(Under development.)
|
|
We plan to provide a performance parameter database format that can be
|
|
filled in either by hand or by an automated training mechanism.
|
|
The analysis module will then use this database instead of the built in.
|
|
generic parameters.
|
|
</para>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.implementation" xreflabel="Implementation"><info><title>Implementation Issues</title></info>
|
|
<?dbhtml filename="profile_mode_impl.html"?>
|
|
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.implementation.stack" xreflabel="Stack Traces"><info><title>Stack Traces</title></info>
|
|
|
|
<para>
|
|
Accurate stack traces are needed during profiling since we group events by
|
|
call context and dynamic instance. Without accurate traces, diagnostics
|
|
may be hard to interpret. For instance, when giving advice to the user
|
|
it is imperative to reference application code, not library code.
|
|
</para>
|
|
<para>
|
|
Currently we are using the libc <code>backtrace</code> routine to get
|
|
stack traces.
|
|
<code>_GLIBCXX_PROFILE_STACK_DEPTH</code> can be set
|
|
to 0 if you are willing to give up call context information, or to a small
|
|
positive value to reduce run time overhead.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.implementation.symbols" xreflabel="Symbolization"><info><title>Symbolization of Instruction Addresses</title></info>
|
|
|
|
<para>
|
|
The profiling and analysis phases use only instruction addresses.
|
|
An external utility such as addr2line is needed to postprocess the result.
|
|
We do not plan to add symbolization support in the profile extension.
|
|
This would require access to symbol tables, debug information tables,
|
|
external programs or libraries and other system dependent information.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.implementation.concurrency" xreflabel="Concurrency"><info><title>Concurrency</title></info>
|
|
|
|
<para>
|
|
Our current model is simplistic, but precise.
|
|
We cannot afford to approximate because some of our diagnostics require
|
|
precise matching of operations to container instance and call context.
|
|
During profiling, we keep a single information table per diagnostic.
|
|
There is a single lock per information table.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.implementation.stdlib-in-proflib" xreflabel="Using the Standard Library in the Runtime Library"><info><title>Using the Standard Library in the Instrumentation Implementation</title></info>
|
|
|
|
<para>
|
|
As much as we would like to avoid uses of libstdc++ within our
|
|
instrumentation library, containers such as unordered_map are very
|
|
appealing. We plan to use them as long as they are named properly
|
|
to avoid ambiguity.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.implementation.malloc-hooks" xreflabel="Malloc Hooks"><info><title>Malloc Hooks</title></info>
|
|
|
|
<para>
|
|
User applications/libraries can provide malloc hooks.
|
|
When the implementation of the malloc hooks uses stdlibc++, there can
|
|
be an infinite cycle between the profile mode instrumentation and the
|
|
malloc hook code.
|
|
</para>
|
|
<para>
|
|
We protect against reentrance to the profile mode instrumentation code,
|
|
which should avoid this problem in most cases.
|
|
The protection mechanism is thread safe and exception safe.
|
|
This mechanism does not prevent reentrance to the malloc hook itself,
|
|
which could still result in deadlock, if, for instance, the malloc hook
|
|
uses non-recursive locks.
|
|
XXX: A definitive solution to this problem would be for the profile extension
|
|
to use a custom allocator internally, and perhaps not to use libstdc++.
|
|
</para>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.implementation.construction-destruction" xreflabel="Construction and Destruction of Global Objects"><info><title>Construction and Destruction of Global Objects</title></info>
|
|
|
|
<para>
|
|
The profiling library state is initialized at the first call to a profiling
|
|
method. This allows us to record the construction of all global objects.
|
|
However, we cannot do the same at destruction time. The trace is written
|
|
by a function registered by <code>atexit</code>, thus invoked by
|
|
<code>exit</code>.
|
|
</para>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.developer" xreflabel="Developer Information"><info><title>Developer Information</title></info>
|
|
<?dbhtml filename="profile_mode_devel.html"?>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.developer.bigpic" xreflabel="Big Picture"><info><title>Big Picture</title></info>
|
|
|
|
|
|
<para>The profile mode headers are included with
|
|
<code>-D_GLIBCXX_PROFILE</code> through preprocessor directives in
|
|
<code>include/std/*</code>.
|
|
</para>
|
|
|
|
<para>Instrumented implementations are provided in
|
|
<code>include/profile/*</code>. All instrumentation hooks are macros
|
|
defined in <code>include/profile/profiler.h</code>.
