From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f169.google.com (mail-ig0-f169.google.com [209.85.213.169]) by kanga.kvack.org (Postfix) with ESMTP id 740B46B0038 for ; Wed, 2 Dec 2015 23:13:18 -0500 (EST) Received: by igcph11 with SMTP id ph11so3324285igc.1 for ; Wed, 02 Dec 2015 20:13:18 -0800 (PST) Received: from lgeamrelo13.lge.com (LGEAMRELO13.lge.com. [156.147.23.53]) by mx.google.com with ESMTPS id cj4si10351360igb.40.2015.12.02.20.13.17 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Wed, 02 Dec 2015 20:13:17 -0800 (PST) Date: Thu, 3 Dec 2015 13:14:11 +0900 From: Joonsoo Kim Subject: Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value Message-ID: <20151203041410.GA1495@js1304-P5Q-DELUXE> References: <1448346123-2699-1-git-send-email-iamjoonsoo.kim@lge.com> <20151125025735.GC9563@js1304-P5Q-DELUXE> <20151126015252.GA13138@js1304-P5Q-DELUXE> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151126015252.GA13138@js1304-P5Q-DELUXE> Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Thu, Nov 26, 2015 at 10:52:52AM +0900, Joonsoo Kim wrote: > On Wed, Nov 25, 2015 at 10:04:44AM -0600, Christoph Lameter wrote: > > > Although vmstat values aren't designed for accuracy, these are already > > > used by some sensitive places so it is better to be more accurate. > > > > The design is to sacrifice accuracy and the time the updates occur for > > performance reasons. This is not the purpose the counters were designed > > for. If you put these demands on the vmstat then you will get complex > > convoluted code and compromise performance. > > I understand design decision, but, it is better to get value as much > as accurate if there is no performance problem. My patch would not > cause much performance degradation because it is just adding one > this_cpu_read(). > > Consider about following example. Current implementation returns > interesting output if someone do following things. > > v1 = zone_page_state(XXX); > mod_zone_page_state(XXX, 1); > v2 = zone_page_state(XXX); > > v2 would be same with v1 in most of cases even if we already update > it. > > This situation could occurs in page allocation path and others. If > some task try to allocate many pages, then watermark check returns > same values until updating vmstat even if some freepage are allocated. > There are some adjustments for this imprecision but why not do it become > accurate? I think that this change is reasonable trade-off. > Christoph, any comment? Thanks. -- 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