Re: [OMPI users] How can I measure synchronization time of MPI_Bcast()

2017-10-23 Thread Gilles Gouaillardet
Konstantinos, i previously suggested you use the profiler interface (aka PMPI) specified in the MPI standard. An example is available at http://mpi-forum.org/docs/mpi-3.1/mpi31-report/node363.htm#Node363 The pro is you simply need to rewrite MPI_Bcast() vs add some code around each MPI_Bc

Re: [OMPI users] How can I measure synchronization time of MPI_Bcast()

2017-10-23 Thread Konstantinos Konstantinidis
I do not completely understand whether that involves changing some MPI code. I have no prior experience with that. But if I get the idea something like this could potentially work (assume that comm is the communicator of the groups that communicates at each iteration): *clock_t total_time = cloc

Re: [OMPI users] How can I measure synchronization time of MPI_Bcast()

2017-10-23 Thread Barrett, Brian via users
Gilles suggested your best next course of action; time the MPI_Bcast and MPI_Barrier calls and see if there’s a non-linear scaling effect as you increase group size. You mention that you’re using m3.large instances; while this isn’t the list for in-depth discussion about EC2 instances (the AWS

Re: [OMPI users] How can I measure synchronization time of MPI_Bcast()

2017-10-23 Thread Gilles Gouaillardet
Konstantions, A simple way is to rewrite MPI_Bcast() and insert timer and PMPI_Barrier() before invoking the real PMPI_Bcast(). time spent in PMPI_Barrier() can be seen as time NOT spent on actual data transmission, and since all tasks are synchronized upon exit, time spent in PMPI_Bcast() can be

Re: [OMPI users] How can I measure synchronization time of MPI_Bcast()

2017-10-23 Thread Konstantinos Konstantinidis
In any case, do you think that the time NOT spent on actual data transmission can impact the total time of the broadcast especially when there are so many groups that communicate (please refer to the numbers I gave before if you want to get an idea). Also, is there any way to quantify this impact

Re: [OMPI users] How can I measure synchronization time of MPI_Bcast()

2017-10-20 Thread Jeff Hammond
Broadcast is collective but not necessarily synchronous in the sense you imply. If you broadcast message size under the eager limit, the root may return before any non-root processes enter the function. Data transfer may happen prior to processes entering the function. Only rendezvous forces synchr

[OMPI users] How can I measure synchronization time of MPI_Bcast()

2017-10-20 Thread Konstantinos Konstantinidis
Hi, I am running some tests on Amazon EC2 and they require a lot of communication among m3.large instances. I would like to give you an idea of what kind of communication takes place. There are 40 m3.large instances. Now, 28672 groups of 5 instances are formed in a specific manner (let's skip the