From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with ESMTP id 119296B00E0 for ; Tue, 5 Jan 2010 12:57:42 -0500 (EST) Date: Tue, 5 Jan 2010 09:55:43 -0800 (PST) From: Linus Torvalds Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault() In-Reply-To: <87wrzwbh0z.fsf@basil.nowhere.org> 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> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Andi Kleen Cc: KAMEZAWA Hiroyuki , Minchan Kim , Peter Zijlstra , "Paul E. McKenney" , Peter Zijlstra , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , cl@linux-foundation.org, "hugh.dickins" , Nick Piggin , Ingo Molnar List-ID: On Tue, 5 Jan 2010, Andi Kleen wrote: > > > Oh well. Somebody who is bored might look at trying to make the wrapper > > code in arch/x86/lib/semaphore_32.S work on x86-64 too. It should make the > > successful rwsem cases much faster. > > Maybe, maybe not. If there is actual contention on the lock, but mainly just readers (which is what the profile indicates: since there is no scheduler footprint, the actual writer-vs-reader case is probably very rare), then the xadd is likely to be _much_ faster than the spinlock. Sure, the cacheline is going to bounce regardless (since it's a shared per-mm data structure), but the spinlock is going to bounce wildly back-and-forth between everybody who _tries_ to get it, while the regular xadd is going to bounce just once per actual successful xadd. So a spinlock is as cheap as an atomic when there is no contention (which is the common single-thread case - the real cost of both lock and atomic is simply the fact that CPU serialization is expensive), but when there is actual lock contention, I bet the atomic xadd is going to be shown to be superior. Remember: we commonly claim that 'spin_unlock' is basically free on x86 - and that's true, but it is _only_ true for the uncontended state. 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