Konstantinos,
I am afraid there is some confusion here.
the plm_rsh_no_tree_spawn is only used at startup time (e.g. when remote
launching one orted daemon per node but the one running mpirun).
there is zero impact on the performances of MPI communications such as
MPI_Bcast()
the coll/tuned module select the broadcast algorithm based on
communicator and message sizes.
you can manually force that with
mpirun --mca coll_tuned_use_dynamic_rules true --mca
coll_tuned_bcast_algo <algo> ./my_test
where <algo> is the algo number as described by ompi_info --all
MCA coll tuned: parameter "coll_tuned_bcast_algorithm"
(current value: "ignore", data source: default, level: 5 tuner/detail,
type: int)
Which bcast algorithm is used. Can be locked
down to choice of: 0 ignore, 1 basic linear, 2 chain, 3: pipeline, 4:
split binary tree, 5: binary tree, 6: binomial tree.
Valid values: 0:"ignore", 1:"basic_linear",
2:"chain", 3:"pipeline", 4:"split_binary_tree", 5:"binary_tree",
6:"binomial"
for some specific communicator and message sizes, you might experience
better performances.
you also have the option to write your own rules (e.g. which algo should
be used based on communicator and message sizes) if you are not happy
with the default rules.
(that would be with the coll_tuned_dynamic_rules_filename MCA option)
note coll/tuned does not take the topology (e.g. inter vs intra node
communications) into consideration when choosing the algorithm.
Cheers,
Gilles
On 10/17/2017 3:30 PM, Konstantinos Konstantinidis wrote:
I have implemented some algorithms in C++ which are greatly affected
by shuffling time among nodes which is done by some broadcast calls.
Up to now, I have been testing them by running something like
mpirun -mca btl ^openib -mca plm_rsh_no_tree_spawn 1 ./my_test
which I think make MPI_Bcast to work serially. Now, I want to improve
the communication time so I have configured the appropriate SSH access
from every node to every other node and I have enabled the binary tree
implementation of Open MPI collective calls by running
mpirun -mca btl ^openib ./my_test
My problem is that throughout various experiments with files of
different sizes, I realized that there is no improvement in terms of
transmission time even though theoretically I would expect a gain of
approximately (log(k))/(k-1) where k is the size of the group that the
communication takes place within.
I compile the code with
mpic++ my_test.cc -o my_test
and all of the experiments are done on Amazon EC2 r3.large or m3.large
machines. I have also set different values of rate limits to avoid
bursty behavior of Amazon's EC2 transmission rate. The Open MPI I have
installed is described on the txt I have attached after running ompi_info.
What can be wrong here?
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