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// -*- 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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