313 lines
9.4 KiB
C++
313 lines
9.4 KiB
C++
// -*- C++ -*-
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// Copyright (C) 2007-2020 Free Software Foundation, Inc.
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//
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// This file is part of the GNU ISO C++ Library. This library is free
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// software; you can redistribute it and/or modify it under the terms
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// of the GNU General Public License as published by the Free Software
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// Foundation; either version 3, or (at your option) any later
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// version.
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// This library is distributed in the hope that it will be useful, but
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// WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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// General Public License for more details.
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// Under Section 7 of GPL version 3, you are granted additional
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// permissions described in the GCC Runtime Library Exception, version
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// 3.1, as published by the Free Software Foundation.
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// You should have received a copy of the GNU General Public License and
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// a copy of the GCC Runtime Library Exception along with this program;
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// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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// <http://www.gnu.org/licenses/>.
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/** @file parallel/workstealing.h
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* @brief Parallelization of embarrassingly parallel execution by
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* means of work-stealing.
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*
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* Work stealing is described in
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*
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* R. D. Blumofe and C. E. Leiserson.
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* Scheduling multithreaded computations by work stealing.
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* Journal of the ACM, 46(5):720-748, 1999.
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*
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* This file is a GNU parallel extension to the Standard C++ Library.
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*/
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// Written by Felix Putze.
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#ifndef _GLIBCXX_PARALLEL_WORKSTEALING_H
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#define _GLIBCXX_PARALLEL_WORKSTEALING_H 1
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#include <parallel/parallel.h>
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#include <parallel/random_number.h>
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#include <parallel/compatibility.h>
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namespace __gnu_parallel
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{
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#define _GLIBCXX_JOB_VOLATILE volatile
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/** @brief One __job for a certain thread. */
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template<typename _DifferenceTp>
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struct _Job
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{
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typedef _DifferenceTp _DifferenceType;
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/** @brief First element.
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*
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* Changed by owning and stealing thread. By stealing thread,
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* always incremented. */
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_GLIBCXX_JOB_VOLATILE _DifferenceType _M_first;
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/** @brief Last element.
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*
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* Changed by owning thread only. */
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_GLIBCXX_JOB_VOLATILE _DifferenceType _M_last;
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/** @brief Number of elements, i.e. @c _M_last-_M_first+1.
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*
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* Changed by owning thread only. */
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_GLIBCXX_JOB_VOLATILE _DifferenceType _M_load;
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};
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/** @brief Work stealing algorithm for random access iterators.
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*
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* Uses O(1) additional memory. Synchronization at job lists is
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* done with atomic operations.
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* @param __begin Begin iterator of element sequence.
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* @param __end End iterator of element sequence.
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* @param __op User-supplied functor (comparator, predicate, adding
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* functor, ...).
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* @param __f Functor to @a process an element with __op (depends on
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* desired functionality, e. g. for std::for_each(), ...).
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* @param __r Functor to @a add a single __result to the already
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* processed elements (depends on functionality).
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* @param __base Base value for reduction.
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* @param __output Pointer to position where final result is written to
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* @param __bound Maximum number of elements processed (e. g. for
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* std::count_n()).
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* @return User-supplied functor (that may contain a part of the result).
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*/
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template<typename _RAIter,
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typename _Op,
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typename _Fu,
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typename _Red,
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typename _Result>
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_Op
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__for_each_template_random_access_workstealing(_RAIter __begin,
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_RAIter __end, _Op __op,
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_Fu& __f, _Red __r,
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_Result __base,
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_Result& __output,
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typename std::iterator_traits<_RAIter>::difference_type __bound)
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{
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_GLIBCXX_CALL(__end - __begin)
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typedef std::iterator_traits<_RAIter> _TraitsType;
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typedef typename _TraitsType::difference_type _DifferenceType;
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const _Settings& __s = _Settings::get();
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_DifferenceType __chunk_size =
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static_cast<_DifferenceType>(__s.workstealing_chunk_size);
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// How many jobs?
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_DifferenceType __length = (__bound < 0) ? (__end - __begin) : __bound;
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// To avoid false sharing in a cache line.
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const int __stride = (__s.cache_line_size * 10
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/ sizeof(_Job<_DifferenceType>) + 1);
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// Total number of threads currently working.
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_ThreadIndex __busy = 0;
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_Job<_DifferenceType> *__job;
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omp_lock_t __output_lock;
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omp_init_lock(&__output_lock);
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// Write base value to output.
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__output = __base;
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// No more threads than jobs, at least one thread.
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_ThreadIndex __num_threads = __gnu_parallel::max<_ThreadIndex>
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(1, __gnu_parallel::min<_DifferenceType>(__length,
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__get_max_threads()));
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# pragma omp parallel shared(__busy) num_threads(__num_threads)
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{
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# pragma omp single
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{
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__num_threads = omp_get_num_threads();
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// Create job description array.
