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

[Mike Waychison <mikew@google.com>]

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.