linux-kernel - Re: [2.6.30.1] Significant latency playing video file from NFS4 share

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Date:	Tue, 21 Jul 2009 13:58:25 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Frans Pop <elendil@...net.nl>
Cc:	linux-nfs@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: [2.6.30.1] Significant latency playing video file from NFS4
 share

On Tue, 2009-07-21 at 19:31 +0200, Frans Pop wrote:
> Any suggestions based on the data below?

Yeah. Sorry for the delay.

> Is the problem maybe that the amount of data transferred each time is so
> big that the network speed becomes a bottleneck and causes the latency?
> I can reproduce the issue with both a 10MBit wired link and with a 54MBit
> wireless link.
> 
> Is this a kernel bug or a configuration issue?

<snip>

> elrond:/david mounted on /srv/fjp/david:
> 
>    op/s         rpc bklog
>    0.80            0.00
> read:             ops/s            kB/s           kB/op         retrans         avg RTT (ms)    avg exe (ms)
>                   0.750         192.196         256.262        0 (0.0%)         1998.933        1999.200
> write:            ops/s            kB/s           kB/op         retrans         avg RTT (ms)    avg exe (ms)
>                   0.000           0.000           0.000        0 (0.0%)           0.000           0.000

This would be where you are losing your performance. 1998.933 ms rtt
means that you are seeing a 2 second delay from the instant the RPC
client pushes the request into the socket to the moment that socket
receives a reply from the server.

Basically, you have either a _very_ slow server, or (more likely) a
networking problem. A tcpdump ought to be able to show what the problem
is: whether it is the packets getting ACKed very slowly, and/or if you
have dropped packet issues or if it is truly a problem with the server
taking a long time to reply.

Cheers
  Trond

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