linux-kernel - Re: [rfc][patch] store-free path walking

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Date:	Fri, 9 Oct 2009 12:40:13 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	David Miller <davem@...emloft.net>
Cc:	torvalds@...ux-foundation.org, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kiran@...lex86.org, peterz@...radead.org
Subject: Re: [rfc][patch] store-free path walking

On Thu, Oct 08, 2009 at 11:15:27PM -0700, David Miller wrote:
> From: Nick Piggin <npiggin@...e.de>
> Date: Fri, 9 Oct 2009 05:50:50 +0200
> 
> > OK, I got rid of this guy from the RCU walk. Basically now hold
> > vfsmount_lock over the entire RCU path walk (which also pins the mnt)
> > and use a seqlock in the fs struct to get a consistent mnt,dentry
> > pair. This also simplifies the walk because we don't need the
> > complexity to avoid mntget/mntput (just do one final mntget on the
> > resulting mnt before dropping vfsmount_lock).
> > 
> > vfsmount_lock adds one per-cpu atomic for the spinlock, and we
> > remove two thread-shared atomics for fs->lock so a net win for
> > both single threaded performance and thread-shared scalability.
> > Latency is no problem because we hold rcu_read_lock for the same
> > length of time anyway.
> > 
> > The parallel git diff workload is improved by serveral percent.
> 
> Sounds sweet Nick, can't wait to play with your next set of
> patches here.

http://www.kernel.org/pub/linux/kernel/people/npiggin/patches/fs-scale/

OK well there will be another rev of the patches there with everything,
also with the d_mounted patch so dentry remains a nice size. Also tried
rearranging some structures in the dentry to help lookups but haven't
quite finished there.

Jens I would be interested to see whether parallel case on the N2 is
improved at all. Profiles from that case might be interesting.

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