[Beowulf] Performance characterising a HPC application

[Christian Bell]

On Wed, 04 Apr 2007, Ashley Pittman wrote:

> GasNet does however get extra credit for having a asynchronous
> collective, namely barrier.  Unfortunately when you read the spec it's
> actually a special case asynchronous reduce which is almost impossible
> to optimise anything like as well as barrier which is a shame.

GASNet's named split-phase barriers cater to a language-level feature
in UPC to generate mismatches if the name doesn't match, which can be
a useful debugging tool.  The same primitive has a flags option to
make it unnamed, which probably has the properties you want (being
split-phase without the need to reduce a value across processors).
This one could be implemented over a faster split-phase notify/wait
if the interconnect can offer one (although yes, this would require
bending the rules somewhat at the language-level since one local proc
doing what it thinks is an unnamed barrier would have to assume that
all of its peers also used an unnamed barrier).

    . . christian


-- 
christian.bell at qlogic.com
(QLogic SIG, formerly Pathscale)