From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Waychison Subject: Re: [PATCH v1 0/8] Deferred dput() and iput() -- reducing lock contention Date: Tue, 20 Jan 2009 22:22:00 -0800 Message-ID: <4976BF08.90306@google.com> References: <20090117022936.20425.43248.stgit@crlf.corp.google.com> <877i4pdx4f.fsf@basil.nowhere.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Andi Kleen Return-path: Received: from smtp-out.google.com ([216.239.45.13]:48479 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751000AbZAUGU6 (ORCPT ); Wed, 21 Jan 2009 01:20:58 -0500 In-Reply-To: <877i4pdx4f.fsf@basil.nowhere.org> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Andi Kleen wrote: > Mike Waychison writes: > >> livelock on dcache_lock/inode_lock (specifically in atomic_dec_and_lock()) > > I'm not sure how something can livelock in atomic_dec_and_lock which > doesn't take a spinlock itself? Are you saying you run into NUMA memory > unfairness here? Or did I misparse you? By atomic_dec_and_lock, I really meant to say _atomic_dec_and_lock(). It takes the spinlock if the cmpxchg hidden inside atomic_dec_unless fails. There are likely NUMA unfairness issues at play, but it's not the main worry at this point. > >> This patchset is an attempt to try and reduce the locking overheads associated >> with final dput() and final iput(). This is done by batching dentries and >> inodes into per-process queues and processing them in 'parallel' to consolidate >> some of the locking. > > I was wondering what this does to the latencies when dput/iput > is only done for very objects. Does it increase costs then > significantly? very objects? > > As a high level comment it seems like a lot of work to work > around global locks, like the inode_lock, where it might be better to > just split the lock up? Mind you I don't have a clear proposal > how to do that, but surely it's doable somehow. > Perhaps.. the only plausible way I can think this would be doable would be to rework the global resources (like the global inode_unused LRU list and deal with inode state transitions), but even then, some sort of consistency needs to happen at the super_block level, which means the smallest I can see the lock becoming would be per-super_block, which doesn't solve the problem afaict.