|
|
</para>
|
|
|
|
<para>All the implementation of the instrumentation hooks is in
|
|
<code>include/profile/impl/*</code>. Although all the code gets included,
|
|
thus is publicly visible, only a small number of functions are called from
|
|
outside this directory. All calls to hook implementations must be
|
|
done through macros defined in <code>profiler.h</code>. The macro
|
|
must ensure (1) that the call is guarded against reentrance and
|
|
(2) that the call can be turned off at compile time using a
|
|
<code>-D_GLIBCXX_PROFILE_...</code> compiler option.
|
|
</para>
|
|
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.developer.howto" xreflabel="How To Add A Diagnostic"><info><title>How To Add A Diagnostic</title></info>
|
|
|
|
|
|
<para>Let's say the diagnostic name is "magic".
|
|
</para>
|
|
|
|
<para>If you need to instrument a header not already under
|
|
<code>include/profile/*</code>, first edit the corresponding header
|
|
under <code>include/std/</code> and add a preprocessor directive such
|
|
as the one in <code>include/std/vector</code>:
|
|
<programlisting>
|
|
#ifdef _GLIBCXX_PROFILE
|
|
# include <profile/vector>
|
|
#endif
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>If the file you need to instrument is not yet under
|
|
<code>include/profile/</code>, make a copy of the one in
|
|
<code>include/debug</code>, or the main implementation.
|
|
You'll need to include the main implementation and inherit the classes
|
|
you want to instrument. Then define the methods you want to instrument,
|
|
define the instrumentation hooks and add calls to them.
|
|
Look at <code>include/profile/vector</code> for an example.
|
|
</para>
|
|
|
|
<para>Add macros for the instrumentation hooks in
|
|
<code>include/profile/impl/profiler.h</code>.
|
|
Hook names must start with <code>__profcxx_</code>.
|
|
Make sure they transform
|
|
in no code with <code>-D_NO_GLIBCXX_PROFILE_MAGIC</code>.
|
|
Make sure all calls to any method in namespace <code>__gnu_profile</code>
|
|
is protected against reentrance using macro
|
|
<code>_GLIBCXX_PROFILE_REENTRANCE_GUARD</code>.
|
|
All names of methods in namespace <code>__gnu_profile</code> called from
|
|
<code>profiler.h</code> must start with <code>__trace_magic_</code>.
|
|
</para>
|
|
|
|
<para>Add the implementation of the diagnostic.
|
|
<itemizedlist>
|
|
<listitem><para>
|
|
Create new file <code>include/profile/impl/profiler_magic.h</code>.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
Define class <code>__magic_info: public __object_info_base</code>.
|
|
This is the representation of a line in the object table.
|
|
The <code>__merge</code> method is used to aggregate information
|
|
across all dynamic instances created at the same call context.
|
|
The <code>__magnitude</code> must return the estimation of the benefit
|
|
as a number of small operations, e.g., number of words copied.
|
|
The <code>__write</code> method is used to produce the raw trace.
|
|
The <code>__advice</code> method is used to produce the advice string.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
Define class <code>__magic_stack_info: public __magic_info</code>.
|
|
This defines the content of a line in the stack table.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
Define class <code>__trace_magic: public __trace_base<__magic_info,
|
|
__magic_stack_info></code>.
|
|
It defines the content of the trace associated with this diagnostic.
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>Add initialization and reporting calls in
|
|
<code>include/profile/impl/profiler_trace.h</code>. Use
|
|
<code>__trace_vector_to_list</code> as an example.
|
|
</para>
|
|
|
|
<para>Add documentation in file <code>doc/xml/manual/profile_mode.xml</code>.
|
|
</para>
|
|
</section>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.diagnostics"><info><title>Diagnostics</title></info>
|
|
<?dbhtml filename="profile_mode_diagnostics.html"?>
|
|
|
|
|
|
<para>
|
|
The table below presents all the diagnostics we intend to implement.
|
|
Each diagnostic has a corresponding compile time switch
|
|
<code>-D_GLIBCXX_PROFILE_<diagnostic></code>.
|
|
Groups of related diagnostics can be turned on with a single switch.
|
|
For instance, <code>-D_GLIBCXX_PROFILE_LOCALITY</code> is equivalent to
|
|
<code>-D_GLIBCXX_PROFILE_SOFTWARE_PREFETCH
|
|
-D_GLIBCXX_PROFILE_RBTREE_LOCALITY</code>.
|
|
</para>
|
|
|
|
<para>
|
|
The benefit, cost, expected frequency and accuracy of each diagnostic
|
|
was given a grade from 1 to 10, where 10 is highest.
|
|
A high benefit means that, if the diagnostic is accurate, the expected
|
|
performance improvement is high.
|
|
A high cost means that turning this diagnostic on leads to high slowdown.
