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Subject: Re: [OMPI users] RE : RE : Latency of 250 microseconds with Open-MPI 1.4.3, Mellanox Infiniband and 256 MPI ranks
From: Yevgeny Kliteynik (kliteyn_at_[hidden])
Date: 2011-09-26 08:46:29


On 26-Sep-11 11:27 AM, Yevgeny Kliteynik wrote:
> On 22-Sep-11 12:09 AM, Jeff Squyres wrote:
>> On Sep 21, 2011, at 4:24 PM, Sébastien Boisvert wrote:
>>
>>>> What happens if you run 2 ibv_rc_pingpong's on each node? Or N ibv_rc_pingpongs?
>>>
>>> With 11 ibv_rc_pingpong's
>>>
>>> http://pastebin.com/85sPcA47
>>>
>>> Code to do that => https://gist.github.com/1233173
>>>
>>> Latencies are around 20 microseconds.
>>
>> This seems to imply that the network is to blame for the higher latency...?
>
> Interesting... I'm getting the same latency with ibv_rc_pingpong.
> I get 8.5 usec for a single ping-pong.

BTW, I've just checked this with performance guys - ibv_rc_pingpong
is not used for performance measurement but only as IB network
sanity check, therefore it was never meant to give optimal performance.

Use ib_write_lat instead.

-- YK
 
> Please run 'ibclearcounters' to reset fabric counters, then
> ibdiagnet to make sure that the fabric is clean.
> If you have 4x QDR cluster, run ibdiagnet as follows:
>
> ibdiagnet --ls 10 --lw 4x
>
> Check that you don't have any errors/warnings.
>
> Then please run your script with ib_write_lat instead of ibv_rc_pingpong.
> Just replace the command in the script and the rest would be fine.
>
> If the fabric is clean, you're supposed to get typical
> latency of ~1.4 usec.
>
> -- YK
>
>
>> I.e., if you run the same pattern with MPI processes and get 20us latency, that would tend to imply that the network itself is not performing well with that IO pattern.
>>
>>> My job seems to do well so far with ofud !
>>>
>>> [sboisver12_at_colosse2 ray]$ qstat
>>> job-ID prior name user state submit/start at queue slots ja-task-ID
>>> -----------------------------------------------------------------------------------------------------------------
>>> 3047460 0.55384 fish-Assem sboisver12 r 09/21/2011 15:02:25 med_at_r104-n58 256
>>
>> I would still be suspicious -- ofud is not well tested, and it can definitely hang if there are network drops.
>>
>