Re: [PATCH v1 0/8] Deferred dput() and iput() -- reducing lockcontention

From: Dave Chinner
Date: Sat Jan 17 2009 - 03:12:28 EST

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

[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,



Dave Chinner
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