On Friday 02 February 2007 14:36, Tim Prince wrote:
> jsquyres_at_[hidden] wrote:
> > On Feb 2, 2007, at 6:02 AM, Davide Cesari wrote:
> >> If the second holds, then your request is pointless, in my knowledge,
> >> LAM does not make anything particular to attach processes to CPUs in a
> >> SMP system, it just starts as many processes as requested, then it
> >> is up
> >> to the operating system to balance them among the available
> >> processors,
> >
> > This is correct. LAM simply starts up the Right number of processes
> > and does not bind them to any particular CPUs.
> >
> >> this is the essence of Symmetric Multi Processing; AFAIK, there is no
> >> such a concept (and no need too) of starting a process on a particular
> >> CPU in a plain SMP system.
> >>
> >> If you are using the Linux kernel, then recent versions should have a
> >> tunable scheduler which tries to attach processes to CPUs as much as
> >> possible (the so-called CPU affinity) to improve performance on
> >> SMP, but
> >> it is not guaranteed either that a given process will always run on
> >> the
> >> same CPU.
> >> If you have a NUMA (Non Uniform Memory Access) system, then things
> >> are
> >> more complex, but I have no direct experience of that.
>
> Most recent linux versions include a useful taskset command:
> mpirun -np 8 taskset -c 8-15 ./a.out
> which should be fairly effective at placing your processes on that group
> of processors within each node. The purpose of using taskset usually is
> to improve efficiency through cache or NUMA memory affinity, but it
> could be used to do what OP appears to be requesting.
Sorry, I must be missing something, but shouldn't this be something the OS
does? I think I recall that last time I recompiled a Linux kernel (a 2.6 one,
for AMD Opteron machine, about 6 months ago?) there was stuff related to
NUMA. I'd feel better if someone doing kernel development takes care of this
rather than having this responsibility myself :-).
Best,
R.
> The latest schedulers, included in RHEL4_U4 and CentOS 4.4, generally
> accomplish efficient scheduling without requiring taskset, but that
> doesn't appear to be what OP is asking.
> _______________________________________________
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