Hello,
I have a question concerning the MPI_BLOCK variable. I am running vasp.4.6.35 on a cluster where each blade consists of 2 quad-core Opteron (Barcelona) CPUs. Blades are connected with Infiniband. When running on 2 or 4 blades (16 or 32 cores), increasing the value of MPI_BLOCK seems to help, e.g. on 4 blades using MPI_BLOCK=8000 can increase performance by about 15 %. Is it safe to use such and even higher values for MPI_BLOCK ? Does anybody have experience with this ?
Many thanks in advance.
Best regards
Roman Martonak
--
Assoc. Prof. Roman Martonak
Department of Experimental Physics
Faculty of Mathematics, Physics and Informatics
Comenius University
Mlynska dolina F2
842 48 Bratislava phone: +421 2 60295467
Slovakia e-mail: martonak at fmph.uniba.sk
MPI_BLOCK
Moderators: Global Moderator, Moderator
-
job
- Jr. Member
- Posts: 55
- Joined: Tue Aug 16, 2005 7:44 am
MPI_BLOCK
I've been using MPI_BLOCK=60000, which was recommended by Cray. To be honest, I don't really see the point of this parameter; In all cases I've seen MPI performance is better with bigger blocks rather than multiple transfers with smaller blocks.
Another thing you can try is to use the use_collective preprocessor flag. That enables use of MPI collective operations rather than emulating them using MPI point-to-point operations. Depending on the network and MPI implementation this might be better.
Another thing you can try is to use the use_collective preprocessor flag. That enables use of MPI collective operations rather than emulating them using MPI point-to-point operations. Depending on the network and MPI implementation this might be better.
Last edited by job on Sat Feb 14, 2009 8:09 am, edited 1 time in total.