[mpich2-dev] question about communication optimization on shared memory

[Shigang Li]

Hi,Darius,
Thank you and this really makes sense. I still have one question. Is it
possible for a MPICH programmer to know which configuration is using, e.g.
posix shared memory,  sys V shared memory or disabled?

Best Regards,
Shigang.

On Mon, Oct 15, 2012 at 10:18 AM, Darius Buntinas <buntinas at mcs.anl.gov>wrote:

>
> MPICH configure will try to use posix shared memory (i.e., mmapping a
> file), and if that fails it will try to use sysv shared memory (i.e.,
> shmat).  If that fails, I think configure aborts (but it may just disable
> shared memory, I'm not sure).  Sysv shared memory typically has tight
> limits on the amount of shared memory that can be allocated per process and
> per node, so you don't really want to use that.
>
> At init time, the processes query the process manager for the topology
> (i.e., which processes are on which nodes), and based on that decide which
> pairs of processes should communicate using shared memory and which should
> use the network.
>
> Currently collectives are implemented using point-to-point messages, so
> some messages will go over shared memory and some over the network.  The
> communication patterns used by the collectives are optimized to take the
> topology into account.  MPICH does have an interface to allow implementors
> to override the default collective algorithms, such as using shared memory,
> but we haven't done this yet.
>
> -d
>
>
> On Oct 14, 2012, at 8:31 PM, Shigang Li wrote:
>
> > Dear Sir or Madam,
> >
> > I'm a student from UIUC, and have some questions about communication
> optimizations on shared memory in state-of-art mpich2.
> >
> > 1) When configure mpich2, does it support shared memory optimization in
> default? If so, can it select the communication pattern automatically
> according to hardware architecture? For example in a SMPs cluster, using
> shared memory optimization within a node and using socket cross nodes?
> >
> > 2) What about collective communication on shared memory? Is it
> implemented on top of the optimized point to point communication or use
> other mechanisms?
> >
> > Best Regards,
> > Shigang.
>
>
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