From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753331Ab0IPBVw (ORCPT ); Wed, 15 Sep 2010 21:21:52 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:40618 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752350Ab0IPBVv (ORCPT ); Wed, 15 Sep 2010 21:21:51 -0400 Subject: Re: [RFC][PATCH] update /proc/sys/vm/drop_caches documentation From: Dave Hansen To: KAMEZAWA Hiroyuki Cc: Tim Pepper , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel In-Reply-To: <20100916091215.ef59acd7.kamezawa.hiroyu@jp.fujitsu.com> References: <20100914234714.8AF506EA@kernel.beaverton.ibm.com> <20100915133303.0b232671.kamezawa.hiroyu@jp.fujitsu.com> <20100915192454.GD5585@tpepper-t61p.dolavim.us> <20100916091215.ef59acd7.kamezawa.hiroyu@jp.fujitsu.com> Content-Type: text/plain; charset="ANSI_X3.4-1968" Date: Wed, 15 Sep 2010 18:21:47 -0700 Message-ID: <1284600107.20776.640.camel@nimitz> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2010-09-16 at 09:12 +0900, KAMEZAWA Hiroyuki wrote: > I hear a customer's case. His server generates 3-80000+ new dentries per day > and dentries will be piled up to 1000000+ in a month. This makes open()'s > performance very bad because Hash-lookup will be heavy. (He has very big memory.) > > What we could ask him was > - rewrite your application. or > - reboot once in a month (and change hash size) or > - drop_cache once in a month > > Because their servers cannot stop, he used drop_caches once in a month > while his server is idle, at night. Changing HashSize cannot be a permanent > fix because he may not stop the server for years. That is a really interesting case. They must have a *ton* of completely extra memory laying around. Do they not have much page cache activity? It usually balances out the dentry/inode caches. Would this user be better off with a smaller dentry hash in general? Is it special hardware that should _have_ a lower default hash size? > For rare users who have 10000000+ of files and tons of free memory, drop_cache > can be an emergency help. In this case, though, would a WARN_ON() in an emergency be such a bad thing? They evidently know what they're doing, and shouldn't be put off by it. -- Dave