Date: Fri, 17 Oct 2014 12:48:54 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-threads@freebsd.org Cc: adrian@freebsd.org Subject: Re: sem_post() performance Message-ID: <7085436.G0Uv0KIisk@ralph.baldwin.cx> In-Reply-To: <201410141731.42666.jhb@freebsd.org> References: <20140921213742.GA46868@stack.nl> <20141014203638.GA23965@stack.nl> <201410141731.42666.jhb@freebsd.org>
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On Tuesday, October 14, 2014 05:31:42 PM John Baldwin wrote: > On Tuesday, October 14, 2014 4:36:38 pm Jilles Tjoelker wrote: > > On Mon, Oct 13, 2014 at 05:35:09PM -0400, John Baldwin wrote: > > > On Sunday, September 21, 2014 11:37:42 PM Jilles Tjoelker wrote: > > > > It has been reported that POSIX semaphores are slow, in contexts such > > > > as > > > > Python. Note that POSIX semaphores are the only synchronization > > > > objects > > > > that support use by different processes in shared memory; this does > > > > not > > > > work for mutexes and condition variables because they are pointers to > > > > the actual data structure. > > > > > > > > In fact, sem_post() unconditionally performs an umtx system call. > > > > > > > > To avoid both lost wakeups and possible writes to a destroyed > > > > semaphore, > > > > an uncontested sem_post() must check the _has_waiters flag atomically > > > > with incrementing _count. > > > > > > > > The proper way to do this would be to take one bit from _count and use > > > > it for the _has_waiters flag; the definition of SEM_VALUE_MAX permits > > > > this. However, this would require a new set of umtx semaphore > > > > operations > > > > and will break ABI of process-shared semaphores (things may break if > > > > an > > > > old and a new libc access the same semaphore over shared memory). > > > > > > Have you thought more about pursuing this option? I think there was a > > > general consensus from earlier in the thread to just break the ABI (at > > > least adjust SEM_MAGIC to give some protection) and fix it. > > > > I think this is a good direction but I haven't gotten around to it yet. > > Ok, I might take a stab at it. I have an implementation of this at https://reviews.freebsd.org/D961 -- John Baldwin
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