parallel.h

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/***************************************************************************
 * include/stxxl/bits/parallel.h
 *
 * Part of the STXXL. See http://stxxl.sourceforge.net
 *
 * Copyright (C) 2008-2010 Andreas Beckmann <[email protected]>
 * Copyright (C) 2011 Johannes Singler <[email protected]>
 *
 * Distributed under the Boost Software License, Version 1.0.
 * (See accompanying file LICENSE_1_0.txt or copy at
 * http://www.boost.org/LICENSE_1_0.txt)
 **************************************************************************/

#ifndef STXXL_PARALLEL_HEADER
#define STXXL_PARALLEL_HEADER

#include <stxxl/bits/config.h>

#undef STXXL_PARALLEL
#undef STXXL_PARALLEL_MODE

#if defined(_GLIBCXX_PARALLEL) || STXXL_PARALLEL_MODE_EXPLICIT
#define STXXL_PARALLEL_MODE
#endif

#if defined(STXXL_PARALLEL_MODE)
#define STXXL_PARALLEL 1
#else
#define STXXL_PARALLEL 0
#endif

#include <cassert>

#ifdef STXXL_PARALLEL_MODE
 #include <omp.h>
#endif

#if STXXL_PARALLEL
 #include <algorithm>
#endif

#include <stxxl/bits/namespace.h>
#include <stxxl/bits/common/settings.h>
#include <stxxl/bits/verbose.h>


#if defined(_GLIBCXX_PARALLEL)
//use _STXXL_FORCE_SEQUENTIAL to tag calls which are not worthwhile parallelizing
#define _STXXL_FORCE_SEQUENTIAL , __gnu_parallel::sequential_tag()
#else
#define _STXXL_FORCE_SEQUENTIAL
#endif

#if 0
// sorting triggers is done sequentially
#define _STXXL_SORT_TRIGGER_FORCE_SEQUENTIAL _STXXL_FORCE_SEQUENTIAL
#else
// sorting triggers may be parallelized
#define _STXXL_SORT_TRIGGER_FORCE_SEQUENTIAL
#endif

#if !STXXL_PARALLEL
#undef STXXL_PARALLEL_MULTIWAY_MERGE
#define STXXL_PARALLEL_MULTIWAY_MERGE 0
#endif

#if defined(STXXL_PARALLEL_MODE) && ((__GNUC__ * 10000 + __GNUC_MINOR__ * 100) < 40400)
#undef STXXL_PARALLEL_MULTIWAY_MERGE
#define STXXL_PARALLEL_MULTIWAY_MERGE 0
#endif

#if !defined(STXXL_PARALLEL_MULTIWAY_MERGE)
#define STXXL_PARALLEL_MULTIWAY_MERGE 1
#endif

#if !defined(STXXL_NOT_CONSIDER_SORT_MEMORY_OVERHEAD)
#define STXXL_NOT_CONSIDER_SORT_MEMORY_OVERHEAD 0
#endif

#if STXXL_PARALLEL_MODE_EXPLICIT
#include <parallel/algorithm>
#else
#include <algorithm>
#endif


STXXL_BEGIN_NAMESPACE

inline unsigned sort_memory_usage_factor()
{
#if STXXL_PARALLEL && !STXXL_NOT_CONSIDER_SORT_MEMORY_OVERHEAD && defined(STXXL_PARALLEL_MODE)
 return (__gnu_parallel::_Settings::get().sort_algorithm == __gnu_parallel::MWMS && omp_get_max_threads() > 1) ? 2 : 1; //memory overhead for multiway mergesort
#else
 return 1; //no overhead
#endif
}

inline void check_sort_settings()
{
#if STXXL_PARALLEL && defined(STXXL_PARALLEL_MODE) && !defined(STXXL_NO_WARN_OMP_NESTED)
 static bool did_warn = false;
 if (!did_warn) {
 if (__gnu_parallel::_Settings::get().sort_algorithm != __gnu_parallel::MWMS) {
 if (omp_get_max_threads() <= 2) {
 did_warn = true; // no problem with at most 2 threads, no need to check again
 } else if (!omp_get_nested()) {
 STXXL_ERRMSG("Inefficient settings detected. To get full potential from your CPU it is recommended to set OMP_NESTED=TRUE in the environment.");
 did_warn = true;
 }
 }
 }
#else
 // nothing to check
#endif
}

inline bool do_parallel_merge()
{
#if STXXL_PARALLEL_MULTIWAY_MERGE && defined(STXXL_PARALLEL_MODE)
 return !stxxl::SETTINGS::native_merge && omp_get_max_threads() >= 1;
#else
 return false;
#endif
}


namespace potentially_parallel {

#if STXXL_PARALLEL_MODE_EXPLICIT
using __gnu_parallel::sort;
using __gnu_parallel::random_shuffle;
#else
using std::sort;
using std::random_shuffle;
#endif

} // namespace potentially_parallel

namespace parallel {

#if STXXL_PARALLEL

/*! Multi-way merging dispatcher.
 * @param seqs_begin Begin iterator of iterator pair input sequence.
 * @param seqs_end End iterator of iterator pair input sequence.
 * @param target Begin iterator out output sequence.
 * @param comp Comparator.
 * @param length Maximum length to merge.
 * @return End iterator of output sequence.
 */
template <typename RandomAccessIteratorPairIterator,
 typename RandomAccessIterator3, typename DiffType, typename Comparator>
RandomAccessIterator3
multiway_merge(RandomAccessIteratorPairIterator seqs_begin,
 RandomAccessIteratorPairIterator seqs_end,
 RandomAccessIterator3 target,
 Comparator comp,
 DiffType length)
{
#if defined(STXXL_PARALLEL_MODE) && ((__GNUC__ * 10000 + __GNUC_MINOR__ * 100) >= 40400)
 return __gnu_parallel::multiway_merge(seqs_begin, seqs_end, target, length, comp);
#elif defined(STXXL_PARALLEL_MODE)
 return __gnu_parallel::multiway_merge(seqs_begin, seqs_end, target, comp, length);
#else
#error "no implementation found for multiway_merge()"
#endif
}

/*! Multi-way merging front-end.
 * @param seqs_begin Begin iterator of iterator pair input sequence.
 * @param seqs_end End iterator of iterator pair input sequence.
 * @param target Begin iterator out output sequence.
 * @param comp Comparator.
 * @param length Maximum length to merge.
 * @return End iterator of output sequence.
 * @pre For each @c i, @c seqs_begin[i].second must be the end marker of the sequence, but also reference the one more sentinel element.
 */
template <typename RandomAccessIteratorPairIterator,
 typename RandomAccessIterator3, typename DiffType, typename Comparator>
RandomAccessIterator3
multiway_merge_sentinel(RandomAccessIteratorPairIterator seqs_begin,
 RandomAccessIteratorPairIterator seqs_end,
 RandomAccessIterator3 target,
 Comparator comp,
 DiffType length)
{
#if defined(STXXL_PARALLEL_MODE) && ((__GNUC__ * 10000 + __GNUC_MINOR__ * 100) >= 40400)
 return __gnu_parallel::multiway_merge_sentinels(seqs_begin, seqs_end, target, length, comp);
#elif defined(STXXL_PARALLEL_MODE)
 return __gnu_parallel::multiway_merge_sentinels(seqs_begin, seqs_end, target, comp, length);
#else
#error "no implementation found for multiway_merge_sentinel()"
#endif
}

#endif

} // namespace parallel

STXXL_END_NAMESPACE

#endif // !STXXL_PARALLEL_HEADER
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