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
Listing the "Blender Foundation" as copyright holder implied the Blender
Foundation holds copyright to files which may include work from many
developers.
While keeping copyright on headers makes sense for isolated libraries,
Blender's own code may be refactored or moved between files in a way
that makes the per file copyright holders less meaningful.
Copyright references to the "Blender Foundation" have been replaced with
"Blender Authors", with the exception of `./extern/` since these this
contains libraries which are more isolated, any changed to license
headers there can be handled on a case-by-case basis.
Some directories in `./intern/` have also been excluded:
- `./intern/cycles/` it's own `AUTHORS` file is planned.
- `./intern/opensubdiv/`.
An "AUTHORS" file has been added, using the chromium projects authors
file as a template.
Design task: #110784
Ref !110783.
A lot of files were missing copyright field in the header and
the Blender Foundation contributed to them in a sense of bug
fixing and general maintenance.
This change makes it explicit that those files are at least
partially copyrighted by the Blender Foundation.
Note that this does not make it so the Blender Foundation is
the only holder of the copyright in those files, and developers
who do not have a signed contract with the foundation still
hold the copyright as well.
Another aspect of this change is using SPDX format for the
header. We already used it for the license specification,
and now we state it for the copyright as well, following the
FAQ:
https://reuse.software/faq/
This patch introduces a new `CacheMutex` which makes it easy to implement
lazily computed caches in e.g. `Curves`. For more details see `BLI_cache_mutex.hh`.
Differential Revision: https://developer.blender.org/D16419
