On 2/5/07, Brian W. Barrett <brbarret_at_[hidden]> wrote:
> On Fri, 2 Feb 2007, Ramon Diaz-Uriarte wrote:
>
> > 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 :-).
>
> You're only missing that Computers Suck (IMHO) :). There is an awful lot
> of code in the Linux kernel to try to make NUMA machines more tolerable.
> But it has it's limitations -- it's designed to provide the best overall
> machine "responsiveness", not the lowest latency for 2 of the 80 processes
> running on the machine. MPI apps tend to want the second one -- a very
> few processes should be privledged over all others.
>
Brian, thanks for your comments. I was obviously missing something
(the Computers Suck or a related concept, I guess :-); maybe I should
play around with taskset.
Best,
R.
>
> Brian
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>
--
Ramon Diaz-Uriarte
Statistical Computing Team
Structural Biology and Biocomputing Programme
Spanish National Cancer Centre (CNIO)
http://ligarto.org/rdiaz
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