Re: LAM: LAM-MPI task placement on an SMP system

jsquyrescisco.com 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.
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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