From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f175.google.com (mail-pd0-f175.google.com [209.85.192.175]) by kanga.kvack.org (Postfix) with ESMTP id 440E66B0035 for ; Thu, 12 Dec 2013 18:40:41 -0500 (EST) Received: by mail-pd0-f175.google.com with SMTP id w10so1371896pde.20 for ; Thu, 12 Dec 2013 15:40:40 -0800 (PST) Received: from mga01.intel.com (mga01.intel.com. [192.55.52.88]) by mx.google.com with ESMTP id ob10si9915pbb.217.2013.12.12.15.40.39 for ; Thu, 12 Dec 2013 15:40:39 -0800 (PST) Date: Thu, 12 Dec 2013 15:40:38 -0800 From: Andi Kleen Subject: Re: [RFC][PATCH 2/3] mm: slab: move around slab ->freelist for cmpxchg Message-ID: <20131212234038.GQ22695@tassilo.jf.intel.com> References: <20131211224022.AA8CF0B9@viggo.jf.intel.com> <20131211224025.70B40B9C@viggo.jf.intel.com> <00000142e7ea519d-8906d225-c99c-44b5-b381-b573c75fd097-000000@email.amazonses.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <00000142e7ea519d-8906d225-c99c-44b5-b381-b573c75fd097-000000@email.amazonses.com> Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: Dave Hansen , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kirill.shutemov@linux.intel.com On Thu, Dec 12, 2013 at 05:46:02PM +0000, Christoph Lameter wrote: > On Wed, 11 Dec 2013, Dave Hansen wrote: > > > > > The write-argument to cmpxchg_double() must be 16-byte aligned. > > We used to align 'struct page' itself in order to guarantee this, > > but that wastes 8-bytes per page. Instead, we take 8-bytes > > internal to the page before page->counters and move freelist > > between there and the existing 8-bytes after counters. That way, > > no matter how 'stuct page' itself is aligned, we can ensure that > > we have a 16-byte area with which to to this cmpxchg. > > Well this adds additional branching to the fast paths. The branch should be predictible and compare the cost of a branch (near nothing on a modern OOO CPU with low IPC code like this when predicted) to the cost of a cache miss (due to larger struct page) -Andi -- ak@linux.intel.com -- Speaking for myself only -- 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