From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 44BFE8D0040 for ; Tue, 29 Mar 2011 21:29:38 -0400 (EDT) Date: Tue, 29 Mar 2011 18:25:44 -0700 From: Andrew Morton Subject: Re: [PATCH]mmap: add alignment for some variables Message-Id: <20110329182544.6ad4eccb.akpm@linux-foundation.org> In-Reply-To: <1301447843.3981.48.camel@sli10-conroe> References: <1301277536.3981.27.camel@sli10-conroe> <1301360054.3981.31.camel@sli10-conroe> <20110329152434.d662706f.akpm@linux-foundation.org> <1301446882.3981.33.camel@sli10-conroe> <20110329180611.a71fe829.akpm@linux-foundation.org> <1301447843.3981.48.camel@sli10-conroe> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Shaohua Li Cc: Andi Kleen , linux-mm , lkml , Rik van Riel , Hugh Dickins On Wed, 30 Mar 2011 09:17:23 +0800 Shaohua Li wrote: > On Wed, 2011-03-30 at 09:06 +0800, Andrew Morton wrote: > > On Wed, 30 Mar 2011 09:01:22 +0800 Shaohua Li wrote: > > > > > +/* > > > + * Make sure vm_committed_as in one cacheline and not cacheline shared with > > > + * other variables. It can be updated by several CPUs frequently. > > > + */ > > > +struct percpu_counter vm_committed_as ____cacheline_internodealigned_in_smp; > > > > The mystery deepens. The only cross-cpu writeable fields in there are > > percpu_counter.lock and its companion percpu_counter.count. If CPUs > > are contending for the lock then that itself is a problem - how does > > adding some padding to the struct help anything? > I had another patch trying to address the lock contention (for case > OVERCOMMIT_GUESS), will send out soon. But thought better to have the > correct alignment for OVERCOMMIT_NEVER case. I still don't understand why adding ____cacheline_internodealigned_in_smp to vm_committed_as improves anything. Here it is: struct percpu_counter { spinlock_t lock; s64 count; #ifdef CONFIG_HOTPLUG_CPU struct list_head list; /* All percpu_counters are on a list */ #endif s32 __percpu *counters; }; and your patch effectively converts this to struct percpu_counter { spinlock_t lock; s64 count; #ifdef CONFIG_HOTPLUG_CPU struct list_head list; /* All percpu_counters are on a list */ #endif s32 __percpu *counters; + char large_waste_of_space[lots]; }; how is it that this improves things? -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org