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

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Message-ID: <87aaolwar8.fsf@basil.nowhere.org>
Date:	Tue, 17 Aug 2010 16:54:03 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Patrick J. LoPresti" <lopresti@...il.com>
Cc:	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

"Patrick J. LoPresti" <lopresti@...il.com> writes:

>
> 1) Anybody who cares about file system performance is already using
> "noatime" or "relatime", which mitigates the hit greatly.

Consider mtime.

> If the above patch is too slow for some architectures, how about
> making it a configuration option?  Call it "CONFIG_1980S_FILE_TICK",
> have it default to YES on the architectures that care and NO on
> anything remotely modern and sane.
>
> OK that's my proposal.  Bash away.

I suspect it will be a performance disaster on x86 for VFS intensive
applications on capable file systems. VFS is very performance
critical. These checks lurk on unexpected places too, e.g. on /dev
access.

Even TSC is much slower than just reading the variable.

Also you should check if the file system granuality 
even supports it, it's completely wasted on a ext3 for example.

Maybe as a optional sysctl, default to off.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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