From: Anthony Howe <ahowe_ca@yahoo.ca>
Subject: RE: slow file opens on nfs mount across high-latency network
Date: Tue, 28 Jun 2005 12:53:30 -0400 (EDT)
Message-ID: <20050628165330.44157.qmail@web21323.mail.yahoo.com>
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I probably should have provided this link to the pdf
document instead of the attachment:

http://ahowe_ca.tripod.com/nfsperformance.pdf

This graph show the hockey stick effect that I
described in the below post.

Anthony

> I have searched through the mail lists and google
and
> have not found material describing the nfs file open
> threshold effect that I am experiencing. =20
>
> I have been experimenting with opening files on NFS
> mounts over varying network latencies.  I notice
that
> there seems to be a threshold on the number of
> concurrent nfs file opens as network latency
> increases.  Up to and including the threshold, nfs
> file open performance is fine.  After the threshold
of
> concurrent opens, performance degrades at a linear
> rate.
>=20
> For example, the graph in the attached files shows
> this threshold effect for various network latencies:
> - 0ms network latency - no max limit
> - 20ms network latency - 40 maximum concurrent opens
> - 40ms network latency - 20 maximum concurrent opens
> - 80ms network latency - 15 maximum concurrent opens
> - 120ms network latency - 5 maximum concurrent opens
>=20
> What would cause this "hockey stick" threshold
effect
> shown in the attached file?
> Are there any settings that would change this
effect?
>=20
> Here are the stats of my experiment:
> - testing using Redhat Enterprise AS servers V3
> connected via a 100Mbps switch
> - inserting latency with nist net
> - experiment process spawns X number of threads set
to
> each open a file on an NFS mount, the time taken for
> each file open is recorded
> - adjusting rsize, wsize does not affect "hockey
> stick" threshold effect
> - adjusting /proc/sys/net/core/rmem* does not affect
> "hockey stick" threshold effect
> - adjusting number of nfsd processes does not affect

> "hockey stick" threshold effect
>=20
> Thanks in advance for any tips or directions where I
> can look for more information on this topic.
>=20
> Regards,
>=20
> Anthony Howe

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