linux-kernel - Re: Proposal: Use hi-res clock for file timestamps

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Message-ID: <20100818185456.GD13050@fieldses.org>
Date:	Wed, 18 Aug 2010 14:54:56 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Neil Brown <neilb@...e.de>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Patrick J. LoPresti" <lopresti@...il.com>,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Proposal: Use hi-res clock for file timestamps

On Wed, Aug 18, 2010 at 07:50:40PM +0200, Andi Kleen wrote:
> >  - nfsd updates it whenever it reads an mtime out of an inode that matches
> >    current_fs_time to the granularity of 1/HZ.
> 
> That means you have a very very hot cache line on a larger system
> if there are a lot of mtime changes. Probably a bad idea.

Only if those mtime changes are also followed immediately by nfsd reads
of the mtime.

That will be the typical case for nfsd writes, though.

--b.
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