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

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Message-ID: <4976BF08.90306@google.com>
Date:	Tue, 20 Jan 2009 22:22:00 -0800
From:	Mike Waychison <mikew@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v1 0/8] Deferred dput() and iput() -- reducing lock contention

Andi Kleen wrote:
> Mike Waychison <mikew@...gle.com> 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.
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