Re: [mpich-discuss] osu_latency test: why 8KB takes less time than 4KB and 2KB takes less time than 1KB?

20 Jun 2018

      Hi Abu,

I think Jeff means that you should run your experiment with more 
iterations in order to get a stable results.
- Increase the iteration of for loop in each execution (I think osu 
benchmark allows you to set it)
- Run the experiments 10 or 100 times, and take the average and standard 
deviation.

If you see a very small standard deviation (e.g., <=5%), then the trend 
is stable and you might not see such gaps.

Best regards,
Min
On 2018/06/20 12:14, Abu Naser wrote:
...
Hello Jeff,
Yes, I am using a switch and other machinesï¿½are also connected with 
that switch.
If I remove other machines and just use my two node with the switch, 
then will it improve the performance by 200 ~ 400 iterations?
Meanwhile I will give a try with a single dedicated cable.
Thank you.
Best Regards,
Abu Naser
------------------------------------------------------------------------
*From:* Jeff Hammond <[email protected]>
*Sent:* Wednesday, June 20, 2018 12:52:06 PM
*To:* MPICH
*Subject:* Re: [mpich-discuss] osu_latency test: why 8KB takes less 
time than 4KB and 2KB takes less time than 1KB?
Is the ethernet connection a single dedicated cable between the two 
machines or are you running through a switch that handles other traffic?
My best guess is that this is noise and that you may be able to avoid 
it by running a very long time, e.g. 10000 iterations.
Jeff
On Wed, Jun 20, 2018 at 6:53 AM, Abu Naser <[email protected] 
<mailto:[email protected]>> wrote:
Good day to all,
I had run point to point osu_latency test in two nodes for 200
    times.ï¿½ Followings are the average time in microsecond for various
    size of the messages -
1KBï¿½ï¿½ ï¿½84.8514 us
    2KBï¿½ï¿½ ï¿½73.52535 us
    4KBï¿½ï¿½ ï¿½272.55275 us
    8KBï¿½ï¿½ ï¿½234.86385 us
    16KBï¿½ï¿½ ï¿½288.88 us
    32KBï¿½ï¿½ ï¿½523.3725 us
    64KBï¿½ï¿½ ï¿½910.4025 us
From the above looks like, 2KB message has less latency than 1 KB
    and 8KB has less latency than 4KB.
I was looking for explanation of this behaviorï¿½ but did not get any.
1. MPIR_CVAR_CH3_EAGER_MAX_MSG_SIZEis set to 128KB. So none of
        the above message size is using Rendezvous protocol. Is there
        any partition inside eager protocol (e.g. 0 - 512 bytes, 1KB -
        8KB, 16KB - 64KB)? If yes then what are the boundaries for
        them? Can I log them with debug-event-logging?
Setup I am using:
- two nodes has intel core i7, one with 16gb memory another one 8gb
- mpich 3.2.1, configured and build to use nemesis tcp
- 1gb Ethernet connection
- NFS is using for sharing
- osu_latency : uses MPI_Send and MPI_Recv
- MPIR_CVAR_CH3_EAGER_MAX_MSG_SIZE= 131072 (128KB)
Can anyone help me on that? Thanks in advance.
Best Regards,
Abu Naser
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-- 
Jeff Hammond
[email protected] <mailto:[email protected]>
http://jeffhammond.github.io/
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Re: [mpich-discuss] osu_latency test: why 8KB takes less time than 4KB and 2KB takes less time than 1KB?

Min Si