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Subject: Re: [OMPI users] MPI_Reduce performance
From: Richard Treumann (treumann_at_[hidden])
Date: 2010-09-09 14:10:13

I was pointing out that most programs have some degree of elastic
synchronization built in. Tasks (or groups or components in a coupled
model) seldom only produce data.they also consume what other tasks produce
and that limits the potential skew.

If step n for a task (or group or coupled component) depends on data
produced by step n-1 in another task (or group or coupled component) then
no task can be farther ahead of the task it depends on than one step. If
there are 2 tasks that each need the others step n-1 result to compute
step n then they can never get farther than one step out of synch. If
there were a rank ordered loop of 8 tasks so each one needs the output of
the prior step on task ((me-1) mod tasks) to compute then you can get
more skew because if
task 5 gets stalled in step 3,
task 6 will finish step 3 and send results to 7 but stall on recv for step
4 (lacking the end of step 3 send by task 5)
task 7 will finish step 4 and send results to 0 but stall on recv for
step 5
task 0 will finish step 5 and send results to 1 but stall on recv for
step 6

In a 2D or 3D grid, the dependency is tighter so the possible skew is
less. but it is still significant on a huge grid In a program with
frequent calls to MPI_Allreduce on COMM_WORLD, the skew is very limited.
The available skew gets harder to predict as the interdependencies grow
more complex.

I call this "elasticity" because the amount of stretch varies but, like a
bungee cord or an waist band, only goes so far. Every parallel program has
some degree of elasticity built into the way its parts interact.

I assume a coupler has some elasticity too. That is, ocean and atmosphere
each model Monday and report in to coupler but neither can model Tuesday
until they get some of the Monday results generated by the other. (I am
pretending granularity is day by day) Wouldn't the right level of
synchronization among component result automatically form the data
dependencies among them?


Dick Treumann - MPI Team
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363

Eugene Loh <eugene.loh_at_[hidden]>
Open MPI Users <users_at_[hidden]>
09/09/2010 12:40 PM
Re: [OMPI users] MPI_Reduce performance
Sent by:

Gus Correa wrote:

> More often than not some components lag behind (regardless of how
> much you tune the number of processors assigned to each component),
> slowing down the whole scheme.
> The coupler must sit and wait for that late component,
> the other components must sit and wait for the coupler,
> and the (vicious) "positive feedback" cycle that
> Ashley mentioned goes on and on.

I think "sit and wait" is the "typical" scenario that Dick mentions.
Someone lags, so someone else has to wait.

In contrast, the "feedback" cycle Ashley mentions is where someone lags
and someone else keeps racing ahead, pumping even more data at the
laggard, forcing the laggard ever further behind.
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