02077ffc42
Before 2019, the entire ConditionVariable system was "giant"-locked: that is, there was a single global lock that all ConditionVariable and ConditionVariableEntry operations had to pass through. This of course was not very performant on multicore systems and when ConditionVariables see significant use, so I reworked it then to have more granular locking. Those patches took a number of attempts to get right, as having two objects in separate threads that can each access the other not turn into a deadlock or use-after-free is not easy to say the least, and the ultimate solution I came up with erased most of the performance gains I initially saw on the first (partially broken) patchsets. So I have wanted to revisit this and see if there was a better way even since then. Recently there have been a few reports of ConditionVariable-related panics (apparently double unlocks), notably #16894, and so that was reason enough to actually revisit this code and see if a better solution could be found. Well, I think I have come up with one: after this commit, Entries no longer have their own lock, and instead accesses to Entry members are almost always atomic; and there is now a case where we spin inside Variable::_NotifyLocked as well as one in Entry::_RemoveFromVariable. This leads to somewhat simpler code (no more lock/unlock dance in Notify), though it is significantly more difficult to understand the nuances of it, so I have left a sizable number of comments explaining the intricacies of the new logic. Note: I initially tried 1000 for "tries", but on a few instances I did see the panic hit, strangely. I don't think the code that is waited on can be reasonably reduced any further, so I have just increased the limit to 10000 (which is still well below what spinlocks use.) Hopefully this suffices. Quick benchmark, x86, compiling HaikuDepot and the mime_db in VMware, 2 cores: before: real 0m23.627s user 0m25.152s sys 0m7.319s after: real 0m23.962s user 0m25.229s sys 0m7.330s Though I occasionally I saw sys times as low as 7.171s, so this seems to be at least not a regression if not a definitive improvement. Change-Id: Id042947976885cd5c1433cc4290bdf41b01ed10e Reviewed-on: https://review.haiku-os.org/c/haiku/+/4727 Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org> Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Haiku
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Haiku is an open-source operating system that specifically targets personal computing. Inspired by the BeOS, Haiku is fast, simple to use, easy to learn and yet very powerful.
Goals
- Sensible defaults with minimal configuration required.
- Clean, clear, concise code.
- Unified desktop environment.
Trying Haiku
Haiku provides pre-built nightly images and release images. Haiku is compatible with a large variety of hardware, but in case you don't want to "take the plunge" and install Haiku on bare metal, you can install it on a virtual machine (VM) instead. If you've never used a VM before, you can follow one of the "Emulating Haiku" guides.
Compiling Haiku
See ReadMe.Compiling.
Contributing
Haiku is a meritocratic open source project with a large variety of tasks. Even if you can't write code, you can still help! Haiku needs designers, (technical) writers, translators, testers... Get involved and help out!
Contributing code
If you're submitting a patch to us, please make sure you're following the patch submitting guidelines.
If you're having trouble finding something in the source tree, you can use one of our web-based source code browsers:
- https://xref.landonf.org/ (OpenGrok, provided by Landon Fuller)
- https://git.haiku-os.org/ (git, provided by Haiku, Inc.)
Contributing documentation
The main piece of documentation that still needs work are the API docs (found
in the tree at docs/user). Just find an undocumented class, write
documentation for it, and submit a patch.
Contributing translations
See wiki:i18n.
Contributing software ports
See HaikuPorts.
Contributing to our infrastructure
See Infrastructure.