From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754068Ab0I3Fgc (ORCPT ); Thu, 30 Sep 2010 01:36:32 -0400 Received: from mail-fx0-f46.google.com ([209.85.161.46]:33303 "EHLO mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753727Ab0I3Fga (ORCPT ); Thu, 30 Sep 2010 01:36:30 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=Pn2n72w3rQLk/1BOO8gqfjJaQ/8C99pRG0ZL2wbIomUbcr3ieq7dcGm2uwQo1o1y+r /aOxgU895G7EQaJB0yALOlUllD+avT5dkO6pAGl2qSgEdv8D28Akg8C58i8ViNkMojIz 87HY4Q9G5uTBFVGTcXE6b1JizOpZR3FB3N43w= Subject: Re: [PATCH 15/17] fs: inode per-cpu last_ino allocator From: Eric Dumazet To: Andrew Morton Cc: Dave Chinner , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20100929215312.5fcb6976.akpm@linux-foundation.org> References: <1285762729-17928-1-git-send-email-david@fromorbit.com> <1285762729-17928-16-git-send-email-david@fromorbit.com> <20100929215312.5fcb6976.akpm@linux-foundation.org> Content-Type: text/plain; charset="UTF-8" Date: Thu, 30 Sep 2010 07:36:22 +0200 Message-ID: <1285824982.5211.675.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le mercredi 29 septembre 2010 à 21:53 -0700, Andrew Morton a écrit : > On Wed, 29 Sep 2010 22:18:47 +1000 Dave Chinner wrote: > > > From: Eric Dumazet > > Please note my new email address, thanks. > > last_ino was converted to an atomic variable to allow the inode_lock > > to go away. However, contended atomics do not scale on large > > machines, and new_inode() triggers excessive contention in such > > situations. > > > > Solve this problem by providing to each cpu a per_cpu variable, > > feeded by the shared last_ino, but once every 1024 allocations. > > This reduces contention on the shared last_ino, and give same > > spreading ino numbers than before (i.e. same wraparound after 2^32 > > allocations). > > > > [npiggin: some extra commenting and use of defines] > > > > ... > > > > +#ifdef CONFIG_SMP > > +#define LAST_INO_BATCH 1024 > > +/* > > + * Each cpu owns a range of LAST_INO_BATCH numbers. > > + * 'shared_last_ino' is dirtied only once out of LAST_INO_BATCH allocations, > > + * to renew the exhausted range. > > + * > > + * This does not significantly increase overflow rate because every CPU can > > + * consume at most LAST_INO_BATCH-1 unused inode numbers. So there is > > + * NR_CPUS*(LAST_INO_BATCH-1) wastage. At 4096 and 1024, this is ~0.1% of the > > + * 2^32 range, and is a worst-case. Even a 50% wastage would only increase > > + * overflow rate by 2x, which does not seem too significant. > > + * > > + * On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW > > + * error if st_ino won't fit in target struct field. Use 32bit counter > > + * here to attempt to avoid that. > > + */ > > +static DEFINE_PER_CPU(unsigned int, last_ino); > > +static atomic_t shared_last_ino; > > + > > +static unsigned int last_ino_get(void) > > +{ > > + unsigned int *p = &get_cpu_var(last_ino); > > + unsigned int res = *p; > > + > > + if (unlikely((res & (LAST_INO_BATCH-1)) == 0)) > > + res = (unsigned int)atomic_add_return(LAST_INO_BATCH, > > + &shared_last_ino) - LAST_INO_BATCH; > > May as well remove the "- LAST_INO_BATCH" there, I think. It'll skew > the results a tad at startup, but why does that matter? Because on x86, atomic_add_return(val, ptr) uses xadd() + val So, using "atomic_add_return(val, ptr) - val" removes one instruction ;) > > > + *p = ++res; > > + put_cpu_var(last_ino); > > + return res; > > +} > > +#else > > +static unsigned int last_ino_get(void) > > +{ > > + static unsigned int last_ino; > > + > > + return ++last_ino; > > +} > > This is racy with CONFIG_PREEMPT on some architectures, I suspect. I'd > suggest conversion to atomic_t with, of course, an explanatory comment ;) > Thanks, I'll rework the patch ! I am pretty happy to see some interest on this patch serie, eventually :)