Namu,
Hyperthreads aren't real cores, they're really just another hardware
thread (two per physical core). You have two CPUs with 6 physical cores
each. If you start 2 simulations, each with 6 MPI processes running,
your 12 physical cores will each be running at 100%. Adding another
simulation with another 6 MPI processes will oversubscribe your physical
cores (you're asking for 150%), which is why you're still seeing 12
processors at 100%, and everything else very low. Your physical cores
are switching hardware threads, but each core can't go any faster.
Hyperthreads only help when your software doesn't load a core to 100%.
Then the other hyperthread on that core can switch in and use leftover
capacity. Hardware thread switching is much faster than software thread
switching, which is why it's there.
Most simulation software will load cores to 100% (even if it doesn't use
that 100% wisely, which is a whole other flame war) and hyperthreading
doesn't help you.
Damien
On 2015-04-10 2:22 PM, namu patel wrote:
Hello All,
I am using OpenMPI 1.8.4 on my workstation with 2 CPUs, each with 12
processors (6 with hyper-threading). When I run simulations using
mpiexec, I'm noticing a strange performance issue. If I run two
simulations, each with 6 processors, then everything is fine and 12
processors are under 100% load. When I start a 3rd simulation with 6
processors, I notice throttling in all 3 simulations and only 12
processors are at 100%, the rest are below 10%. My guess is that
somehow the processes from the 3rd simulations are doubling onto the
already busy processors. How can I be certain that this is the case?
Thanks,
namu
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