[Wrf-users] Results variability depending on processor count

[Jan Ploski]

"James Correia" <jimmyc42 at gmail.com> schrieb am 09/18/2008 04:12:15 PM:

> Hi-
> It is more matter of truncation at a certain number of processors 
> specifically involving domain decomposition.

James,

Thanks for responding. Can you explain in more detail what you mean by 
"truncation"? After all, the domain decomposition happens at a grid point 
boundary, so in my imagination it should be "clean" without any scraps 
falling off the table. I realize that to compute a grid point's value you 
need all the neighbors, some of which may only be present in another tile, 
but then, it is possible to obtain their exact values by communicating 
with one of the peer processes, so that not a satisfactory explanation 
either.

> That is using, say 64 processors, you will always get the same answer
> on a particular machine.

Yes.

> But if you use 32 processors, the domain decomposition and 
> subsequent integration produces slight differences

It is the causal link between decomposition and the differences that I'm 
puzzled about. The after-effects are clear.

I think the kind of answer I am looking for is a white-box example of 2 
different ways of decomposition on some domain inevitably producing 2 
different results in the subsequent computation. If it is indeed just the 
initial decomposition which matters, then I suppose that already the 
initial integration step would yield different results.

> I have seen this issue before and even in small time period 
> integrations. The key is the number of processors. I would imagine 
> this is also compiler dependent (i.e. Exactly how functions are 
> calculated and at what precision).

It could be, but I'm more interested in the case where we hold every 
parameter except the number of MPI processes constant.

Regards,
Jan Ploski

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