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__job = new _Job<_DifferenceType>[__num_threads * __stride];
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}
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// Initialization phase.
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// Flags for every thread if it is doing productive work.
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bool __iam_working = false;
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// Thread id.
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_ThreadIndex __iam = omp_get_thread_num();
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// This job.
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_Job<_DifferenceType>& __my_job = __job[__iam * __stride];
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// Random number (for work stealing).
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_ThreadIndex __victim;
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// Local value for reduction.
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_Result __result = _Result();
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// Number of elements to steal in one attempt.
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_DifferenceType __steal;
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// Every thread has its own random number generator
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// (modulo __num_threads).
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_RandomNumber __rand_gen(__iam, __num_threads);
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// This thread is currently working.
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# pragma omp atomic
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++__busy;
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__iam_working = true;
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// How many jobs per thread? last thread gets the rest.
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__my_job._M_first = static_cast<_DifferenceType>
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(__iam * (__length / __num_threads));
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__my_job._M_last = (__iam == (__num_threads - 1)
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? (__length - 1)
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: ((__iam + 1) * (__length / __num_threads) - 1));
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__my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
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// Init result with _M_first value (to have a base value for reduction)
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if (__my_job._M_first <= __my_job._M_last)
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{
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// Cannot use volatile variable directly.
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_DifferenceType __my_first = __my_job._M_first;
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__result = __f(__op, __begin + __my_first);
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++__my_job._M_first;
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--__my_job._M_load;
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}
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_RAIter __current;
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# pragma omp barrier
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// Actual work phase
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// Work on own or stolen current start
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while (__busy > 0)
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{
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// Work until no productive thread left.
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# pragma omp flush(__busy)
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// Thread has own work to do
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while (__my_job._M_first <= __my_job._M_last)
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{
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// fetch-and-add call
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// Reserve current job block (size __chunk_size) in my queue.
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_DifferenceType __current_job =
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__fetch_and_add<_DifferenceType>(&(__my_job._M_first),
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__chunk_size);
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// Update _M_load, to make the three values consistent,
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// _M_first might have been changed in the meantime
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__my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
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for (_DifferenceType __job_counter = 0;
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__job_counter < __chunk_size
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&& __current_job <= __my_job._M_last;
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++__job_counter)
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{
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// Yes: process it!
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__current = __begin + __current_job;
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++__current_job;
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// Do actual work.
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__result = __r(__result, __f(__op, __current));
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}
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# pragma omp flush(__busy)
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}
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// After reaching this point, a thread's __job list is empty.
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if (__iam_working)
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{
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// This thread no longer has work.
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# pragma omp atomic
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--__busy;
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__iam_working = false;
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}
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_DifferenceType __supposed_first, __supposed_last,
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__supposed_load;
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do
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{
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// Find random nonempty deque (not own), do consistency check.
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__yield();
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# pragma omp flush(__busy)
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__victim = __rand_gen();
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__supposed_first = __job[__victim * __stride]._M_first;
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__supposed_last = __job[__victim * __stride]._M_last;
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__supposed_load = __job[__victim * __stride]._M_load;
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}
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while (__busy > 0
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&& ((__supposed_load <= 0)
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|| ((__supposed_first + __supposed_load - 1)
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!= __supposed_last)));
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if (__busy == 0)
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break;
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if (__supposed_load > 0)
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{
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// Has work and work to do.
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// Number of elements to steal (at least one).
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__steal = (__supposed_load < 2) ? 1 : __supposed_load / 2;
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// Push __victim's current start forward.
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_DifferenceType __stolen_first =
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__fetch_and_add<_DifferenceType>
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(&(__job[__victim * __stride]._M_first), __steal);
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_DifferenceType __stolen_try = (__stolen_first + __steal
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- _DifferenceType(1));
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__my_job._M_first = __stolen_first;
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__my_job._M_last = __gnu_parallel::min(__stolen_try,
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__supposed_last);
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__my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
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// Has potential work again.
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# pragma omp atomic
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++__busy;
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__iam_working = true;
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# pragma omp flush(__busy)
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}
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# pragma omp flush(__busy)
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} // end while __busy > 0
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// Add accumulated result to output.
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omp_set_lock(&__output_lock);
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__output = __r(__output, __result);
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omp_unset_lock(&__output_lock);
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}
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delete[] __job;
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// Points to last element processed (needed as return value for
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// some algorithms like transform)
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__f._M_finish_iterator = __begin + __length;
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omp_destroy_lock(&__output_lock);
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return __op;
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}
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} // end namespace
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#endif /* _GLIBCXX_PARALLEL_WORKSTEALING_H */
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