# workstealing.h

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    00001 // -*- C++ -*-
    00002
    00003 // Copyright (C) 2007, 2008, 2009, 2010 Free Software Foundation, Inc.
    00004 //
    00005 // This file is part of the GNU ISO C++ Library.  This library is free
    00006 // software; you can redistribute it and/or modify it under the terms
    00007 // of the GNU General Public License as published by the Free Software
    00008 // Foundation; either version 3, or (at your option) any later
    00009 // version.
    00010
    00011 // This library is distributed in the hope that it will be useful, but
    00012 // WITHOUT ANY WARRANTY; without even the implied warranty of
    00013 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    00014 // General Public License for more details.
    00015
    00016 // Under Section 7 of GPL version 3, you are granted additional
    00017 // permissions described in the GCC Runtime Library Exception, version
    00018 // 3.1, as published by the Free Software Foundation.
    00019
    00020 // You should have received a copy of the GNU General Public License and
    00021 // a copy of the GCC Runtime Library Exception along with this program;
    00022 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
    00023 // <http://www.gnu.org/licenses/>.
    00024
    00025 /** @file parallel/workstealing.h
    00026  *  @brief Parallelization of embarrassingly parallel execution by
    00027  *  means of work-stealing.
    00028  *
    00029  *  Work stealing is described in
    00030  *
    00031  *  R. D. Blumofe and C. E. Leiserson.
    00032  *  Scheduling multithreaded computations by work stealing.
    00033  *  Journal of the ACM, 46(5):720–748, 1999.
    00034  *
    00035  *  This file is a GNU parallel extension to the Standard C++ Library.
    00036  */
    00037
    00038 // Written by Felix Putze.
    00039
    00040 #ifndef _GLIBCXX_PARALLEL_WORKSTEALING_H
    00041 #define _GLIBCXX_PARALLEL_WORKSTEALING_H 1
    00042
    00043 #include <parallel/parallel.h>
    00044 #include <parallel/random_number.h>
    00045 #include <parallel/compatibility.h>
    00046
    00047 namespace __gnu_parallel
    00048 {
    00049
    00050 #define _GLIBCXX_JOB_VOLATILE volatile
    00051
    00052   /** @brief One __job for a certain thread. */
    00053   template<typename _DifferenceTp>
    00054     struct _Job
    00055     {
    00056       typedef _DifferenceTp _DifferenceType;
    00057
    00058       /** @brief First element.
    00059        *
    00060        *  Changed by owning and stealing thread. By stealing thread,
    00061        *  always incremented. */
    00062       _GLIBCXX_JOB_VOLATILE _DifferenceType _M_first;
    00063
    00064       /** @brief Last element.
    00065        *
    00066        *  Changed by owning thread only. */
    00067       _GLIBCXX_JOB_VOLATILE _DifferenceType _M_last;
    00068
    00069       /** @brief Number of elements, i.e. @c _M_last-_M_first+1.
    00070        *
    00071        *  Changed by owning thread only. */
    00072       _GLIBCXX_JOB_VOLATILE _DifferenceType _M_load;
    00073     };
    00074
    00075   /** @brief Work stealing algorithm for random access iterators.
    00076     *
    00077     *  Uses O(1) additional memory. Synchronization at job lists is
    00078     *  done with atomic operations.
    00079     *  @param __begin Begin iterator of element sequence.
    00080     *  @param __end End iterator of element sequence.
    00081     *  @param __op User-supplied functor (comparator, predicate, adding
    00082     *  functor, ...).
    00083     *  @param __f Functor to @a process an element with __op (depends on
    00084     *  desired functionality, e. g. for std::for_each(), ...).
    00085     *  @param __r Functor to @a add a single __result to the already
    00086     *  processed elements (depends on functionality).
    00087     *  @param __base Base value for reduction.
    00088     *  @param __output Pointer to position where final result is written to
    00089     *  @param __bound Maximum number of elements processed (e. g. for
    00090     *  std::count_n()).
    00091     *  @return User-supplied functor (that may contain a part of the result).
