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Subject: Re: [OMPI users] timing + /usr/bin/time
From: Raymond Wan (rwan_at_[hidden])
Date: 2008-11-14 04:32:12


Hi Fabian,

Thank you for clarifying things and confirming some of the things that I
thought. I guess I have a clearer understanding now.

Fabian Hänsel wrote:
>> Hmmmm, I guess user time does not matter since it is real time that
>> we are interested in reducing.
>>
>
> Right. Even if we *could* measure user time of every MPI worker process
> correctly this was not what you are interested in: Depending on the
> algorithm a significant amount of time could get spend waiting for MPI
> messages to arrive - and that time would not count as user time, but
> also was not 'wasted' as something important happens.
>

The reason why I was wondering is that some people in research papers
compare their algorithm (system) with another one by measuring user time
since it removes some of the effects of what the system does on behalf
of the user's process. And some people, I guess, see this as a fairer
comparison.

On the other hand, I guess I've realized the obvious -- that Open MPI
doesn't reduce the efficiency of the algorithm. Even worse, increases
in user time is an artifact of Open MPI, so it is somewhat misleading if
we are analyzing an algorithm. What MPI should do (if properly used) is
to reduce the real time and that's what we should be reporting...even if
it includes other things that we did not want previously, like the time
spent by the OS in swapping memory, etc.

[Papers I've read with graphs that have "time" on the y-axis and
"processors" on the x-axis rarely mention what time they are
measuring...but it seems obviously now that it must be real time since
user time should [???] increase with more processors.....I think...of
course, assuming we can total the user time across machines accurately.]

Thank you for your message(s)! Think I got it now... :-)

Ray