LAM SMP performance

[Patrick Geoffray]

On Fri, 8 Dec 2000, Josip Loncaric wrote:

> I believe that you are thinking of sysv (semaphores).  LAM compiled with
> usysv uses spinlocks, and the peak 266 Mbyte/s bandwidth is reached for
> 8KB cache-to-cache copies.  Memory gets involved only for larger message
> sizes, and then the bandwidth drops to 127 Mbyte/s.  See my raw data

Ok, 2 us with MPICH and a longer critical path would be 1 us with LAM,
it makes sense.

For the bandwidth measurement, it's a good occasion to talk about a good
way to measure SMP bandwidth : some people do not accept cache-to-cache
performance values because they do not show the real memory bus capacity,
some others do. 

I believe that it gives a more accurate result as an application usually
write the message to send just before to send it, so the data is in the
sender cache. On another hand, the message can be asynchronous and the
cache can be trashed on the receiving side before the user application
uses the payload.

What do you think ?

Patrick Geoffray