    00092     */
    00093   template<typename _RAIter,
    00094            typename _Op,
    00095            typename _Fu,
    00096            typename _Red,
    00097            typename _Result>
    00098     _Op
    00099     __for_each_template_random_access_workstealing(_RAIter __begin,
    00100                            _RAIter __end, _Op __op,
    00101                            _Fu& __f, _Red __r,
    00102                            _Result __base,
    00103                            _Result& __output,
    00104       typename std::iterator_traits<_RAIter>::difference_type __bound)
    00105     {
    00106       _GLIBCXX_CALL(__end - __begin)
    00107
    00108       typedef std::iterator_traits<_RAIter> _TraitsType;
    00109       typedef typename _TraitsType::difference_type _DifferenceType;
    00110
    00111       const _Settings& __s = _Settings::get();
    00112
    00113       _DifferenceType __chunk_size =
    00114           static_cast<_DifferenceType>(__s.workstealing_chunk_size);
    00115
    00116       // How many jobs?
    00117       _DifferenceType __length = (__bound < 0) ? (__end - __begin) : __bound;
    00118
    00119       // To avoid false sharing in a cache line.
    00120       const int __stride = (__s.cache_line_size * 10
    00121                 / sizeof(_Job<_DifferenceType>) + 1);
    00122
    00123       // Total number of threads currently working.
    00124       _ThreadIndex __busy = 0;
    00125
    00126       _Job<_DifferenceType> *__job;
    00127
    00128       omp_lock_t __output_lock;
    00129       omp_init_lock(&__output_lock);
    00130
    00131       // Write base value to output.
    00132       __output = __base;
    00133
    00134       // No more threads than jobs, at least one thread.
    00135       _ThreadIndex __num_threads = __gnu_parallel::max<_ThreadIndex>
    00136     (1, __gnu_parallel::min<_DifferenceType>(__length,
    00137                          __get_max_threads()));
    00138
    00139 #     pragma omp parallel shared(__busy) num_threads(__num_threads)
    00140       {
    00141 #       pragma omp single
    00142     {
    00143       __num_threads = omp_get_num_threads();
    00144
    00145       // Create job description array.
    00146       __job = new _Job<_DifferenceType>[__num_threads * __stride];
    00147     }
    00148
    00149     // Initialization phase.
    00150
    00151     // Flags for every thread if it is doing productive work.
    00152     bool __iam_working = false;
    00153
    00154     // Thread id.
    00155     _ThreadIndex __iam = omp_get_thread_num();
    00156
    00157     // This job.
    00158     _Job<_DifferenceType>& __my_job = __job[__iam * __stride];
    00159
    00160     // Random number (for work stealing).
    00161     _ThreadIndex __victim;
    00162
    00163     // Local value for reduction.
    00164     _Result __result = _Result();
    00165
    00166     // Number of elements to steal in one attempt.
    00167     _DifferenceType __steal;
    00168
    00169     // Every thread has its own random number generator
    00170     // (modulo __num_threads).
    00171     _RandomNumber __rand_gen(__iam, __num_threads);
    00172
    00173     // This thread is currently working.
    00174 #       pragma omp atomic
    00175     ++__busy;
    00176
    00177     __iam_working = true;
    00178
    00179     // How many jobs per thread? last thread gets the rest.
    00180     __my_job._M_first = static_cast<_DifferenceType>
    00181       (__iam * (__length / __num_threads));
    00182
    00183     __my_job._M_last = (__iam == (__num_threads - 1)
    00184                 ? (__length - 1)
    00185                 : ((__iam + 1) * (__length / __num_threads) - 1));
    00186     __my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
    00187
    00188     // Init result with _M_first value (to have a base value for reduction)
    00189     if (__my_job._M_first <= __my_job._M_last)
    00190       {
    00191         // Cannot use volatile variable directly.
    00192         _DifferenceType __my_first = __my_job._M_first;
    00193         __result = __f(__op, __begin + __my_first);
    00194         ++__my_job._M_first;
    00195         --__my_job._M_load;
    00196       }
    00197
    00198     _RAIter __current;
    00199
    00200 #       pragma omp barrier
    00201
    00202     // Actual work phase
    00203     // Work on own or stolen current start
    00204     while (__busy > 0)
    00205       {
    00206         // Work until no productive thread left.
