When you run multiple processes in a processor and they want to
communicate between them, the only way or the only form that they can
communicate or do some computation is with the same processor, for that
reason take more time than in many processors.
for example, I have 4 processes and only one processor, they have to do
some computation and some communication (reduce the information to process
0). The processes can not communicate betwem them after do their
computation; so, each process (p0, p1, p2, p3) is executed in the
processor. The processor can not execute 2 processes at same time, it can
do by time slice. For that reason, the last processes that are in the
queue have to wait until the first processes free the processor.
This happens in the communications, the processes need the processor to
send or receive information, if you are going to use MPI_Reduce, and the
process p0 is the last to use the processor, the first processes should
send the information to the process p0 but the process 0 should not ready.
now, what happen if you have four processors and four processes? There is
one processor for each process; then, the computation is executed at same
time, they don't share their processors, and they don't wait to use a
processor. When they finish their computation, they start to send the
information to other. Using MPI_Reduce, they start to send the
information to one process (for example p0) that belong to a processor.
In this case the the process that is receiving the information is ready to
receive the data from the others (some times is not ready but depend on
many characteristics);and the others are sending the information at same
time (depend on hardware configuration).
This one reason from many that the communication and computation time in
an environment of many processes and processors is less than many
processes in one processor.
This is good link:
http://www.cs.rit.edu/~ncs/parallel.html#books
thanks
> Hi,
>
> I run them on a cluster of machines when I say processes assigned to
> multiple machines. It is obvious that the computation cost will be
> increased if multiple processes running on the same host. But why the
> communication cost is also increased a lot? Could you please give a
> detail explaination?
>
> thanks
>
> ----- Original Message -----
> From: Roberto Pasianot <pasianot_at_[hidden]>
> Date: Monday, March 29, 2004 1:25 pm
> Subject: Re: LAM: is that possible to reduce the communication cost by
> assigning processes on the same node
>
>>
>> Hi,
>>
>> This might be a stupid question but anyway : are you running on
>> a multiprocessor ?. Otherwise what you get is exactly what should be
>> expected.
>>
>> Bye. Roberto.
>>
>>
>> On Mon, 29 Mar 2004, Ming Wu wrote:
>>
>> > Hi,
>> >
>> > I assigned multiple processes on one machine instead of several
>> machines.
>> > In this way, I expect the communication cost will be reduced
>> compared> with assigning them to several machines. However, the
>> result is weird.
>> > both computation cost and communication cost are increased sharply,
>> especially MPI_Reduce, MPI_Sendrecv. It seems that the
>> underlying
>> > implementation of lam_mpi doesn't favor multiple processes on
>> the same
>> > host.
>> >
>> > Your help will be greatly appreciated.
>> >
>> > thanks
>> >
>> > _______________________________________________
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>> >
>>
>> _______________________________________________
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>
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