From: Pekka Enberg Subject: Re: [PATCH] ext2/ext3: allocate ->s_blockgroup_lock separately to avoid wasting space Date: Mon, 17 Nov 2008 23:27:47 +0200 Message-ID: <4921E1D3.8040700@cs.helsinki.fi> References: <1226840289.8172.8.camel@lappy.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org, cl@linux-foundation.org, mpm@selenic.com, eduard.munteanu@linux360.ro To: Peter Zijlstra Return-path: Received: from courier.cs.helsinki.fi ([128.214.9.1]:36636 "EHLO mail.cs.helsinki.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751802AbYKQVbk (ORCPT ); Mon, 17 Nov 2008 16:31:40 -0500 In-Reply-To: <1226840289.8172.8.camel@lappy.programming.kicks-ass.net> Sender: linux-ext4-owner@vger.kernel.org List-ID: Peter Zijlstra wrote: > On Fri, 2008-11-14 at 11:17 +0200, Pekka J Enberg wrote: >> From: Pekka Enberg >> >> As spotted by kmemtrace, struct ext2_sb_info is 17024 bytes and ext3_sb_info is >> 17152 bytes on 64-bit which makes them a very bad fit for SLAB allocators. In >> fact, both allocations are round up to the next available page size of >> order 3 which is 32 KB. >> >> The culprit if the wasted memory is the ->s_blockgroup_lock which can be as big >> as 16 KB when CONFIG_NR_CPUS is set to 32. As struct blockgroup_lock is a >> perfect fit for order 2 page in the worst case, allocate ->s_blockgroup_lock >> separately to avoid wasting space. > > And here I was thinking that NR_CPUS=4096 is currently our worst > case ;-) Sure but look at . NR_BG_LOCKS is capped to 128 for >= 32 CPUs.