Date: Tue, 14 Oct 2014 17:31:42 -0400 From: John Baldwin <jhb@freebsd.org> To: Jilles Tjoelker <jilles@stack.nl> Cc: adrian@freebsd.org, freebsd-threads@freebsd.org Subject: Re: sem_post() performance Message-ID: <201410141731.42666.jhb@freebsd.org> In-Reply-To: <20141014203638.GA23965@stack.nl> References: <20140921213742.GA46868@stack.nl> <17114533.JBQOYsdsdz@ralph.baldwin.cx> <20141014203638.GA23965@stack.nl>
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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. > > > This diff only affects 32-bit aligned but 64-bit misaligned semaphores > > > on 64-bit systems, and changes _count and _has_waiters atomically using > > > a 64-bit atomic operation. It probably needs a may_alias attribute for > > > correctness, but <sys/cdefs.h> does not have a wrapper for that. > > > It does have one bug: > > > > + if (atomic_cmpset_rel_64((uint64_t *)&sem->_kern._count, > > > + oldval, newval)) > > > This needs to be '&_has_waiters'. Right now it changes _count and _flags, > > but not _has_waiters. > > This is probably because I was mistaken about the order of _count and > _has_waiters, and only partially corrected that. > > Anyway, the strange alignment requirements make the patch of little > practical use. Correct, though it can be used for an immediate fix on amd64 in a pinch. -- John Baldwin
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