Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755013Ab0AES2S (ORCPT ); Tue, 5 Jan 2010 13:28:18 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753702Ab0AES2P (ORCPT ); Tue, 5 Jan 2010 13:28:15 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:41655 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752848Ab0AES2L (ORCPT ); Tue, 5 Jan 2010 13:28:11 -0500 Date: Tue, 5 Jan 2010 10:25:43 -0800 (PST) From: Linus Torvalds X-X-Sender: torvalds@localhost.localdomain To: Christoph Lameter cc: Andi Kleen , KAMEZAWA Hiroyuki , Minchan Kim , Peter Zijlstra , "Paul E. McKenney" , Peter Zijlstra , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "hugh.dickins" , Nick Piggin , Ingo Molnar Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault() In-Reply-To: Message-ID: References: <20100104182429.833180340@chello.nl> <20100104182813.753545361@chello.nl> <20100105092559.1de8b613.kamezawa.hiroyu@jp.fujitsu.com> <28c262361001042029w4b95f226lf54a3ed6a4291a3b@mail.gmail.com> <20100105134357.4bfb4951.kamezawa.hiroyu@jp.fujitsu.com> <20100105143046.73938ea2.kamezawa.hiroyu@jp.fujitsu.com> <20100105163939.a3f146fb.kamezawa.hiroyu@jp.fujitsu.com> <87wrzwbh0z.fsf@basil.nowhere.org> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1324 Lines: 30 On Tue, 5 Jan 2010, Christoph Lameter wrote: > > If the critical section protected by the spinlock is small then the > delay will keep the cacheline exclusive until we hit the unlock. This > is the case here as far as I can tell. I hope somebody can time it. Because I think the idle reads on all the (unsuccessful) spinlocks will kill it. Think of it this way: under heavy contention, you'll see a lot of people waiting for the spinlocks and one of them succeeds at writing it, reading the line. So you get an O(n^2) bus traffic access pattern. In contrast, with an xadd, you get O(n) behavior - everybody does _one_ acquire-for- write bus access. Remember: the critical section is small, but since you're contending on the spinlock, that doesn't much _help_. The readers are all hitting the lock (and you can try to solve the O(n*2) issue with back-off, but quite frankly, anybody who does that has basically already lost - I'm personally convinced you should never do lock backoff, and instead look at what you did wrong at a higher level instead). Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/