|
|
A high frequency means that we expect this to occur relatively often.
|
|
A high accuracy means that the diagnostic is unlikely to be wrong.
|
|
These grades are not perfect. They are just meant to guide users with
|
|
specific needs or time budgets.
|
|
</para>
|
|
|
|
<table frame="all" xml:id="table.profile_diagnostics">
|
|
<title>Profile Diagnostics</title>
|
|
|
|
<tgroup cols="7" align="left" colsep="1" rowsep="1">
|
|
<colspec colname="c1"/>
|
|
<colspec colname="c2"/>
|
|
<colspec colname="c3"/>
|
|
<colspec colname="c4"/>
|
|
<colspec colname="c5"/>
|
|
<colspec colname="c6"/>
|
|
<colspec colname="c7"/>
|
|
|
|
<thead>
|
|
<row>
|
|
<entry>Group</entry>
|
|
<entry>Flag</entry>
|
|
<entry>Benefit</entry>
|
|
<entry>Cost</entry>
|
|
<entry>Freq.</entry>
|
|
<entry>Implemented</entry>
|
|
</row>
|
|
</thead>
|
|
<tbody>
|
|
<row>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.containers">
|
|
CONTAINERS</link></entry>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.hashtable_too_small">
|
|
HASHTABLE_TOO_SMALL</link></entry>
|
|
<entry>10</entry>
|
|
<entry>1</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>yes</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.hashtable_too_large">
|
|
HASHTABLE_TOO_LARGE</link></entry>
|
|
<entry>5</entry>
|
|
<entry>1</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>yes</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.inefficient_hash">
|
|
INEFFICIENT_HASH</link></entry>
|
|
<entry>7</entry>
|
|
<entry>3</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>yes</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.vector_too_small">
|
|
VECTOR_TOO_SMALL</link></entry>
|
|
<entry>8</entry>
|
|
<entry>1</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>yes</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.vector_too_large">
|
|
VECTOR_TOO_LARGE</link></entry>
|
|
<entry>5</entry>
|
|
<entry>1</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>yes</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.vector_to_hashtable">
|
|
VECTOR_TO_HASHTABLE</link></entry>
|
|
<entry>7</entry>
|
|
<entry>7</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>no</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.hashtable_to_vector">
|
|
HASHTABLE_TO_VECTOR</link></entry>
|
|
<entry>7</entry>
|
|
<entry>7</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>no</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.vector_to_list">
|
|
VECTOR_TO_LIST</link></entry>
|
|
<entry>8</entry>
|
|
<entry>5</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>yes</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.list_to_vector">
|
|
LIST_TO_VECTOR</link></entry>
|
|
<entry>10</entry>
|
|
<entry>5</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>no</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.assoc_ord_to_unord">
|
|
ORDERED_TO_UNORDERED</link></entry>
|
|
<entry>10</entry>
|
|
<entry>5</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>only map/unordered_map</entry>
|
|
</row>
|
|
<row>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.algorithms">
|
|
ALGORITHMS</link></entry>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.algorithms.sort">
|
|
SORT</link></entry>
|
|
<entry>7</entry>
|
|
<entry>8</entry>
|
|
<entry/>
|
|
<entry>7</entry>
|
|
<entry>no</entry>
|
|
</row>
|
|
<row>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.locality">
|
|
LOCALITY</link></entry>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.locality.sw_prefetch">
|
|
SOFTWARE_PREFETCH</link></entry>
|
|
<entry>8</entry>
|
|
<entry>8</entry>
|
|
<entry/>
|
|
<entry>5</entry>
|
|
<entry>no</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.locality.linked">
|
|
RBTREE_LOCALITY</link></entry>
|
|
<entry>4</entry>
|
|
<entry>8</entry>
|
|
<entry/>
|
|
<entry>5</entry>
|
|
<entry>no</entry>
|
|
</row>
|
|
<row>
|
|
<entry/>
|
|
<entry><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="#manual.ext.profile_mode.analysis.mthread.false_share">
|
|
FALSE_SHARING</link></entry>
|
|
<entry>8</entry>
|
|
<entry>10</entry>
|
|
<entry/>
|
|
<entry>10</entry>
|
|
<entry>no</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.template" xreflabel="Template"><info><title>Diagnostic Template</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_<diagnostic></code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> What problem will it diagnose?
