[mpich-discuss] MPI_Allgatherv complexity

Rajeev Thakur thakur at mcs.anl.gov
Mon Nov 2 18:08:48 CST 2009

You can see the algorithms used for different messages sizes in MPICH2
, but Cray could be using a modified version. What is the total message
size being gathered and how many processes are you running on?


From: mpich-discuss-bounces at mcs.anl.gov
[mailto:mpich-discuss-bounces at mcs.anl.gov] On Behalf Of Justin Luitjens
Sent: Monday, November 02, 2009 5:59 PM
To: mpich-discuss at mcs.anl.gov
Subject: [mpich-discuss] MPI_Allgatherv complexity


I'm wondering what is the parallel complexity of MPI_Allgatherv.  I
would suspect in the worst case it would be something like N log P but
from the timings i'm collecting this doesn't seem to be the case.  I'm
running tests on Kraken
) and it looks like it might be something like P log P.


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