From: Trond Myklebust <trond.myklebust@fys.uio.no>
Subject: Re: [NFS][PATCH] Making sure negative lookup entries don't
	exist
Date: Sat, 29 Jan 2005 11:32:44 -0800
Message-ID: <1107027164.11814.9.camel@lade.trondhjem.org>
References: <41FA88D5.6010904@RedHat.com>
	 <1106967908.10024.19.camel@lade.trondhjem.org>
	 <41FB88C7.3060903@RedHat.com>
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Cc: linux-nfs <nfs@lists.sourceforge.net>
To: Steve Dickson <SteveD@redhat.com>
In-Reply-To: <41FB88C7.3060903@RedHat.com>
Sender: nfs-admin@lists.sourceforge.net
Errors-To: nfs-admin@lists.sourceforge.net

lau den 29.01.2005 Klokka 07:59 (-0500) skreiv Steve Dickson:
> Trond Myklebust wrote:
> 
> >If you are going to do a GETATTR on each call, why cache negative
> >dentries in the first place? Better then to simply remove the relevant
> >lines in nfs_lookup() and replace all occurrences of d_delete() with
> >d_drop().
> >  
> >
> Well I believe that would cause an otw lookup which is much more expensive
> than a cheap getattr to ensure the correct mtime of the directory....

It is not that much more expensive (for Linux servers at least).

> >Note, though, that both patches will seriously kill performance on
> >static, frequently-accessed directories (think $PATH, /usr/share etc.),
> >  
> >
> Well in my testing, which consisted of running the cthon04 tests a 
> number of
> times and counting the opts with nfsstat, the getattrs only went up by 
> 3% and the
> lookups when down by about 2%.... which was a bit surprising....

I can believe it for Connectathon, but that doesn't really test the sort
of scenario I described where you are repeatedly searching through
read-only directories for frequently-used files.

Try looking at an nfsroot setup with and without your patch. That should
illustrate more clearly what I meant.

> >so we're certainly not applying that kind of patch to the mainline
> >kernel...
> >  
> >
> hmm... I hate when you say things like that... :-)  I just trying to 
> come up with

In this case it's a question of "been there, done that, had the Torvalds
chastise me for my stupidity"... 8-)

Cheers,
  Trond
-- 
Trond Myklebust <trond.myklebust@fys.uio.no>


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