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85f64f3851
Thanks to @autumnjolitz (in #876) the Cosmopolitan codebase is now acquainted with Apple's outstanding ulock system calls which offer something much closer to futexes than Grand Central Dispatch which wasn't quite as good, since its wait function can't be interrupted by signals (therefore necessitating a busy loop) and it also needs semaphore objects to be created and freed. Even though ulock is an internal Apple API, strictly speaking, the benefits of futexes are so great that it's worth the risk for now especially since we have the GCD implementation still as a quick escape hatch if it changes Here's why this change is important for x86 XNU users. Cosmo has a suboptimal polyfill when the operating system doesn't offer an API that let's us implement futexes properly. Sadly we had to use that on X86 XNU until now. The polyfill works using clock_nanosleep, to poll the futex in a busy loop with exponential backoff. On XNU x86 clock_nanosleep suffers from us not being able to use a fast clock gettime implementation, which had a compounding effect that's made the polyfill function even more poorly. On X86 XNU we also need to polyfill sched_yield() using select(), which made things even more troublesome. Now that we have futexes we don't have any busy loops anymore for both condition variables and thread joining so optimal performance is attained. To demonstrate, consider these benchmarks Before: $ ./lockscale_test.com -b consumed 38.8377 seconds real time and 0.087131 seconds cpu time After: $ ./lockscale_test.com -b consumed 0.007955 seconds real time and 0.011515 seconds cpu time Fixes #876
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41 lines
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DESCRIPTION
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*NSYNC is a synchronization primitives library.
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LICENSE
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Apache 2.0
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ORIGIN
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git@github.com:google/nsync
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commit ac5489682760393fe21bd2a8e038b528442412a7
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Author: Mike Burrows <m3b@google.com>
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Date: Wed Jun 1 16:47:52 2022 -0700
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LOCAL CHANGES
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- Time APIs were so good that they're now in libc
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- Double linked list API was so good that it's now in libc
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- Support Apple's ulock futexes which are internal but nicer than GCD
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- Ensure resources such as POSIX semaphores are are released on fork.
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- Modified *NSYNC to allocate waiter objects on the stack. We need it
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because we use *NSYNC mutexes to implement POSIX mutexes, which are
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too low-level to safely depend on malloc, or even mmap in our case.
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- Rewrote most of the semaphore and futex system call support code so
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it works well with Cosmopolitan's fat runtime portability. *NSYNC's
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unit test suite passes on all supported platforms. However the BSDs
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currently appear to be overutilizing CPU time compared with others.
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This appears to be the fault of the OSes rather than *NSYNC / Cosmo
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- Support POSIX thread cancellation. APIs that wait on condition vars
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are now cancellation points. In PTHREAD_CANCEL_MASKED mode they may
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return ECANCELED. In PTHREAD_CANCEL_DEFERRED mode the POSIX threads
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library will unwind the stack to re-acquire locks and free waiters.
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On the other hand the *NSYNC APIs for mutexes will now safely block
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thread cancellation, but you can still use *NSYNC notes to do that.
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