From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261271AbULWT3o (ORCPT ); Thu, 23 Dec 2004 14:29:44 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261289AbULWT3o (ORCPT ); Thu, 23 Dec 2004 14:29:44 -0500 Received: from omx3-ext.sgi.com ([192.48.171.20]:48559 "EHLO omx3.sgi.com") by vger.kernel.org with ESMTP id S261271AbULWT3Y (ORCPT ); Thu, 23 Dec 2004 14:29:24 -0500 Date: Thu, 23 Dec 2004 11:29:10 -0800 (PST) From: Christoph Lameter X-X-Sender: clameter@schroedinger.engr.sgi.com cc: akpm@osdl.org, linux-ia64@vger.kernel.org, torvalds@osdl.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Prezeroing V2 [0/3]: Why and When it works In-Reply-To: Message-ID: References: <41C20E3E.3070209@yahoo.com.au> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII To: unlisted-recipients:; (no To-header on input) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Change from V1 to V2: o Add explanation--and some bench results--as to why and when this optimization works and why other approaches have not worked. o Instead of zero_page(p,order) extend clear_page to take second argument o Update all architectures to accept second argument for clear_pages o Extensive removal of all page allocs/clear_page combination from all archs o Blank / typo fixups o SGI BTE zero driver update: Use node specific variables instead of cpu specific since a cpu may be responsible for multiple nodes. The patches increasing the page fault rate (introduction of atomic pte operations and anticipatory prefaulting) do so by reducing the locking overhead and are therefore mainly of interest for applications running in SMP systems with a high number of cpus. The single thread performance does just show minor increases. Only the performance of multi-threaded applications increase significantly. The most expensive operation in the page fault handler is (apart of SMP locking overhead) the zeroing of the page. This zeroing means that all cachelines of the faulted page (on Altix that means all 128 cachelines of 128 byte each) must be loaded and later written back. This patch allows to avoid having to load all cachelines if only a part of the cachelines of that page is needed immediately after the fault. Thus the patch will only be effective for sparsely accessed memory which is typicalfor anonymous memory and pte maps. Prezeroed pages will be used for those purposes. Unzeroed pages will be used as usual for the other purposes. Others have also thought that prezeroing could be a benefit and have tried provide a way to provide zeroed pages to the page fault handler: http://marc.theaimsgroup.com/?t=109914559100004&r=1&w=2 http://marc.theaimsgroup.com/?t=109777267500005&r=1&w=2 http://marc.theaimsgroup.com/?l=linux-kernel&m=104931944213955&w=2 However, these attempt have tried to zero pages soon to be accessed (and which may already have recently been accessed). Elements of these pages are thus already in the cache. Approaches like that will only shift processing a bit and not yield performance benefits. Prezeroing only makes sense for pages that are not currently needed and that are not in the cpu caches. Pages that have recently been touched and that soon will be touched again are better hot zeroed since the zeroing will largely be done to cachelines already in the cpu caches. The patch makes prezeroing very effective by: 1. Aggregating zeroing operations to only apply to pages of higher order, which results in many pages that will later become order 0 to be zeroed in one go. For that purpose the existing clear_page function is extended and made to take an additional argument specifying the order of the page to be cleared. 2. Hardware support for offloading zeroing from the cpu. This avoids the invalidation of the cpu caches by extensive zeroing operations. The result is a significant increase of the page fault performance even for single threaded applications: w/o patch: Gb Rep Threads User System Wall flt/cpu/s fault/wsec 4 3 1 0.146s 11.155s 11.030s 69584.896 69566.852 w/patch Gb Rep Threads User System Wall flt/cpu/s fault/wsec 1 1 1 0.014s 0.110s 0.012s524292.194 517665.538 The performance can only be upheld if enough zeroed pages are available. In a heavy memory intensive benchmarks the system could potentially run out of zeroed pages but the efficient algorithm for page zeroing still shows this to be a winner: (8 way system with 6 GB RAM, no hardware zeroing support) w/o patch: Gb Rep Threads User System Wall flt/cpu/s fault/wsec 4 3 1 0.146s 11.155s 11.030s 69584.896 69566.852 4 3 2 0.170s 14.909s 7.097s 52150.369 98643.687 4 3 4 0.181s 16.597s 5.079s 46869.167 135642.420 4 3 8 0.166s 23.239s 4.037s 33599.215 179791.120 w/patch Gb Rep Threads User System Wall flt/cpu/s fault/wsec 4 3 1 0.183s 2.750s 2.093s268077.996 267952.890 4 3 2 0.185s 4.876s 2.097s155344.562 263967.292 4 3 4 0.150s 6.617s 2.097s116205.793 264774.080 4 3 8 0.186s 13.693s 3.054s 56659.819 221701.073 Note that zeroing of pages makes no sense if the application touches all cache lines of a page allocated (there is no influence of prezeroing on benchmarks like lmbench for that reason) since the extensive caching of modern cpus means that the zeroes written to a hot zeroed page will then be overwritten by the application in the cpu cache and thus the zeros will never make it to memory! The test program used above only touches one 128 byte cache line of a 16k page (ia64). Here is another test in order to gauge the influence of the number of cache lines touched on the performance of the prezero enhancements: Gb Rep Thr CLine User System Wall flt/cpu/s fault/wsec 1 1 1 1 0.01s 0.12s 0.01s500813.853 497925.891 1 1 1 2 0.01s 0.11s 0.01s493453.103 472877.725 1 1 1 4 0.02s 0.10s 0.01s479351.658 471507.415 1 1 1 8 0.01s 0.13s 0.01s424742.054 416725.013 1 1 1 16 0.05s 0.12s 0.01s347715.359 336983.834 1 1 1 32 0.12s 0.13s 0.02s258112.286 256246.731 1 1 1 64 0.24s 0.14s 0.03s169896.381 168189.283 1 1 1 128 0.49s 0.14s 0.06s102300.257 101674.435 The benefits of prezeroing become smaller the more cache lines of a page are touched. Prezeroing can only be effective if memory is not immediately touched after the anonymous page fault. The patch is composed of 4 parts: [1/4] Introduce __GFP_ZERO Modifies the page allocator to be able to take the __GFP_ZERO flag and returns zeroed memory on request. Modifies locations throughout the linux sources that retrieve a page and then zero it to request a zeroed page. [2/4] Architecture specific clear_page updates Adds second order argument to clear_page and updates all arches. Note: The two first pages may be used alone if no zeroing engine is wanted. [3/4] Page Zeroing Adds management of ZEROED and NOT_ZEROED pages and a background daemon called scrubd. scrubd is disabled by default but can be enabled by writing an order number to /proc/sys/vm/scrub_start. If a page is coalesced of that order or higher then the scrub daemon will start zeroing until all pages of order /proc/sys/vm/scrub_stop and higher are zeroed and then go back to sleep. In an SMP environment the scrub daemon is typically running on the most idle cpu. Thus a single threaded application running on one cpu may have the other cpu zeroing pages for it etc. The scrub daemon is hardly noticable and usually finished zeroing quickly since most processors are optimized for linear memory filling. [4/4] SGI Altix Block Transfer Engine Support Implements a driver to shift the zeroing off the cpu into hardware. With hardware support there will be minimal impact of zeroing on the performance of the system.