From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with SMTP id B52426B003D for ; Tue, 24 Mar 2009 11:23:29 -0400 (EDT) Message-ID: <49C8FDD4.7070900@wpkg.org> Date: Tue, 24 Mar 2009 16:35:48 +0100 From: Tomasz Chmielewski MIME-Version: 1.0 Subject: Re: why my systems never cache more than ~900 MB? References: <49C89CE0.2090103@wpkg.org> <200903250220.45575.nickpiggin@yahoo.com.au> In-Reply-To: <200903250220.45575.nickpiggin@yahoo.com.au> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Nick Piggin Cc: linux-mm@kvack.org List-ID: Nick Piggin schrieb: > On Tuesday 24 March 2009 19:42:08 Tomasz Chmielewski wrote: >> On my (32 bit) systems with more than 1 GB memory it is impossible to cache >> more than about 900 MB. Why? >> >> Caching never goes beyond ~900 MB (i.e. when I read a mounted drive with >> dd): > > Because blockdev mappings are limited to lowmem due to sharing their > cache with filesystem metadata cache, which needs kernel mapped memory. > It will >900MB of pagecache data OK (data from regular files) Does not help me, as what interests me here on these machines is mainly caching block device data; they are iSCSI targets and access block devices directly. (...) >> Same behaviour on 32 bit machines with 4 GB RAM. >> >> No problems on 64 bit machines. >> I have one 32 bit machine that caches beyond ~900 MB without problems. > > Does it have a different user/kernel split? Yes it does: CONFIG_VMSPLIT_2G_OPT=y What split should I choose to enable blockdev mapping on the whole memory on 32 bit system with 3 or 4 GB RAM? Is it possible with 4 GB RAM at all? >> Is it some kernel/proc/sys setting that I'm missing? > > No, it just can't be done without changing code. -- Tomasz Chmielewski http://wpkg.org -- 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