linux-kernel - Re: Massive slowdown when re-querying large nfs dir

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Date:	Wed, 7 Nov 2007 12:36:26 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Neil Brown <neilb@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Massive slowdown when re-querying large nfs dir

Neil Brown wrote:
> On Tuesday November 6, akpm@...ux-foundation.org wrote:
> > > On Tue, 6 Nov 2007 14:28:11 +0300 Al Boldi <a1426z@...ab.com> wrote:
> > > Al Boldi wrote:
> > > > There is a massive (3-18x) slowdown when re-querying a large nfs dir
> > > > (2k+ entries) using a simple ls -l.
> > > >
> > > > On 2.6.23 client and server running userland rpc.nfs.V2:
> > > > first  try: time -p ls -l <2k+ entry dir>  in ~2.5sec
> > > > more tries: time -p ls -l <2k+ entry dir>  in ~8sec
> > > >
> > > > first  try: time -p ls -l <5k+ entry dir>  in ~9sec
> > > > more tries: time -p ls -l <5k+ entry dir>  in ~180sec
> > > >
> > > > On 2.6.23 client and 2.4.31 server running userland rpc.nfs.V2:
> > > > first  try: time -p ls -l <2k+ entry dir>  in ~2.5sec
> > > > more tries: time -p ls -l <2k+ entry dir>  in ~7sec
> > > >
> > > > first  try: time -p ls -l <5k+ entry dir>  in ~8sec
> > > > more tries: time -p ls -l <5k+ entry dir>  in ~43sec
> > > >
> > > > Remounting the nfs-dir on the client resets the problem.
> > > >
> > > > Any ideas?
> > >
> > > Ok, I played some more with this, and it turns out that nfsV3 is a lot
> > > faster.  But, this does not explain why the 2.4.31 kernel is still
> > > over 4-times faster than 2.6.23.
> > >
> > > Can anybody explain what's going on?
> >
> > Sure, Neil can! ;)

Thanks Andrew!

> Nuh.
> He said "userland rpc.nfs.Vx".  I only do "kernel-land NFS".  In these
> days of high specialisation, each line of code is owned by a different
> person, and finding the right person is hard....
>
> I would suggest getting a 'tcpdump -s0' trace and seeing (with
> wireshark) what is different between the various cases.

Thanks Neil for looking into this.  Your suggestion has already been answered 
in a previous post, where the difference has been attributed to "ls -l" 
inducing lookup for the first try, which is fast, and getattr for later 
tries, which is super-slow.

Now it's easy to blame the userland rpc.nfs.V2 server for this, but what's 
not clear is how come 2.4.31 handles getattr faster than 2.6.23?


Thanks!

--
Al

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