54 lines
2.1 KiB
Markdown
54 lines
2.1 KiB
Markdown
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# Synchronization Barriers
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OSX does not implement pthread_barrier as its an optional part
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of the POSIX standard and they probably want to drive people to libdispatch/Grand Central Dispatch.
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So we need to roll our own with a POSIX compatible API.
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## Glibc barriers, design bug and implementation
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> Note: due to GPL licensing, do not lift the code.
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> Not that we can as it is heavily dependent on futexes
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> which are not available on OSX
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We need to make sure that we don't hit the same bug
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as glibc: https://sourceware.org/bugzilla/show_bug.cgi?id=13065
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which seems to be an issue in some of the barrier implementations
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in the wild.
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The design of Glibc barriers is here:
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https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/DESIGN-barrier.txt;h=23463c6b7e77231697db3e13933b36ce295365b1;hb=HEAD
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And implementation:
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- https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_barrier_destroy.c;h=76957adef3ee751e5b0cfa429fcf4dd3cfd80b2b;hb=HEAD
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- https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_barrier_init.c;h=c8ebab3a3cb5cbbe469c0d05fb8d9ca0c365b2bb;hb=HEAD`
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- https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_barrier_wait.c;h=49fcfd370c1c4929fdabdf420f2f19720362e4a0;hb=HEAD
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## Synchronization barrier techniques
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This article goes over the techniques of
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"pool barrier" and "ticket barrier"
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https://locklessinc.com/articles/barriers/
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to reach 2x to 20x the speed of pthreads barrier
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This course https://cs.anu.edu.au/courses/comp8320/lectures/aux/comp422-Lecture21-Barriers.pdf
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goes over
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- centralized barrier with sense reversal
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- combining tree barrier
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- dissemination barrier
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- tournament barrier
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- scalable tree barrier
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More courses:
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- http://www.cs.rochester.edu/u/sandhya/csc458/seminars/jb_Barrier_Methods.pdf
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It however requires lightweight mutexes like Linux futexes
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that OSX lacks.
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This post goes over lightweight mutexes like Benaphores (from BeOS)
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https://preshing.com/20120226/roll-your-own-lightweight-mutex/
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This gives a few barrier implementations
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http://gallium.inria.fr/~maranget/MPRI/02.pdf
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and refers to Cubible paper for formally verifying synchronization barriers
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http://cubicle.lri.fr/papers/jfla2014.pdf (in French)
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