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Date: Tue, 26 Aug 2014 18:35:19 +0200
From: Jens Gustedt <>
Subject: Re: Multi-threaded performance progress

Am Dienstag, den 26.08.2014, 09:04 +0200 schrieb Jens Gustedt:
> Am Montag, den 25.08.2014, 23:43 -0400 schrieb Rich Felker:
> > This release cycle looks like it's going to be huge for multi-threaded
> > performance issues. So far the cumulative improvement on my main
> > development system, as measured by the cond_bench.c by Timo Teräs, is
> > from ~250k signals in 2 seconds up to ~3.7M signals in 2 seconds.
> > That's comparable to what glibc gets on similar hardware with a cond
> > var implementation that's much less correct. The improvements are a
> > result of adding private futex support, redesigning the cond var
> > implementation, and improvements to the spin-before-futex-wait
> > behavior.
> Very impressive!

I reviewed the new pthread_cond code closely and found it to be really
rock solid.

I have some minor things, that might still improve things (or
not). They make the code a bit longer, but they attempt to gain things
here and there:

 - Tighten the lock on _c_lock such that the critical section
   contains the least necessary.

 - Have all the update of the list of waiters done by the signaling
   or broadcasting thread. This work is serialized by the lock on the
   cv, anyhow, so let the main work be done by a thread that already
   holds the lock and is scheduled.

 - In case of broadcast, work on head and tail of the list
   first. These are the only ones that would change the _c_head and _c_tail
   entries of the cv.

 - Try to reduce the number of futex calls. Threads that are leaving
   don't have to regain the lock when there is known contention with a
   signaler, now that the signaler is doing the main work in that

   Also only wake up the signaling thread at the end when he is known
   to be inside a futex call.

There are perhaps other possibilities, like doing some spinning in
"lock" before going into __wait.


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