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>.
|
|
What is the fundamental reason why this is a problem</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>
|
|
Percentage reduction in execution time. When reduction is more than
|
|
a constant factor, describe the reduction rate formula.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>
|
|
What would the advise look like?</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis>
|
|
What stdlibc++ components need to be instrumented?</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
How do we decide when to issue the advice?</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
How do we measure benefits? Math goes here.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
program code
|
|
...
|
|
advice sample
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.containers" xreflabel="Containers"><info><title>Containers</title></info>
|
|
|
|
|
|
<para>
|
|
<emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_CONTAINERS</code>.
|
|
</para>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.hashtable_too_small" xreflabel="Hashtable Too Small"><info><title>Hashtable Too Small</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_HASHTABLE_TOO_SMALL</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect hashtables with many
|
|
rehash operations, small construction size and large destruction size.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis> Rehash is very expensive.
|
|
Read content, follow chains within bucket, evaluate hash function, place at
|
|
new location in different order.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis> 36%.
|
|
Code similar to example below.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>
|
|
Set initial size to N at construction site S.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis>
|
|
<code>unordered_set, unordered_map</code> constructor, destructor, rehash.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
For each dynamic instance of <code>unordered_[multi]set|map</code>,
|
|
record initial size and call context of the constructor.
|
|
Record size increase, if any, after each relevant operation such as insert.
|
|
Record the estimated rehash cost.</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Number of individual rehash operations * cost per rehash.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 unordered_set<int> us;
|
|
2 for (int k = 0; k < 1000000; ++k) {
|
|
3 us.insert(k);
|
|
4 }
|
|
|
|
foo.cc:1: advice: Changing initial unordered_set size from 10 to 1000000 saves 1025530 rehash operations.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.hashtable_too_large" xreflabel="Hashtable Too Large"><info><title>Hashtable Too Large</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_HASHTABLE_TOO_LARGE</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect hashtables which are
|
|
never filled up because fewer elements than reserved are ever
|
|
inserted.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis> Save memory, which
|
|
is good in itself and may also improve memory reference performance through
|
|
fewer cache and TLB misses.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis> unknown.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>
|
|
Set initial size to N at construction site S.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis>
|
|
<code>unordered_set, unordered_map</code> constructor, destructor, rehash.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
For each dynamic instance of <code>unordered_[multi]set|map</code>,
|
|
record initial size and call context of the constructor, and correlate it
|
|
with its size at destruction time.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Number of iteration operations + memory saved.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 vector<unordered_set<int>> v(100000, unordered_set<int>(100)) ;
|
|
2 for (int k = 0; k < 100000; ++k) {
|
|
3 for (int j = 0; j < 10; ++j) {
|
|
4 v[k].insert(k + j);
|
|
5 }
|
|
6 }
|
|
|
|
foo.cc:1: advice: Changing initial unordered_set size from 100 to 10 saves N
|
|
bytes of memory and M iteration steps.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.inefficient_hash" xreflabel="Inefficient Hash"><info><title>Inefficient Hash</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_INEFFICIENT_HASH</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect hashtables with polarized
|
|
distribution.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis> A non-uniform
|
|
distribution may lead to long chains, thus possibly increasing complexity
|
|
by a factor up to the number of elements.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis> factor up
|
|
to container size.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis> Change hash function
|
|
for container built at site S. Distribution score = N. Access score = S.
|
|
Longest chain = C, in bucket B.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis>
|
|
<code>unordered_set, unordered_map</code> constructor, destructor, [],
|
|
insert, iterator.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
Count the exact number of link traversals.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Total number of links traversed.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
class dumb_hash {
|
|
public:
|
|
size_t operator() (int i) const { return 0; }
|
|
};
|
|
...
|
|
unordered_set<int, dumb_hash> hs;
|
|
...
|
|
for (int i = 0; i < COUNT; ++i) {
|
|
hs.find(i);
|
|
}
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.vector_too_small" xreflabel="Vector Too Small"><info><title>Vector Too Small</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_VECTOR_TOO_SMALL</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis>Detect vectors with many
|
|
resize operations, small construction size and large destruction size..
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>Resizing can be expensive.
|
|
Copying large amounts of data takes time. Resizing many small vectors may
|
|
have allocation overhead and affect locality.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>
|
|
Set initial size to N at construction site S.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis><code>vector</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
For each dynamic instance of <code>vector</code>,
|
|
record initial size and call context of the constructor.
