From owner-freebsd-threads@FreeBSD.ORG Tue Oct 14 20:36:42 2014 Return-Path: Delivered-To: freebsd-threads@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 1A5C3C81; Tue, 14 Oct 2014 20:36:42 +0000 (UTC) Received: from mx1.stack.nl (relay04.stack.nl [IPv6:2001:610:1108:5010::107]) (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits)) (Client CN "mailhost.stack.nl", Issuer "CA Cert Signing Authority" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id D5889208; Tue, 14 Oct 2014 20:36:41 +0000 (UTC) Received: from snail.stack.nl (snail.stack.nl [IPv6:2001:610:1108:5010::131]) by mx1.stack.nl (Postfix) with ESMTP id 94107B840F; Tue, 14 Oct 2014 22:36:38 +0200 (CEST) Received: by snail.stack.nl (Postfix, from userid 1677) id 835B428494; Tue, 14 Oct 2014 22:36:38 +0200 (CEST) Date: Tue, 14 Oct 2014 22:36:38 +0200 From: Jilles Tjoelker To: John Baldwin Subject: Re: sem_post() performance Message-ID: <20141014203638.GA23965@stack.nl> References: <20140921213742.GA46868@stack.nl> <17114533.JBQOYsdsdz@ralph.baldwin.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <17114533.JBQOYsdsdz@ralph.baldwin.cx> User-Agent: Mutt/1.5.21 (2010-09-15) Cc: adrian@freebsd.org, freebsd-threads@freebsd.org X-BeenThere: freebsd-threads@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: Threading on FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 14 Oct 2014 20:36:42 -0000 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. > > 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 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. -- Jilles Tjoelker