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

[Jeff Hammond]

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 <an16e at my.fsu.edu> wrote:

>
> Good day to all,
>
>
> I had run point to point osu_latency test in two nodes for 200 times.
> Followings are the average time in microsecond for various size of the
> messages -
>
> 1KB    84.8514 us
> 2KB    73.52535 us
> 4KB    272.55275 us
> 8KB    234.86385 us
> 16KB    288.88 us
> 32KB    523.3725 us
> 64KB    910.4025 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 behavior  but did not get any.
>
>
>
>    1. MPIR_CVAR_CH3_EAGER_MAX_MSG_SIZE is 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
jeff.science at gmail.com
http://jeffhammond.github.io/
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