On 13 Sep 2010, at 12:20, Georges Markomanolis wrote: > Dear all, > > Hi again, after using MPI_Ssend seems to be what I was looking for but I > would like to know more about MPI_Send. > > For example sending 1 byte with MPI_Send it takes 8.69 microsec but with > MPI_Ssend it takes 152.9 microsec. I understand the difference but it seems > that from one message's size and after their difference is not so big like > trying for 518400 bytes where it needs 3515.78 microsec with MPI_Send and > 3584.1 microsec with MPI_Ssend.
It sounds like you are measuring send overhead rather than latency, in fact as far as I know it's impossible to measure the send latency as you have no way of being able to know when to 'stop the clock', this is why ping-pong latency is always quoted. I suspect the underlying latency of the two sends is very similar to each other in practice. > So has is there any rule to figure out (of course it depends on the hardware) > the threshold that after this size the difference between the timings of > MPI_Send and MPI_Send is not so big or at least how to find it for my > hardware? Yes there is but I'm not familiar enough with OMPI to be able to tell you, I'm sure somebody can though. If my suspicion above is correct I have doubt knowing what this value is would help you at all though in terms of application performance. Ashley. -- Ashley Pittman, Bath, UK. Padb - A parallel job inspection tool for cluster computing http://padb.pittman.org.uk
