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 ESMTP id 308316B0044 for ; Mon, 29 Dec 2008 23:02:31 -0500 (EST) Date: Tue, 30 Dec 2008 05:02:25 +0100 From: Nick Piggin Subject: Re: [rfc][patch 1/2] mnt_want_write speedup 1 Message-ID: <20081230040225.GB27679@wotan.suse.de> References: <20081219061937.GA16268@wotan.suse.de> <1229669697.17206.602.camel@nimitz> <20081219070311.GA26419@wotan.suse.de> <1229700721.17206.634.camel@nimitz> <20081222043526.GC13406@wotan.suse.de> <1230591637.19452.129.camel@nimitz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1230591637.19452.129.camel@nimitz> Sender: owner-linux-mm@kvack.org To: Dave Hansen Cc: Linux Memory Management List , linux-fsdevel@vger.kernel.org List-ID: On Mon, Dec 29, 2008 at 03:00:37PM -0800, Dave Hansen wrote: > On Mon, 2008-12-22 at 05:35 +0100, Nick Piggin wrote: > > > Is there a real good reason to allocate the percpu counters dynamically? > > > Might as well stick them in the vfsmount and let the one > > > kmem_cache_zalloc() in alloc_vfsmnt() do a bit larger of an allocation. > > > Did you think that was going to bloat it to a compound allocation or > > > something? I hate the #ifdefs. :) > > > > Distros want to ship big NR_CPUS kernels and have them run reasonably on > > small num_possible_cpus() systems. But also, it would help to avoid > > cacheline bouncing from false sharing (allocpercpu.c code can also mess > > this bug for small objects like these counters, but that's a problem > > with the allocpercpu code which should be fixed anyway). > > I guess we could also play the old trick: > > struct vfsmount > { > ... > int mnt_writers[0]; > }; > > And just > > void __init mnt_init(void) > { > ... > int size = sizeof(struct vfsmount) + num_possible_cpus() * sizeof(int) > > - mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct vfsmount), > + mnt_cache = kmem_cache_create("mnt_cache", size, > 0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL); > > That should save us the dereference from the pointer and still let it be > pretty flexible. Still results in cacheline contention, however... -- 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