[mesa-users] OpenMP vs. MPI

[Ryan O'Leary]

Hi Josiah,

Did you turn hyperthreading off in the bios when doing the test on <=4
cores?  It depends on the code, compiler, problem, cpu, etc., but
typically that is the more practical test for checking if
hyperthreading is faster or slower.  I have do not recall my past
tests with MESA, but I have had codes that show a 10-20% slowdown with
hyperthreading turned on, but typically this manifested itself only by
changing the bios setting*.  That is, you might find MESA runs faster
on four cores with HT turned off than with 4 or 8 cores with HT turned
on in the bios.

Most computing clusters that I have used have hyperthreading turned
off, so you typically want to use all of the cores unless memory
becomes an issue.

*It might be possible to do this test without changing the bios with
GOMP_CPU_AFFINITY or turning cpu cores offline e.g, echo 0 >
/sys/devices/system/cpu/cpuX/online I have not tried this and can't at
the moment.

-Ryan.


On Thu, Sep 25, 2014 at 11:12 AM, Josiah Schwab <jwschwab at berkeley.edu> wrote:
> Hi All,
>
>> In reality, you may wish to compare the performance to see just how much
>> slower the cluster cores are.  In my case, my desktop is only about 10-30%
>> faster, but I have 8 cores (4 physical cores, hyperthreaded), compared to
>> being able to muster many hundreds on the cluster when necessary.
>
> Speaking of hyperthreading (HT), Bill and I had a conversation at the
> MESA Summer school which I have long meant to follow up on.
>
> Should you set OMP_NUM_THREADS to the number of physical cores or the
> number of hyperthreaded cores?  If a program is doing an excellent job
> of keeping the CPU busy, you don't expect HT to get you any benefit.
>
> Years ago, I did some quick benchmarks and determined HT helped enough
> that I've been setting OMP_NUM_THREADS to the number of virtual cores.
>
> During the summer school I did a simple test with the latest MESA
> version.  Here's a plot of how long the 1M_pre_ms_to_WD test case took
> on my 4 physical core (8 virtual w/ HT) desktop.
>
>
>
> So on this problem, on a 4 year old Xeon processor, hyperthreading sped
> things up by about 20%.
>
> There's no reason to expect that this is a problem-independent,
> machine-independent result, though it seems clear HT won't ever buy you
> order unity speed improvements.  Users may want to do simple benchmarks
> and see what is working best for them.
>
> Josiah
>
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