From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with ESMTP id DBC59600533 for ; Thu, 7 Jan 2010 11:21:20 -0500 (EST) Date: Thu, 7 Jan 2010 08:19:56 -0800 (PST) From: Linus Torvalds Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault() In-Reply-To: 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: Christoph Lameter Cc: Arjan van de Ven , 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 List-ID: On Thu, 7 Jan 2010, Christoph Lameter wrote: > > > > 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. You're missing what Arjan said - the jav workload does a lot of memory allocations too, causing mmap/munmap. So now some paths are indeed holding it for writing (or need to wait for it to become writable). And the fairness of rwsems quite possibly then impacts throughput a _lot_.. (Side note: I wonder if we should wake up _all_ readers when we wake up any. Right now, we wake up all readers - but only until we hit a writer. Which is the _fair_ thing to do, but it does mean that we can end up in horrible patterns of alternating readers/writers, when it could be much better to just say "release the hounds" and let all pending readers go after a writer has had its turn). Linus -- 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