Performance drop in iozone for reading large files

[Dean Hildebrand]

Just to clarify, are you referring to client memory below?  Does the 
amount of server memory affect the situation?  Did you try a 4GB file on 
the 4GB ram system?

Dean

Bryce Harrington wrote:
> There is a performance issue in READ operations we've noticed in iozone
> runs for a while now.  If you look at this plot, of NFSv4 with 1G mem:
>
>    http://crucible.osdl.org/runs/1604/test_output/iozone.sys.log.png
>
> You can see that we're getting fairly steady performance up until the
> 1048576kb case, where read performance drops to about a third.
>
> Is anyone else experiencing similar effects?
>
> Testing against NFSv3 shows similar behavior. 
>
> Also, from our tests it appears to be memory-related.  We booted with
> less memory and see the issue occuring for lower file sizes:
>
>  NFSv3 w/ 256M mem:
>     http://crucible.osdl.org/runs/1607/test_output/iozone.nfsv3.log.png
>     -> Drops off at the 131072k file size
>
>  NFSv3 w/ 512M mem:
>     http://crucible.osdl.org/runs/1612/test_output/iozone.nfsv3.log.png
>     -> Drops off at the 262144k file size
>
> On our 64-bit x86 systems with 4G RAM, we are not seeing the issue,
> presumably due to the larger memory size.
>
>  NFSv4 w/ 4G mem:
>     http://crucible.osdl.org/runs/1511/test_output/iozone.sys.log.png
>     -> No drop off
>
> This issue has been present for a while now, but other issues had hidden
> it in recent iozone runs.
>
> Does anyone have an idea why the dropoff is occuring?  Is the memory
> size expected to affect NFS performance this way?  Is anyone else
> experiencing similar effects?
>
> Thanks,
> Bryce
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-- 
Dean Hildebrand
Ph.D. Candidate
University of Michigan