--- /dev/null
+// -*- C++ -*-
+
+// Copyright (C) 2007, 2008, 2009 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the terms
+// of the GNU General Public License as published by the Free Software
+// Foundation; either version 3, or (at your option) any later
+// version.
+
+// This library is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+// General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file parallel/workstealing.h
+ * @brief Parallelization of embarrassingly parallel execution by
+ * means of work-stealing.
+ *
+ * Work stealing is described in
+ *
+ * R. D. Blumofe and C. E. Leiserson.
+ * Scheduling multithreaded computations by work stealing.
+ * Journal of the ACM, 46(5):720–748, 1999.
+ *
+ * This file is a GNU parallel extension to the Standard C++ Library.
+ */
+
+// Written by Felix Putze.
+
+#ifndef _GLIBCXX_PARALLEL_WORKSTEALING_H
+#define _GLIBCXX_PARALLEL_WORKSTEALING_H 1
+
+#include <parallel/parallel.h>
+#include <parallel/random_number.h>
+#include <parallel/compatibility.h>
+
+namespace __gnu_parallel
+{
+
+#define _GLIBCXX_JOB_VOLATILE volatile
+
+/** @brief One job for a certain thread. */
+template<typename _DifferenceTp>
+ struct Job
+ {
+ typedef _DifferenceTp difference_type;
+
+ /** @brief First element.
+ *
+ * Changed by owning and stealing thread. By stealing thread,
+ * always incremented. */
+ _GLIBCXX_JOB_VOLATILE difference_type first;
+
+ /** @brief Last element.
+ *
+ * Changed by owning thread only. */
+ _GLIBCXX_JOB_VOLATILE difference_type last;
+
+ /** @brief Number of elements, i. e. @c last-first+1.
+ *
+ * Changed by owning thread only. */
+ _GLIBCXX_JOB_VOLATILE difference_type load;
+ };
+
+/** @brief Work stealing algorithm for random access iterators.
+ *
+ * Uses O(1) additional memory. Synchronization at job lists is
+ * done with atomic operations.
+ * @param begin Begin iterator of element sequence.
+ * @param end End iterator of element sequence.
+ * @param op User-supplied functor (comparator, predicate, adding
+ * functor, ...).
+ * @param f Functor to "process" an element with op (depends on
+ * desired functionality, e. g. for std::for_each(), ...).
+ * @param r Functor to "add" a single result to the already
+ * processed elements (depends on functionality).
+ * @param base Base value for reduction.
+ * @param output Pointer to position where final result is written to
+ * @param bound Maximum number of elements processed (e. g. for
+ * std::count_n()).
+ * @return User-supplied functor (that may contain a part of the result).
+ */
+template<typename RandomAccessIterator,
+ typename Op,
+ typename Fu,
+ typename Red,
+ typename Result>
+ Op
+ for_each_template_random_access_workstealing(RandomAccessIterator begin,
+ RandomAccessIterator end,
+ Op op, Fu& f, Red r,
+ Result base, Result& output,
+ typename std::iterator_traits
+ <RandomAccessIterator>::
+ difference_type bound)
+ {
+ _GLIBCXX_CALL(end - begin)
+
+ typedef std::iterator_traits<RandomAccessIterator> traits_type;
+ typedef typename traits_type::difference_type difference_type;
+
+ const _Settings& __s = _Settings::get();
+
+ difference_type chunk_size = static_cast<difference_type>(__s.workstealing_chunk_size);
+
+ // How many jobs?
+ difference_type length = (bound < 0) ? (end - begin) : bound;
+
+ // To avoid false sharing in a cache line.
+ const int stride = __s.cache_line_size * 10 / sizeof(Job<difference_type>) + 1;
+
+ // Total number of threads currently working.
+ thread_index_t busy = 0;
+
+ Job<difference_type> *job;
+
+ omp_lock_t output_lock;
+ omp_init_lock(&output_lock);
+
+ // Write base value to output.
+ output = base;
+
+ // No more threads than jobs, at least one thread.
+ thread_index_t num_threads =
+ __gnu_parallel::max<thread_index_t>(1,
+ __gnu_parallel::min<difference_type>(length, get_max_threads()));
+
+# pragma omp parallel shared(busy) num_threads(num_threads)
+ {
+
+# pragma omp single
+ {
+ num_threads = omp_get_num_threads();
+
+ // Create job description array.
+ job = new Job<difference_type>[num_threads * stride];
+ }
+
+ // Initialization phase.
