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

[Mike Waychison]

Dave Chinner wrote:
> 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.....

I'll dig through the archives and try to come up with a response later 
today.

> > Besides various workload testing,
>
> Details?

I ran a couple different workloads, though I was looking for 
stability/correctness rather than performance.  I ran iozone (ext2 + 
ext4/nojournal), dbench (ext2), tbench (ext2), specjbb, unixbench, 
kernbench as well as a couple internal benchmarks.
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