From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1AFB2C433B4 for ; Tue, 18 May 2021 10:39:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EC7DE6124C for ; Tue, 18 May 2021 10:39:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1348566AbhERKk5 (ORCPT ); Tue, 18 May 2021 06:40:57 -0400 Received: from mx2.suse.de ([195.135.220.15]:50620 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241525AbhERKkt (ORCPT ); Tue, 18 May 2021 06:40:49 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C35E7AF19; Tue, 18 May 2021 10:39:30 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id B00521F2C6E; Tue, 18 May 2021 12:39:28 +0200 (CEST) Date: Tue, 18 May 2021 12:39:28 +0200 From: Jan Kara To: chi wu Cc: Jan Kara , Andrew Morton , linux-kernel@vger.kernel.org, tan.hu@zte.com.cn Subject: Re: [PATCH] lib/flex_proportions.c: Use abs() when percpu_counter is negative. Message-ID: <20210518103928.GC28667@quack2.suse.cz> References: <20210517155306.11113-1-wuchi.zero@gmail.com> <20210518085955.GB28667@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 18-05-21 18:22:05, chi wu wrote: > Jan Kara 于2021年5月18日周二 下午4:59写道： > > > > On Tue 18-05-21 11:42:53, chi wu wrote: > > > Chi Wu 于2021年5月17日周一 下午11:53写道： > > > > > > > > The value of percpu_counter_read() may become negative after > > > > running percpu_counter_sum() in fprop_reflect_period_percpu(). > > > > The value of variable 'num' will be zero in fprop_fraction_percpu() > > > > when using percpu_counter_read_positive(), but if using the abs of > > > > percpu_counter_read() will be close to the correct value. > > > > > > > > > > I realized that I was wrong as follow: > > > (a) the decay rule is broken, the negative means the difference in > > > decay here. > > > (b) as the target event increasing, proportion of the event will > > > decrease to 0 firstly and then it will increase. The logic is bad. > > > 1. abs(-50) / abs(100) = 50% //+50 to 2 > > > 2. abs(0) / abs(150) = 0 % //+50 to 3 > > > 3. abs(50)/abs(200) = 25% > > > > > > Anyway, the percpu_counter_sum() had cost a lost performance, > > > may be we could get a little benefits from that. So could we add a > > > variable to stroe the decay value, we will get the value when > > > percpu_counter_read() is negative? > > > > The result of percpu_counter_read() is inherently inexact (but fast! ;). It > > can be upto number_of_cpus * counter_batch away from the real counter > > value. But do you observe any practical problems with this inaccuracy on > > your system? Sure, cache memory won't be split among devices exactly > > according to writeout proportion but that usually does not matter. > > > > Honza > > > > Thanks, Got it. > Just try to optimize the fuse (with strictlimit feature)performance > issue: The writing thread will be paused and runs slowly, when the > proportion of fuse-bdi is 0. > The issue is normal,and one of reasons is the characteristics of > percpu_counter batch. Even the pages are writeout, we may be could not > get the real proportion value due to side effects of counter > performance. It's just a slight disappointment. Well, you can tune 'min_ratio' of the fuse-bdi to avoid problems with these near-zero states... To always give the bdi some breathing room for ramping up. Honza -- Jan Kara SUSE Labs, CR