307 lines
9.9 KiB
C++
307 lines
9.9 KiB
C++
// -*- C++ -*-
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// Copyright (C) 2007, 2008, 2009 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 difference_type;
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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 difference_type 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 difference_type last;
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/** @brief Number of elements, i. e. @c last-first+1.
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*
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* Changed by owning thread only. */
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_GLIBCXX_JOB_VOLATILE difference_type 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 "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 "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 RandomAccessIterator,
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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(RandomAccessIterator begin,
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RandomAccessIterator end,
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Op op, Fu& f, Red r,
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Result base, Result& output,
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typename std::iterator_traits
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<RandomAccessIterator>::
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difference_type bound)
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{
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_GLIBCXX_CALL(end - begin)
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typedef std::iterator_traits<RandomAccessIterator> traits_type;
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typedef typename traits_type::difference_type difference_type;
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const _Settings& __s = _Settings::get();
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difference_type chunk_size = static_cast<difference_type>(__s.workstealing_chunk_size);
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// How many jobs?
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difference_type 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 / sizeof(Job<difference_type>) + 1;
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// Total number of threads currently working.
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thread_index_t busy = 0;
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Job<difference_type> *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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thread_index_t num_threads =
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__gnu_parallel::max<thread_index_t>(1,
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__gnu_parallel::min<difference_type>(length, 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<difference_type>[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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thread_index_t iam = omp_get_thread_num();
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// This job.
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Job<difference_type>& my_job = job[iam * stride];
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// Random number (for work stealing).
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thread_index_t 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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difference_type steal;
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// Every thread has its own random number generator
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// (modulo num_threads).
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random_number 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.first =
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static_cast<difference_type>(iam * (length / num_threads));
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my_job.last = (iam == (num_threads - 1)) ?
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(length - 1) : ((iam + 1) * (length / num_threads) - 1);
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my_job.load = my_job.last - my_job.first + 1;
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// Init result with first value (to have a base value for reduction).
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if (my_job.first <= my_job.last)
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{
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// Cannot use volatile variable directly.
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difference_type my_first = my_job.first;
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result = f(op, begin + my_first);
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++my_job.first;
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--my_job.load;
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}
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RandomAccessIterator current;
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# pragma omp barrier
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// Actual work phase
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// Work on own or stolen 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.first <= my_job.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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difference_type current_job =
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fetch_and_add<difference_type>(&(my_job.first), chunk_size);
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// Update load, to make the three values consistent,
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// first might have been changed in the meantime
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my_job.load = my_job.last - my_job.first + 1;
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for (difference_type job_counter = 0;
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job_counter < chunk_size && current_job <= my_job.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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difference_type supposed_first, supposed_last, 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].first;
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supposed_last = job[victim * stride].last;
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supposed_load = job[victim * stride].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) != 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 start forward.
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difference_type stolen_first =
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fetch_and_add<difference_type>(
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&(job[victim * stride].first), steal);
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difference_type stolen_try =
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stolen_first + steal - difference_type(1);
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my_job.first = stolen_first;
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my_job.last = __gnu_parallel::min(stolen_try, supposed_last);
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my_job.load = my_job.last - my_job.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.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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