[FLASH-USERS] Oddness in results when using uneven number of parallel processors

[Seyit Hocuk]

Hi Nathan,

No, I understand what Mpi_allreduce does and am also trying to work out 
mpi_allgather (for blockList). Either I have to use these to make the 
block loop complete again, or find another way to write the code without 
making use of (especially) blockList. Since it was already there, it was 
so easy to make use of them, however, I never had considered the 
parallellisation of the code.

Suggestions on how to make a complete block loop within a partial block 
loop are always welcome of course. :)

Thanks Nathan,
Seyit




Nathan C. Hearn wrote:
> Hi Seyit,
>
>     MPI_allReduce collects a value from each MPI process, applies an 
> operation (like summation) on the set of values, and reports the 
> result back to all processes.  If it looks like you will  be writing 
> your own code to communicate between processes, I  would recommend 
> getting a copy of one of the "Using MPI" books from MIT Press -- they 
> provide a good tutorial for MPI and explanations of the various MPI 
> functions.
>
>
> - Nathan
>
>
> On Sep 15, 2009, at 4:54, Seyit Hocuk <seyit at astro.rug.nl> wrote:
>
>>
>> Hi Anshu,
>>
>> I realise now that my second block loop is also reduced by the number 
>> of processors which may be the problem. That is, blockCount and 
>> blockList are divided(reduced). The first block loop can be 
>> parallelized, however, I want that each block/zone to do the complete 
>> block loop again, not a part of it. The fact that I noticed no 
>> problem when I used 2^x procs, must come from the way that I do 
>> things in the code and that with 2^x procs the box is divided 
>> symmetricly. Like you said, it was just a coincidence.
>>
>> Im trying to understand what MPI_allreduce function does. Does it 
>> exactly do what I want, i.e., make blockList and blockCount 
>> complete/undivided again?
>>
>> Thanks,
>> Seyit
>>