From: Romain Dolbeau <romain.dolbeau@caps-entreprise.com>
Subject: Latency problem with some clients but not others
Date: Wed, 15 Aug 2007 19:19:19 +0200
Message-ID: <46C33597.3030004@caps-entreprise.com>
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Hello all,

I have a strange latency problem that affect some clients
but not others. The server is a x86_64 Debian machine.

I've created a test case with just 2 64 bits clients. Both use
the same kernel, and have the same packages installed. They both
mount the same filesystems at the mountpoint through amd (the
maps are distributed through NIS).

The user is the same, with one single logging through SSH.
Nothing is running (except kdm) on either clients.
All machines are directly hooked to the same gigabit switch.
The network traffic was extremely low during the test.
Both clients were freshly rebooted.

One of the client is a dual Xeon 5130 system, with an on-board
intel NIC (module e1000). The other is a single Core 2 Duo 6320,
with an on-board ??? NIC (module r8169).

When doing a ./configure (lots of small r/w accesses) inside one
of the NFS mounted filesystem, the first system is fairly fast,
while the other is much slower - each line of the configure script
takes up to a second to display a result. *But*, pure throughput is
fine - if I use dd to write or read a large file, the speed is what
I would expect from the wire.

The problem is reproductible to all similar clients to the
second system, but I also have other clients (for instance
old 32 bits systems with 3c59x cards) that do not exhibit
the problem. In fact, it seems that all my 32 bits clients
are fast (well, as fast as they can be :-), and all my 64 bits
are slow, except two : the one above, and an old Pentium D
machine with an e100 card.

Any idea where I should look ? My only clue is that all my slow
64 bits client uses the same driver (r8169), whereas the fast one
uses e1000 and e100, could that be the source of the problem ?
(I don't havea spare NIC to try) ; is there any know problem
with such cheap on-board NIC ? How could I tell ?

Thanks in advance for any help.

P.S. just in case it's significant...

* mount display : "type nfs 
(nodev,nosuid,nounmount,noatime,rsize=8192,wsize=8192,vers=3,proto=tcp)"
  for all filesystems on all clients.
* kernel is current Debian stable (2.6.18-4) or testing (2.6.21-2),
  same symptoms for both.
* I've tried both the included r8169 driver and the one from Realtek,
  same symptoms for both.

-- 
Romain Dolbeau
<romain.dolbeau@caps-entreprise.com>


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