|
|
Record size increase, if any, after each relevant operation such as
|
|
<code>push_back</code>. Record the estimated resize cost.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Total number of words copied * time to copy a word.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 vector<int> v;
|
|
2 for (int k = 0; k < 1000000; ++k) {
|
|
3 v.push_back(k);
|
|
4 }
|
|
|
|
foo.cc:1: advice: Changing initial vector size from 10 to 1000000 saves
|
|
copying 4000000 bytes and 20 memory allocations and deallocations.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.vector_too_large" xreflabel="Vector Too Large"><info><title>Vector Too Large</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_VECTOR_TOO_LARGE</code>
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis>Detect vectors which are
|
|
never filled up because fewer elements than reserved are ever
|
|
inserted.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>Save memory, which
|
|
is good in itself and may also improve memory reference performance through
|
|
fewer cache and TLB misses.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>
|
|
Set initial size to N at construction site S.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis><code>vector</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
For each dynamic instance of <code>vector</code>,
|
|
record initial size and call context of the constructor, and correlate it
|
|
with its size at destruction time.</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Total amount of memory saved.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 vector<vector<int>> v(100000, vector<int>(100)) ;
|
|
2 for (int k = 0; k < 100000; ++k) {
|
|
3 for (int j = 0; j < 10; ++j) {
|
|
4 v[k].insert(k + j);
|
|
5 }
|
|
6 }
|
|
|
|
foo.cc:1: advice: Changing initial vector size from 100 to 10 saves N
|
|
bytes of memory and may reduce the number of cache and TLB misses.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.vector_to_hashtable" xreflabel="Vector to Hashtable"><info><title>Vector to Hashtable</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_VECTOR_TO_HASHTABLE</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect uses of
|
|
<code>vector</code> that can be substituted with <code>unordered_set</code>
|
|
to reduce execution time.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
Linear search in a vector is very expensive, whereas searching in a hashtable
|
|
is very quick.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>factor up
|
|
to container size.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>Replace
|
|
<code>vector</code> with <code>unordered_set</code> at site S.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis><code>vector</code>
|
|
operations and access methods.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
For each dynamic instance of <code>vector</code>,
|
|
record call context of the constructor. Issue the advice only if the
|
|
only methods called on this <code>vector</code> are <code>push_back</code>,
|
|
<code>insert</code> and <code>find</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Cost(vector::push_back) + cost(vector::insert) + cost(find, vector) -
|
|
cost(unordered_set::insert) + cost(unordered_set::find).
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 vector<int> v;
|
|
...
|
|
2 for (int i = 0; i < 1000; ++i) {
|
|
3 find(v.begin(), v.end(), i);
|
|
4 }
|
|
|
|
foo.cc:1: advice: Changing "vector" to "unordered_set" will save about 500,000
|
|
comparisons.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.hashtable_to_vector" xreflabel="Hashtable to Vector"><info><title>Hashtable to Vector</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_HASHTABLE_TO_VECTOR</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect uses of
|
|
<code>unordered_set</code> that can be substituted with <code>vector</code>
|
|
to reduce execution time.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
Hashtable iterator is slower than vector iterator.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>95%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>Replace
|
|
<code>unordered_set</code> with <code>vector</code> at site S.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis><code>unordered_set</code>
|
|
operations and access methods.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
For each dynamic instance of <code>unordered_set</code>,
|
|
record call context of the constructor. Issue the advice only if the
|
|
number of <code>find</code>, <code>insert</code> and <code>[]</code>
|
|
operations on this <code>unordered_set</code> are small relative to the
|
|
number of elements, and methods <code>begin</code> or <code>end</code>
|
|
are invoked (suggesting iteration).</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Number of .</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 unordered_set<int> us;
|
|
...
|
|
2 int s = 0;
|
|
3 for (unordered_set<int>::iterator it = us.begin(); it != us.end(); ++it) {
|
|
4 s += *it;
|
|
5 }
|
|
|
|
foo.cc:1: advice: Changing "unordered_set" to "vector" will save about N
|
|
indirections and may achieve better data locality.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.vector_to_list" xreflabel="Vector to List"><info><title>Vector to List</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_VECTOR_TO_LIST</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect cases where
|
|
<code>vector</code> could be substituted with <code>list</code> for
|
|
better performance.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
Inserting in the middle of a vector is expensive compared to inserting in a
|
|
list.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>factor up to
|
|
container size.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>Replace vector with list
|
|
at site S.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis><code>vector</code>
|
|
operations and access methods.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
For each dynamic instance of <code>vector</code>,
|
|
record the call context of the constructor. Record the overhead of each
|
|
<code>insert</code> operation based on current size and insert position.
