[Beowulf] interconnect and compiler ?

Michael H. Frese Michael.Frese at NumerEx-LLC.com
Fri Jan 30 10:00:06 EST 2009


At 07:07 AM 1/30/2009, Michael H. Frese wrote:
>For 64 bit floating point that's about 2000 numbers.

Oops.  Of course, I should have said "500 numbers."


Mike

At 07:07 AM 1/30/2009, Michael H. Frese wrote:
>At 07:05 PM 1/29/2009, Greg Lindahl wrote:
>>On Thu, Jan 29, 2009 at 07:22:10PM -0500, Mark Hahn wrote:
>> > I'll bite: suppose I run large MPI jobs (say, 1k rank)
>> > and have 8 cores/node and 1 nic/node.  under what circumstances
>> > would a node be primarily worried about message rate, rather than latency?
>>
>>Well, let's say that you're doing a stencil computation on a 3D grid,
>>diagonals included. Then each cycle each core needs to send to 26
>>neighbors, and then receive from 26 neighbors. Even if you have
>>fat-ish nodes (~ 8 cores) and a clever layout of the cores onto the 3D
>>grid, that's a lot of off-node messages in a row. And that's message
>>rate.
>
>Johnn Adams said "Facts are stubborn things," and there just aren't 
>enough of them in your example to determine whether bandwidth or 
>latency dominates communication time.  I have a 3-d code that does 
>quite a lot of that.  Assuming each processor has a 100 x 100 x 100 
>grid, the communication to the six face neighbors of 10,000 elements 
>may dominate.  If the basic grid dimension is 10 instead of 100, the 
>communication to the 12 edge neighbors and the 8 corner neighbors 
>may take three times as those 6 messages.
>
>The critical thing from the hardware is the size of the message that 
>requires twice the latency time to transmit.  For GigE at 30 
>microseconds, and this is 30 kilobits.  For 64 bit floating point 
>that's about 2000 numbers.  For messages smaller than that the 
>latency dominates, while for messages longer than that the bandwidth does.
>
>The latency dominates in time-stepping finite-difference codes where 
>you need to do a time step in a few seconds.  For steady state 
>finite-difference codes where you can spend an hour on a single 
>solution, the bandwidth determines how big a problem you can do.
>
>As the list is wont to say, YMMV.
>
>
>Mike
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