[Beowulf] Using Autoparallel compilers or Multi-Threaded libraries with MPI

Toon Knapen toon.knapen at gmail.com
Sun Dec 2 09:51:53 EST 2007


Mark Hahn wrote:
>> IMHO the hybris approach (MPI+threads) is interesting in case every
>> MPI-process has lots of local data.
> 
> yes.  but does this happen a lot?  the appealing case would be threads 
> that make lots of heavy use of some large data, _but_
> without needing synchronization/locking.  once you need locking
> among the threads, message passing starts to catch up.

Direct solvers (for Finite Elements for instance) need a lot of data. 
Additionally distributing the matrix generate interfaces (between the 
different submatrices) which are hard to solve. In such situation, one 
tries to minimize the number of interfaces (by having one submatrix per 
MPI-process) and speed up the solving of each submatrix using threads.

Finance is another example. Financial applications need to evaluate a 
large number of open positions based on the simulated, current or past 
market-data. There are many dependencies between all the different data 
which makes that it is hard to decompose the data in largely independent 
chunks.



> 
>> latter is simpler because it only requires MPI-parallelism but if the 
>> code
>> is memory-bound and every mpi-process has much of the same data, it 
>> will be
>> better to share this common data with all processes on the same cpu 
>> and thus
>> use threads intra-node.
> 
> what kind of applications behave like that?  I agree that if your MPI 
> app is keeping huge amounts of (static) data replicated in each rank,
> you should rethink your design.
> 


See above.

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