|
|
Report instance with high insertion overhead.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
(Sum(cost(vector::method)) - Sum(cost(list::method)), for
|
|
method in [push_back, insert, erase])
|
|
+ (Cost(iterate vector) - Cost(iterate list))</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 vector<int> v;
|
|
2 for (int i = 0; i < 10000; ++i) {
|
|
3 v.insert(v.begin(), i);
|
|
4 }
|
|
|
|
foo.cc:1: advice: Changing "vector" to "list" will save about 5,000,000
|
|
operations.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.list_to_vector" xreflabel="List to Vector"><info><title>List to Vector</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_LIST_TO_VECTOR</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect cases where
|
|
<code>list</code> could be substituted with <code>vector</code> for
|
|
better performance.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
Iterating through a vector is faster than through a list.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>64%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>Replace list with vector
|
|
at site S.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis><code>vector</code>
|
|
operations and access methods.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
Issue the advice if there are no <code>insert</code> operations.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
(Sum(cost(vector::method)) - Sum(cost(list::method)), for
|
|
method in [push_back, insert, erase])
|
|
+ (Cost(iterate vector) - Cost(iterate list))</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 list<int> l;
|
|
...
|
|
2 int sum = 0;
|
|
3 for (list<int>::iterator it = l.begin(); it != l.end(); ++it) {
|
|
4 sum += *it;
|
|
5 }
|
|
|
|
foo.cc:1: advice: Changing "list" to "vector" will save about 1000000 indirect
|
|
memory references.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.list_to_slist" xreflabel="List to Forward List"><info><title>List to Forward List (Slist)</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_LIST_TO_SLIST</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect cases where
|
|
<code>list</code> could be substituted with <code>forward_list</code> for
|
|
better performance.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
The memory footprint of a forward_list is smaller than that of a list.
|
|
This has beneficial effects on memory subsystem, e.g., fewer cache misses.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>40%.
|
|
Note that the reduction is only noticeable if the size of the forward_list
|
|
node is in fact larger than that of the list node. For memory allocators
|
|
with size classes, you will only notice an effect when the two node sizes
|
|
belong to different allocator size classes.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>Replace list with
|
|
forward_list at site S.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis><code>list</code>
|
|
operations and iteration methods.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
Issue the advice if there are no <code>backwards</code> traversals
|
|
or insertion before a given node.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Always true.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 list<int> l;
|
|
...
|
|
2 int sum = 0;
|
|
3 for (list<int>::iterator it = l.begin(); it != l.end(); ++it) {
|
|
4 sum += *it;
|
|
5 }
|
|
|
|
foo.cc:1: advice: Change "list" to "forward_list".
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.assoc_ord_to_unord" xreflabel="Ordered to Unordered Associative Container"><info><title>Ordered to Unordered Associative Container</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_ORDERED_TO_UNORDERED</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect cases where ordered
|
|
associative containers can be replaced with unordered ones.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
Insert and search are quicker in a hashtable than in
|
|
a red-black tree.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>52%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>
|
|
Replace set with unordered_set at site S.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis>
|
|
<code>set</code>, <code>multiset</code>, <code>map</code>,
|
|
<code>multimap</code> methods.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
Issue the advice only if we are not using operator <code>++</code> on any
|
|
iterator on a particular <code>[multi]set|map</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
(Sum(cost(hashtable::method)) - Sum(cost(rbtree::method)), for
|
|
method in [insert, erase, find])
|
|
+ (Cost(iterate hashtable) - Cost(iterate rbtree))</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 set<int> s;
|
|
2 for (int i = 0; i < 100000; ++i) {
|
|
3 s.insert(i);
|
|
4 }
|
|
5 int sum = 0;
|
|
6 for (int i = 0; i < 100000; ++i) {
|
|
7 sum += *s.find(i);
|
|
8 }
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.algorithms" xreflabel="Algorithms"><info><title>Algorithms</title></info>
|
|
|
|
|
|
<para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_ALGORITHMS</code>.
|
|
</para>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.algorithms.sort" xreflabel="Sorting"><info><title>Sort Algorithm Performance</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_SORT</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Give measure of sort algorithm
|
|
performance based on actual input. For instance, advise Radix Sort over
|
|
Quick Sort for a particular call context.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
See papers:
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://dl.acm.org/citation.cfm?doid=1065944.1065981">
|
|
A framework for adaptive algorithm selection in STAPL</link> and
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://ieeexplore.ieee.org/document/4228227/">
|
|
Optimizing Sorting with Machine Learning Algorithms</link>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>60%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis> Change sort algorithm
|
|
at site S from X Sort to Y Sort.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis> <code>sort</code>
|
|
algorithm.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
Issue the advice if the cost model tells us that another sort algorithm
|
|
would do better on this input. Requires us to know what algorithm we
|
|
are using in our sort implementation in release mode.</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Runtime(algo) for algo in [radix, quick, merge, ...]</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.locality" xreflabel="Data Locality"><info><title>Data Locality</title></info>
|
|
|
|
|
|
<para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_LOCALITY</code>.
|
|
</para>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.locality.sw_prefetch" xreflabel="Need Software Prefetch"><info><title>Need Software Prefetch</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_SOFTWARE_PREFETCH</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Discover sequences of indirect
|
|
memory accesses that are not regular, thus cannot be predicted by
|
|
hardware prefetchers.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
Indirect references are hard to predict and are very expensive when they
|
|
miss in caches.</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>25%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis> Insert prefetch
|
|
instruction.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis> Vector iterator and
|
|
access operator [].
