Re: [Starpu-devel] MPI scaling

[Xavier Lacoste <xavier.lacoste@inria.fr>]

Le 24 juin 2014 à 14:17, Samuel Thibault <samuel.thibault@ens-lyon.org> a 
écrit :

> Hello,
> 
> Xavier Lacoste, le Mon 23 Jun 2014 09:19:19 +0200, a écrit :
>> I'm using eager scheduler has I have no GPUs
> 
> Maybe you'd want to give a try to the lws scheduler.

Ok I'll have a try.

I don't see it in starpu 1.1.2, is it new in 1.2 ? Is it different from ws ?

The variable STARPU_SCHED can be set to one of the following strings:
eager   -> eager policy with a central queue
prio    -> eager (with priorities)
random  -> weighted random based on worker overall performance
ws      -> work stealing
dm      -> performance model
dmda    -> data-aware performance model
dmdar   -> data-aware performance model (ready)
dmdas   -> data-aware performance model (sorted)
pheft   -> parallel HEFT
> 
>> I attach the 4x8 execution trace. I would like to know is their is a mean 
>> to see if a communication is triggered as soon as the last local update as 
>> been performed.
> 
> We can see big idle areas indeed. Is "local update" a particular kind
> of task? You could make vite show it in a different color and thus see
> where it is. Perhaps the problem here is rather that these tasks are
> scheduled late, and giving them a priority and using the prio scheduler
> would help.
> 

This local update task is the same task as the update to local GEMMs (except 
the written data is temporary buffer and will have to be send over MPI). I 
could rename them to have a different color.

I already tried using eager and giving non zero priority to these update 
tasks (as I saw in online documentation that the non zero priority tasks are 
put in front of the queue). Is this different from prio scheduler behaviour ?

>> Have you got any advice to try to improve MPI scaling.
> 
> Well, it's the matter of Marc's thesis :)
> Here it seems to me rather a problem of task order.
> 
>> In my case here, using the whole 8 cores of a node is worse than using 7 
>> threads per node... Have you already experience that ?
> 
> That may not be surprising indeed, since this lets the MPI thread have
> its own core.
> 
> Samuel

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