From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx181.postini.com [74.125.245.181]) by kanga.kvack.org (Postfix) with SMTP id 32BDE6B0002 for ; Mon, 18 Feb 2013 13:08:30 -0500 (EST) MIME-Version: 1.0 Message-ID: <5120c705-5fcf-4e33-8562-22e8ad4b6c54@default> Date: Mon, 18 Feb 2013 10:07:59 -0800 (PST) From: Dan Magenheimer Subject: RE: [PATCH v2] zsmalloc: Add Kconfig for enabling PTE method References: <1360117028-5625-1-git-send-email-minchan@kernel.org> <51207655.5000209@gmail.com> In-Reply-To: <51207655.5000209@gmail.com> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Ric Mason , Minchan Kim Cc: Greg Kroah-Hartman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Seth Jennings , Nitin Gupta , Konrad Rzeszutek Wilk > From: Ric Mason [mailto:ric.masonn@gmail.com] > Sent: Saturday, February 16, 2013 11:19 PM > To: Minchan Kim > Cc: Greg Kroah-Hartman; linux-mm@kvack.org; linux-kernel@vger.kernel.org;= Andrew Morton; Seth > Jennings; Nitin Gupta; Dan Magenheimer; Konrad Rzeszutek Wilk > Subject: Re: [PATCH v2] zsmalloc: Add Kconfig for enabling PTE method >=20 > On 02/06/2013 10:17 AM, Minchan Kim wrote: > > Zsmalloc has two methods 1) copy-based and 2) pte-based to access > > allocations that span two pages. You can see history why we supported > > two approach from [1]. > > > > In summary, copy-based method is 3 times fater in x86 while pte-based > > is 6 times faster in ARM. >=20 > Why in some arches copy-based method is better and in the other arches > pte-based is better? What's the root reason? Minchan, if you post another version, I think these precise numbers (of "times faster") should be removed. The speed is very data dependent, because the copy-based method is copying a zpage which may vary widely in size from ~100 bytes to nearly PAGE_SIZE bytes, a factor of 40x or more. Please at least say "up to 3 times" or "approximately 3x faster for an average compressed page". Ric, the copy-based method does an extra copy of N bytes (where N is the compressed size of a page). The pte-based method requires extra TLB actions. The relative speed of TLB operations vs copying is very architecture-dependent. It is also probably dependent on the specific implementation of the architecture (i.e x86 sandybridge is likely very different than x86 nehalem) and, as noted above, dependent on N which is unpredictable. So it makes sense to have both choices, but it's not at all clear how to select which one to use! -- 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