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
First, get cache line size and page size from system.
|
|
Then record iterator dereference sequences for which the value is a pointer.
|
|
For each sequence within a container, issue a warning if successive pointer
|
|
addresses are not within cache lines and do not form a linear pattern
|
|
(otherwise they may be prefetched by hardware).
|
|
If they also step across page boundaries, make the warning stronger.
|
|
</para>
|
|
<para>The same analysis applies to containers other than vector.
|
|
However, we cannot give the same advice for linked structures, such as list,
|
|
as there is no random access to the n-th element. The user may still be
|
|
able to benefit from this information, for instance by employing frays (user
|
|
level light weight threads) to hide the latency of chasing pointers.
|
|
</para>
|
|
<para>
|
|
This analysis is a little oversimplified. A better cost model could be
|
|
created by understanding the capability of the hardware prefetcher.
|
|
This model could be trained automatically by running a set of synthetic
|
|
cases.
|
|
</para>
|
|
</listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Total distance between pointer values of successive elements in vectors
|
|
of pointers.</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 int zero = 0;
|
|
2 vector<int*> v(10000000, &zero);
|
|
3 for (int k = 0; k < 10000000; ++k) {
|
|
4 v[random() % 10000000] = new int(k);
|
|
5 }
|
|
6 for (int j = 0; j < 10000000; ++j) {
|
|
7 count += (*v[j] == 0 ? 0 : 1);
|
|
8 }
|
|
|
|
foo.cc:7: advice: Insert prefetch instruction.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.locality.linked" xreflabel="Linked Structure Locality"><info><title>Linked Structure Locality</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_RBTREE_LOCALITY</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Give measure of locality of
|
|
objects stored in linked structures (lists, red-black trees and hashtables)
|
|
with respect to their actual traversal patterns.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>Allocation can be tuned
|
|
to a specific traversal pattern, to result in better data locality.
|
|
See paper:
|
|
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://parasol.tamu.edu/publications/download.php?file_id=570">
|
|
Custom Memory Allocation for Free</link> by Jula and Rauchwerger.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>30%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis>
|
|
High scatter score N for container built at site S.
|
|
Consider changing allocation sequence or choosing a structure conscious
|
|
allocator.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis> Methods of all
|
|
containers using linked structures.</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
First, get cache line size and page size from system.
|
|
Then record the number of successive elements that are on different line
|
|
or page, for each traversal method such as <code>find</code>. Give advice
|
|
only if the ratio between this number and the number of total node hops
|
|
is above a threshold.</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Sum(same_cache_line(this,previous))</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 set<int> s;
|
|
2 for (int i = 0; i < 10000000; ++i) {
|
|
3 s.insert(i);
|
|
4 }
|
|
5 set<int> s1, s2;
|
|
6 for (int i = 0; i < 10000000; ++i) {
|
|
7 s1.insert(i);
|
|
8 s2.insert(i);
|
|
9 }
|
|
...
|
|
// Fast, better locality.
|
|
10 for (set<int>::iterator it = s.begin(); it != s.end(); ++it) {
|
|
11 sum += *it;
|
|
12 }
|
|
// Slow, elements are further apart.
|
|
13 for (set<int>::iterator it = s1.begin(); it != s1.end(); ++it) {
|
|
14 sum += *it;
|
|
15 }
|
|
|
|
foo.cc:5: advice: High scatter score NNN for set built here. Consider changing
|
|
the allocation sequence or switching to a structure conscious allocator.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.mthread" xreflabel="Multithreaded Data Access"><info><title>Multithreaded Data Access</title></info>
|
|
|
|
|
|
<para>
|
|
The diagnostics in this group are not meant to be implemented short term.
|
|
They require compiler support to know when container elements are written
|
|
to. Instrumentation can only tell us when elements are referenced.
|
|
</para>
|
|
|
|
<para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_MULTITHREADED</code>.
