linux-kernel - Re: [PATCH v1 0/8] Deferred dput() and iput() -- reducing lock contention

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Date:	Sat, 17 Jan 2009 19:12:10 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Mike Waychison <mikew@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v1 0/8] Deferred dput() and iput() -- reducing lock
	contention

On Fri, Jan 16, 2009 at 06:29:36PM -0800, Mike Waychison wrote:
> We've noticed that at times it can become very easy to have a system begin to
> livelock on dcache_lock/inode_lock (specifically in atomic_dec_and_lock()) when
> a lot of dentries are getting finalized at the same time (massive delete and
> large fdtable destructions are two paths I've seen cause problems).
> 
> 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.

Hmmmm. This deferring of dput/iput will have the same class of
effects on filesystems as the recent reverted changes to make
generic_delete_inode() an asynchronous process. That is, it
temporally separates the transaction for namespace deletion (i.e.
unlink) from the transaction that completes the inode deletion that
occurs, typically, during ->clear_inode. See the recent thread
titled:

[PATCH] async: Don't call async_synchronize_full_special() while holding sb_lock

For more details.

I suspect that change is likely to cause worse problems than the
async changes in that it doesn't have a cap on the number of
deferred operations.....

> Besides various workload testing,

Details?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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