From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with SMTP id 2F4336B007B for ; Thu, 7 Jan 2010 11:12:15 -0500 (EST) Date: Thu, 7 Jan 2010 10:11:07 -0600 (CST) From: Christoph Lameter Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault() In-Reply-To: <20100105192243.1d6b2213@infradead.org> Message-ID: References: <20100104182429.833180340@chello.nl> <20100104182813.753545361@chello.nl> <20100105054536.44bf8002@infradead.org> <20100105192243.1d6b2213@infradead.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Arjan van de Ven Cc: Peter Zijlstra , "Paul E. McKenney" , Peter Zijlstra , KAMEZAWA Hiroyuki , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "minchan.kim@gmail.com" , "hugh.dickins" , Nick Piggin , Ingo Molnar , Linus Torvalds List-ID: On Tue, 5 Jan 2010, Arjan van de Ven wrote: > On Tue, 5 Jan 2010 09:17:11 -0600 (CST) > Christoph Lameter wrote: > > > On Tue, 5 Jan 2010, Arjan van de Ven wrote: > > > > > while I appreciate the goal of reducing contention on this lock... > > > wouldn't step one be to remove the page zeroing from under this > > > lock? that's by far (easily by 10x I would guess) the most > > > expensive thing that's done under the lock, and I would expect a > > > first order of contention reduction just by having the zeroing of a > > > page not done under the lock... > > > > The main issue is cacheline bouncing. mmap sem is a rw semaphore and > > only held for read during a fault. > > depends on the workload; on a many-threads-java workload, you also get > it for write quite a bit (lots of malloc/frees in userspace in addition > to pagefaults).. at which point you do end up serializing on the > zeroing. > > There's some real life real big workloads that show this pretty badly; > so far the workaround is to have glibc batch up a lot of the free()s.. > but that's just pushing it a little further out. Again mmap_sem is a rwsem and only a read lock is held. Zeroing in do_anonymous_page can occur concurrently on multiple processors in the same address space. The pte lock is intentionally taken *after* zeroing to allow concurrent zeroing to occur. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org