Date: Thu, 30 Jul 2015 09:46:49 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: New optimized normal-type mutex? On Thu, Jul 30, 2015 at 02:37:13PM +0300, Alexander Monakov wrote: > On Thu, 30 Jul 2015, Jens Gustedt wrote: > > Am Donnerstag, den 30.07.2015, 12:36 +0300 schrieb Alexander Monakov: > > > That sounds like your testcase simulates a load where you'd be better off with > > > a spinlock in the first place, no? > > > > Hm, this is not a "testcase" in the sense that this is the real code > > that I'd like to use for the generic atomic lock-full stuff. My test > > is just using this atomic lock-full thing, with a lot of threads that > > use the same head of a "lock-free" FIFO implementation. There the > > inner part in the critical section is just memcpy of some bytes. For > > reasonable uses of atomics this should be about 16 to 32 bytes that > > are copied. > > > > So this is really a use case that I consider important, and that I > > would like to see implemented with similar performance. > > I acknowledge that that seems like an important case, but you have not > addressed my main point. With so little work in the critical section, it does > not make sense to me that you would use something like a normal-type futex-y > mutex. Even a call/return to grab it gives you some overhead. I'd expect you > would use a fully inlined spinlock acquisition/release around the memory copy. No, spinlocks are completely unusable in a POSIX libc that implements priorities. They will deadlock whenever a lower-priority thread gets preempted by a higher-priority one while holding the lock. Rich
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