|
|
</para>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.mthread.ddtest" xreflabel="Dependence Violations at Container Level"><info><title>Data Dependence Violations at Container Level</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_DDTEST</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect container elements
|
|
that are referenced from multiple threads in the parallel region or
|
|
across parallel regions.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis>
|
|
Sharing data between threads requires communication and perhaps locking,
|
|
which may be expensive.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>?%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis> Change data
|
|
distribution or parallel algorithm.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis> Container access methods
|
|
and iterators.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
Keep a shadow for each container. Record iterator dereferences and
|
|
container member accesses. Issue advice for elements referenced by
|
|
multiple threads.
|
|
See paper: <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://dl.acm.org/citation.cfm?id=207110.207148">
|
|
The LRPD test: speculative run-time parallelization of loops with
|
|
privatization and reduction parallelization</link>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Number of accesses to elements referenced from multiple threads
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.mthread.false_share" xreflabel="False Sharing"><info><title>False Sharing</title></info>
|
|
|
|
<itemizedlist>
|
|
<listitem><para><emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_FALSE_SHARING</code>.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Goal:</emphasis> Detect elements in the
|
|
same container which share a cache line, are written by at least one
|
|
thread, and accessed by different threads.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Fundamentals:</emphasis> Under these assumptions,
|
|
cache protocols require
|
|
communication to invalidate lines, which may be expensive.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Sample runtime reduction:</emphasis>68%.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Recommendation:</emphasis> Reorganize container
|
|
or use padding to avoid false sharing.</para></listitem>
|
|
<listitem><para><emphasis>To instrument:</emphasis> Container access methods
|
|
and iterators.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Analysis:</emphasis>
|
|
First, get the cache line size.
|
|
For each shared container, record all the associated iterator dereferences
|
|
and member access methods with the thread id. Compare the address lists
|
|
across threads to detect references in two different threads to the same
|
|
cache line. Issue a warning only if the ratio to total references is
|
|
significant. Do the same for iterator dereference values if they are
|
|
pointers.</para></listitem>
|
|
<listitem><para><emphasis>Cost model:</emphasis>
|
|
Number of accesses to same cache line from different threads.
|
|
</para></listitem>
|
|
<listitem><para><emphasis>Example:</emphasis>
|
|
<programlisting>
|
|
1 vector<int> v(2, 0);
|
|
2 #pragma omp parallel for shared(v, SIZE) schedule(static, 1)
|
|
3 for (i = 0; i < SIZE; ++i) {
|
|
4 v[i % 2] += i;
|
|
5 }
|
|
|
|
OMP_NUM_THREADS=2 ./a.out
|
|
foo.cc:1: advice: Change container structure or padding to avoid false
|
|
sharing in multithreaded access at foo.cc:4. Detected N shared cache lines.
|
|
</programlisting>
|
|
</para></listitem>
|
|
</itemizedlist>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
|
|
<section xml:id="manual.ext.profile_mode.analysis.statistics" xreflabel="Statistics"><info><title>Statistics</title></info>
|
|
|
|
|
|
<para>
|
|
<emphasis>Switch:</emphasis>
|
|
<code>_GLIBCXX_PROFILE_STATISTICS</code>.
|
|
</para>
|
|
|
|
<para>
|
|
In some cases the cost model may not tell us anything because the costs
|
|
appear to offset the benefits. Consider the choice between a vector and
|
|
a list. When there are both inserts and iteration, an automatic advice
|
|
may not be issued. However, the programmer may still be able to make use
|
|
of this information in a different way.
|
|
</para>
|
|
<para>
|
|
This diagnostic will not issue any advice, but it will print statistics for
|
|
each container construction site. The statistics will contain the cost
|
|
of each operation actually performed on the container.
|
|
</para>
|
|
|
|
</section>
|
|
|
|
|
|
</section>
|
|
|
|
|
|
<bibliography xml:id="profile_mode.biblio"><info><title>Bibliography</title></info>
|
|
|
|
|
|
<biblioentry>
|
|
<citetitle>
|
|
Perflint: A Context Sensitive Performance Advisor for C++ Programs
|
|
</citetitle>
|
|
|
|
<author><personname><firstname>Lixia</firstname><surname>Liu</surname></personname></author>
|
|
<author><personname><firstname>Silvius</firstname><surname>Rus</surname></personname></author>
|
|
|
|
<copyright>
|
|
<year>2009</year>
|
|
<holder/>
|
|
</copyright>
|
|
|
|
<publisher>
|
|
<publishername>
|
|
Proceedings of the 2009 International Symposium on Code Generation
|
|
and Optimization
|
|
</publishername>
|
|
</publisher>
|
|
</biblioentry>
|
|
</bibliography>
|
|
|
|
|
|
</chapter>
|