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iothread: document -object iothread on man page 

Add -object iothread documentation to the man page, including references
to the query-iothread QMP command and qom-set syntax for adjusting
adaptive polling parameters at run-time.

Reported-by: Zhenyu Ye <yezhenyu2@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20191025122236.29815-1-stefanha@redhat.com
Message-Id: <20191025122236.29815-1-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2019-10-25 14:22:36 +02:00
parent 5f2585772f
commit 86d2a49b41
1 changed files with 38 additions and 0 deletions
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38
qemu-options.hx
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@ -4926,6 +4926,44 @@ access
CN=laptop.example.com,O=Example Home,L=London,ST=London,C=GB
@end example
@item -object iothread,id=@var{id},poll-max-ns=@var{poll-max-ns},poll-grow=@var{poll-grow},poll-shrink=@var{poll-shrink}
Creates a dedicated event loop thread that devices can be assigned to. This is
known as an IOThread. By default device emulation happens in vCPU threads or
the main event loop thread. This can become a scalability bottleneck.
IOThreads allow device emulation and I/O to run on other host CPUs.
The @option{id} parameter is a unique ID that will be used to reference this
IOThread from @option{-device ...,iothread=@var{id}}. Multiple devices can be
assigned to an IOThread. Note that not all devices support an
@option{iothread} parameter.
The @code{query-iothreads} QMP command lists IOThreads and reports their thread
IDs so that the user can configure host CPU pinning/affinity.
IOThreads use an adaptive polling algorithm to reduce event loop latency.
Instead of entering a blocking system call to monitor file descriptors and then
pay the cost of being woken up when an event occurs, the polling algorithm
spins waiting for events for a short time. The algorithm's default parameters
are suitable for many cases but can be adjusted based on knowledge of the
workload and/or host device latency.
The @option{poll-max-ns} parameter is the maximum number of nanoseconds to busy
wait for events. Polling can be disabled by setting this value to 0.
The @option{poll-grow} parameter is the multiplier used to increase the polling
time when the algorithm detects it is missing events due to not polling long
enough.
The @option{poll-shrink} parameter is the divisor used to decrease the polling
time when the algorithm detects it is spending too long polling without
encountering events.
The polling parameters can be modified at run-time using the @code{qom-set} command (where @code{iothread1} is the IOThread's @code{id}):
@example
(qemu) qom-set /objects/iothread1 poll-max-ns 100000
@end example
@end table