+
+ // Flags for every thread if it is doing productive work.
+ bool iam_working = false;
+
+ // Thread id.
+ thread_index_t iam = omp_get_thread_num();
+
+ // This job.
+ Job<difference_type>& my_job = job[iam * stride];
+
+ // Random number (for work stealing).
+ thread_index_t victim;
+
+ // Local value for reduction.
+ Result result = Result();
+
+ // Number of elements to steal in one attempt.
+ difference_type steal;
+
+ // Every thread has its own random number generator
+ // (modulo num_threads).
+ random_number rand_gen(iam, num_threads);
+
+ // This thread is currently working.
+# pragma omp atomic
+ ++busy;
+
+ iam_working = true;
+
+ // How many jobs per thread? last thread gets the rest.
+ my_job.first =
+ static_cast<difference_type>(iam * (length / num_threads));
+
+ my_job.last = (iam == (num_threads - 1)) ?
+ (length - 1) : ((iam + 1) * (length / num_threads) - 1);
+ my_job.load = my_job.last - my_job.first + 1;
+
+ // Init result with first value (to have a base value for reduction).
+ if (my_job.first <= my_job.last)
+ {
+ // Cannot use volatile variable directly.
+ difference_type my_first = my_job.first;
+ result = f(op, begin + my_first);
+ ++my_job.first;
+ --my_job.load;
+ }
+
+ RandomAccessIterator current;
+
+# pragma omp barrier
+
+ // Actual work phase
+ // Work on own or stolen start
+ while (busy > 0)
+ {
+ // Work until no productive thread left.
+# pragma omp flush(busy)
+
+ // Thread has own work to do
+ while (my_job.first <= my_job.last)
+ {
+ // fetch-and-add call
+ // Reserve current job block (size chunk_size) in my queue.
+ difference_type current_job =
+ fetch_and_add<difference_type>(&(my_job.first), chunk_size);
+
+ // Update load, to make the three values consistent,
+ // first might have been changed in the meantime
+ my_job.load = my_job.last - my_job.first + 1;
+ for (difference_type job_counter = 0;
+ job_counter < chunk_size && current_job <= my_job.last;
+ ++job_counter)
+ {
+ // Yes: process it!
+ current = begin + current_job;
+ ++current_job;
+
+ // Do actual work.
+ result = r(result, f(op, current));
+ }
+
+# pragma omp flush(busy)
+ }
+
+ // After reaching this point, a thread's job list is empty.
+ if (iam_working)
+ {
+ // This thread no longer has work.
+# pragma omp atomic
+ --busy;
+
+ iam_working = false;
+ }
+
+ difference_type supposed_first, supposed_last, supposed_load;
+ do
+ {
+ // Find random nonempty deque (not own), do consistency check.
+ yield();
+# pragma omp flush(busy)
+ victim = rand_gen();
+ supposed_first = job[victim * stride].first;
+ supposed_last = job[victim * stride].last;
+ supposed_load = job[victim * stride].load;
+ }
+ while (busy > 0
+ && ((supposed_load <= 0)
+ || ((supposed_first + supposed_load - 1) != supposed_last)));
+
+ if (busy == 0)
+ break;
+
+ if (supposed_load > 0)
+ {
+ // Has work and work to do.
+ // Number of elements to steal (at least one).
+ steal = (supposed_load < 2) ? 1 : supposed_load / 2;
+
+ // Push victim's start forward.
+ difference_type stolen_first =
+ fetch_and_add<difference_type>(
+ &(job[victim * stride].first), steal);
+ difference_type stolen_try =
+ stolen_first + steal - difference_type(1);
+
+ my_job.first = stolen_first;
+ my_job.last = __gnu_parallel::min(stolen_try, supposed_last);
+ my_job.load = my_job.last - my_job.first + 1;
+
+ // Has potential work again.
+# pragma omp atomic
+ ++busy;
+ iam_working = true;
+
+# pragma omp flush(busy)
+ }
+# pragma omp flush(busy)
+ } // end while busy > 0
+ // Add accumulated result to output.
+ omp_set_lock(&output_lock);
+ output = r(output, result);
+ omp_unset_lock(&output_lock);
+ }
+
+ delete[] job;
+
+ // Points to last element processed (needed as return value for
+ // some algorithms like transform)
+ f.finish_iterator = begin + length;
+
+ omp_destroy_lock(&output_lock);
+
+ return op;
+ }
+} // end namespace
+
+#endif /* _GLIBCXX_PARALLEL_WORKSTEALING_H */
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