[mpich-discuss] Modifying ssh calls for MPICH2

[Jed Brown]

On Tue, Jan 17, 2012 at 16:19, Reuti <reuti at staff.uni-marburg.de> wrote:

> >   printf("Hello World from Node %d.\n", node);
> >       for(j=0; j <= 100000; j++)
> >           for(i=0;i <= 100000; i++)
> >               f=i*2.718281828*i+i+i*3.141592654;
> >
> > Well this is an odd spin loop because the compiler will happily remove
> it for you (note that MPICH2's mpicc wrapper very frequently has
> optimization flags in it). I would suggest sleep(3) instead, or, if you
> must, force the loop to actually be executed (e.g. seed the loop with the
> rank and use the result).
>
> Not for my gcc version and mpich2-1.4, I can see a  clear load.


Add -O1 and it will go away. *Many* people build MPICH2 with some
optimization in CFLAGS (hoping to build MPICH2 with optimization) in which
case those flags also get written into the wrappers. I see this from users
wondering why their debugging session isn't working as expected every few
weeks, so it's definitely a common situation.


> Of course, you can include the rank to modify it, but a clever compiler
> should then also detect that f is never used again.
>

That's why my comment included "and use the result". ;-)
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