Re: slow file opens on nfs mount across high-latency network

[Peter Staubach <staubach@redhat.com>]

Neil Horman wrote:

>On Tue, Jun 28, 2005 at 11:30:17AM -0400, Anthony Howe wrote:
>  
>
>>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.  
>>
>>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.
>>
>>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
>>
>>What would cause this "hockey stick" threshold effect
>>shown in the attached file?
>>Are there any settings that would change this effect?
>>
>>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
>>
>>Thanks in advance for any tips or directions where I
>>can look for more information on this topic.
>>
>>Regards,
>>
>>Anthony Howe
>>
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>>    
>>
>
>
>do you have tcpdumps taken at all these data points?  It kind of sounds to me
>like your getting a geared slowdown from additional congestion caused by rpc
>retransmits (i.e., on a high latency link, you'll wind up with more rpc
>retransmits, resulting in more congestion, resulting in more lost packets,
>resulting in more retransmits, etc.).
>
Some questions --

These are mounts done using the NFSv3 protocol?  Over TCP or over UDP?

Each thread is opening an independent file?  In independent directories
or in a common directory?

    Thanx...

       ps


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