| 1 | /* Parallel for loops |
| 2 | |
| 3 | Copyright (C) 2019-2020 Free Software Foundation, Inc. |
| 4 | |
| 5 | This file is part of GDB. |
| 6 | |
| 7 | This program is free software; you can redistribute it and/or modify |
| 8 | it under the terms of the GNU General Public License as published by |
| 9 | the Free Software Foundation; either version 3 of the License, or |
| 10 | (at your option) any later version. |
| 11 | |
| 12 | This program is distributed in the hope that it will be useful, |
| 13 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 14 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 15 | GNU General Public License for more details. |
| 16 | |
| 17 | You should have received a copy of the GNU General Public License |
| 18 | along with this program. If not, see <http://www.gnu.org/licenses/>. */ |
| 19 | |
| 20 | #ifndef GDBSUPPORT_PARALLEL_FOR_H |
| 21 | #define GDBSUPPORT_PARALLEL_FOR_H |
| 22 | |
| 23 | #include <algorithm> |
| 24 | #if CXX_STD_THREAD |
| 25 | #include <thread> |
| 26 | #include "gdbsupport/thread-pool.h" |
| 27 | #endif |
| 28 | |
| 29 | namespace gdb |
| 30 | { |
| 31 | |
| 32 | /* A very simple "parallel for". This splits the range of iterators |
| 33 | into subranges, and then passes each subrange to the callback. The |
| 34 | work may or may not be done in separate threads. |
| 35 | |
| 36 | This approach was chosen over having the callback work on single |
| 37 | items because it makes it simple for the caller to do |
| 38 | once-per-subrange initialization and destruction. */ |
| 39 | |
| 40 | template<class RandomIt, class RangeFunction> |
| 41 | void |
| 42 | parallel_for_each (RandomIt first, RandomIt last, RangeFunction callback) |
| 43 | { |
| 44 | #if CXX_STD_THREAD |
| 45 | /* So we can use a local array below. */ |
| 46 | const size_t local_max = 16; |
| 47 | size_t n_threads = std::min (thread_pool::g_thread_pool->thread_count (), |
| 48 | local_max); |
| 49 | size_t n_actual_threads = 0; |
| 50 | std::future<void> futures[local_max]; |
| 51 | |
| 52 | size_t n_elements = last - first; |
| 53 | if (n_threads > 1) |
| 54 | { |
| 55 | /* Arbitrarily require that there should be at least 10 elements |
| 56 | in a thread. */ |
| 57 | if (n_elements / n_threads < 10) |
| 58 | n_threads = std::max (n_elements / 10, (size_t) 1); |
| 59 | size_t elts_per_thread = n_elements / n_threads; |
| 60 | n_actual_threads = n_threads - 1; |
| 61 | for (int i = 0; i < n_actual_threads; ++i) |
| 62 | { |
| 63 | RandomIt end = first + elts_per_thread; |
| 64 | auto task = [=] () |
| 65 | { |
| 66 | callback (first, end); |
| 67 | }; |
| 68 | |
| 69 | futures[i] = gdb::thread_pool::g_thread_pool->post_task (task); |
| 70 | first = end; |
| 71 | } |
| 72 | } |
| 73 | #endif /* CXX_STD_THREAD */ |
| 74 | |
| 75 | /* Process all the remaining elements in the main thread. */ |
| 76 | callback (first, last); |
| 77 | |
| 78 | #if CXX_STD_THREAD |
| 79 | for (int i = 0; i < n_actual_threads; ++i) |
| 80 | futures[i].wait (); |
| 81 | #endif /* CXX_STD_THREAD */ |
| 82 | } |
| 83 | |
| 84 | } |
| 85 | |
| 86 | #endif /* GDBSUPPORT_PARALLEL_FOR_H */ |