b7a1325c3c
This patch adds a new `BLI_mutex.hh` header which adds `blender::Mutex` as alias for either `tbb::mutex` or `std::mutex` depending on whether TBB is enabled. Description copied from the patch: ``` /** * blender::Mutex should be used as the default mutex in Blender. It implements a subset of the API * of std::mutex but has overall better guaranteed properties. It can be used with RAII helpers * like std::lock_guard. However, it is not compatible with e.g. std::condition_variable. So one * still has to use std::mutex for that case. * * The mutex provided by TBB has these properties: * - It's as fast as a spin-lock in the non-contended case, i.e. when no other thread is trying to * lock the mutex at the same time. * - In the contended case, it spins a couple of times but then blocks to avoid draining system * resources by spinning for a long time. * - It's only 1 byte large, compared to e.g. 40 bytes when using the std::mutex of GCC. This makes * it more feasible to have many smaller mutexes which can improve scalability of algorithms * compared to using fewer larger mutexes. Also it just reduces "memory slop" across Blender. * - It is *not* a fair mutex, i.e. it's not guaranteed that a thread will ever be able to lock the * mutex when there are always more than one threads that try to lock it. In the majority of * cases, using a fair mutex just causes extra overhead without any benefit. std::mutex is not * guaranteed to be fair either. */ ``` The performance benchmark suggests that the impact is negilible in almost all cases. The only benchmarks that show interesting behavior are the once testing foreach zones in Geometry Nodes. These tests are explicitly testing overhead, which I still have to reduce over time. So it's not unexpected that changing the mutex has an impact there. What's interesting is that on macos the performance improves a lot while on linux it gets worse. Since that overhead should eventually be removed almost entirely, I don't really consider that blocking. Links: * Documentation of different mutex flavors in TBB: https://www.intel.com/content/www/us/en/docs/onetbb/developer-guide-api-reference/2021-12/mutex-flavors.html * Older implementation of a similar mutex by me: https://archive.blender.org/developer/differential/0016/0016711/index.html * Interesting read regarding how a mutex can be this small: https://webkit.org/blog/6161/locking-in-webkit/ Pull Request: https://projects.blender.org/blender/blender/pulls/138370
52 lines
2.0 KiB
C++
52 lines
2.0 KiB
C++
/* SPDX-FileCopyrightText: 2025 Blender Authors
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*
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* SPDX-License-Identifier: GPL-2.0-or-later */
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#pragma once
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/* Always include that so that `BLI_mutex.hh` can be used as replacement to including <mutex>.
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* Otherwise it might be confusing if both are included explicitly in a file. That also makes the
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* difference between compiling with and without TBB smaller. */
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#include <mutex> // IWYU pragma: export
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#ifdef WITH_TBB
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# include <tbb/mutex.h>
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#endif
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namespace blender {
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#ifdef WITH_TBB
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/**
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* blender::Mutex should be used as the default mutex in Blender. It implements a subset of the API
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* of std::mutex but has overall better guaranteed properties. It can be used with RAII helpers
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* like std::lock_guard. However, it is not compatible with e.g. std::condition_variable. So one
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* still has to use std::mutex for that case.
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*
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* The mutex provided by TBB has these properties:
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* - It's as fast as a spin-lock in the non-contended case, i.e. when no other thread is trying to
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* lock the mutex at the same time.
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* - In the contended case, it spins a couple of times but then blocks to avoid draining system
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* resources by spinning for a long time.
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* - It's only 1 byte large, compared to e.g. 40 bytes when using the std::mutex of GCC. This makes
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* it more feasible to have many smaller mutexes which can improve scalability of algorithms
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* compared to using fewer larger mutexes. Also it just reduces "memory slop" across Blender.
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* - It is *not* a fair mutex, i.e. it's not guaranteed that a thread will ever be able to lock the
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* mutex when there are always more than one threads that try to lock it. In the majority of
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* cases, using a fair mutex just causes extra overhead without any benefit. std::mutex is not
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* guaranteed to be fair either.
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*/
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using Mutex = tbb::mutex;
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/* If this is not true anymore at some point, the comment above needs to be updated. */
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static_assert(sizeof(Mutex) == 1);
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#else
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/** Use std::mutex as a fallback when compiling without TBB. */
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using Mutex = std::mutex;
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#endif
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} // namespace blender
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