    00207 #           pragma omp flush(__busy)
    00208
    00209         // Thread has own work to do
    00210         while (__my_job._M_first <= __my_job._M_last)
    00211           {
    00212         // fetch-and-add call
    00213         // Reserve current job block (size __chunk_size) in my queue.
    00214         _DifferenceType __current_job =
    00215           __fetch_and_add<_DifferenceType>(&(__my_job._M_first),
    00216                            __chunk_size);
    00217
    00218         // Update _M_load, to make the three values consistent,
    00219         // _M_first might have been changed in the meantime
    00220         __my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
    00221         for (_DifferenceType __job_counter = 0;
    00222              __job_counter < __chunk_size
    00223                && __current_job <= __my_job._M_last;
    00224              ++__job_counter)
    00225           {
    00226             // Yes: process it!
    00227             __current = __begin + __current_job;
    00228             ++__current_job;
    00229
    00230             // Do actual work.
    00231             __result = __r(__result, __f(__op, __current));
    00232           }
    00233
    00234 #               pragma omp flush(__busy)
    00235           }
    00236
    00237         // After reaching this point, a thread's __job list is empty.
    00238         if (__iam_working)
    00239           {
    00240         // This thread no longer has work.
    00241 #               pragma omp atomic
    00242         --__busy;
    00243
    00244         __iam_working = false;
    00245           }
    00246
    00247         _DifferenceType __supposed_first, __supposed_last,
    00248                         __supposed_load;
    00249         do
    00250           {
    00251         // Find random nonempty deque (not own), do consistency check.
    00252         __yield();
    00253 #               pragma omp flush(__busy)
    00254         __victim = __rand_gen();
    00255         __supposed_first = __job[__victim * __stride]._M_first;
    00256         __supposed_last = __job[__victim * __stride]._M_last;
    00257         __supposed_load = __job[__victim * __stride]._M_load;
    00258           }
    00259         while (__busy > 0
    00260            && ((__supposed_load <= 0)
    00261                || ((__supposed_first + __supposed_load - 1)
    00262                != __supposed_last)));
    00263
    00264         if (__busy == 0)
    00265           break;
    00266
    00267         if (__supposed_load > 0)
    00268           {
    00269         // Has work and work to do.
    00270         // Number of elements to steal (at least one).
    00271         __steal = (__supposed_load < 2) ? 1 : __supposed_load / 2;
    00272
    00273         // Push __victim's current start forward.
    00274         _DifferenceType __stolen_first =
    00275           __fetch_and_add<_DifferenceType>
    00276           (&(__job[__victim * __stride]._M_first), __steal);
    00277         _DifferenceType __stolen_try = (__stolen_first + __steal
    00278                         - _DifferenceType(1));
    00279
    00280         __my_job._M_first = __stolen_first;
    00281         __my_job._M_last = __gnu_parallel::min(__stolen_try,
    00282                                __supposed_last);
    00283         __my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
    00284
    00285         // Has potential work again.
    00286 #               pragma omp atomic
    00287         ++__busy;
    00288         __iam_working = true;
    00289
    00290 #               pragma omp flush(__busy)
    00291           }
    00292 #           pragma omp flush(__busy)
    00293       } // end while __busy > 0
    00294     // Add accumulated result to output.
    00295     omp_set_lock(&__output_lock);
    00296     __output = __r(__output, __result);
    00297     omp_unset_lock(&__output_lock);
    00298       }
    00299
    00300       delete[] __job;
    00301
    00302       // Points to last element processed (needed as return value for
    00303       // some algorithms like transform)
    00304       __f._M_finish_iterator = __begin + __length;
    00305
    00306       omp_destroy_lock(&__output_lock);
    00307
    00308       return __op;
    00309     }
    00310 } // end namespace
    00311
    00312 #endif /* _GLIBCXX_PARALLEL_WORKSTEALING_H */