On 16 Sep 2002, Nathan Parker wrote:
> I'm using MPI::Request::Waitany() in a while loop to collect data from 4
> other nodes. After data from all nodes has been received, the data is
> processed, and the Waitany() loop is restarted. I expected that most
> this process will sit idle most of the time, waiting for data. Instead,
> it appears to spin on the CPU (99% utilization). Is this a known bug?
> If I do "mpirun -lamd," the problem goes away, but lamd then uses 15% of
> the cpu.
This is not a bug - it's a feature! Seriously, you might want to read the
README file included with LAM distributions. The usysv transport "spins"
fairly tightly when it is waiting for anything that might come from a
local node (ie, over the shared memory channel). If you use the sysv
transport, you should see significantly less CPU utilization in these
cases.
Note, of course, that LAM *has* to poll in ANY_SOURCE or Waitany cases
when shared memory is in use. LAM has to look at a series of socket
connections and a shared memory descriptor to check for available data.
The only way to do this is polling. So, even with the sysv rpi, you are
going to see a non-trivial amount of CPU utilization going to this
polling. However, it should be much less than 99% of the CPU.
Brian
--
Brian Barrett
LAM/MPI developer and all around nice guy
Have a LAM/MPI day: http://www.lam-mpi.org/
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