<div>Mark,</div>
<div>Thanks, that led me (with a bit of wandering) to e.g. <a href="http://www.cs.virginia.edu/stream/top20/Balance.html">http://www.cs.virginia.edu/stream/top20/Balance.html</a>.</div>
<div>My immediate concern is for an app that is worse than embarassingly parallel; it can't (currently) trade memory for time, and can't really use any memory or network effectively, by the list's standards. Basically I want a zillion CPUs and they can communicate by crayon on postcard. That's not practical, but my initial valuator is just GHz/$.
</div>
<div>I care about the memory sharing and message passing efficiency issues only in that I want to smarten up my app to take advantage of other economies.</div>
<div>Peter<br><br> </div>
<div><span class="gmail_quote">On 3/8/07, <b class="gmail_sendername">Mark Hahn</b> <<a href="mailto:hahn@mcmaster.ca">hahn@mcmaster.ca</a>> wrote:</span>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">> Great thanks. That was clear and the takeaway is that I should pay attention<br>> to the number of memory channels per core (which may be less than
1.0)<br><br>I think the takeaway is a bit more acute: if your code is cache-friendly,<br>simply pay attention to cores * clock * flops/cycle.<br><br>otherwise (ie, when your models are large), pay attention to the "balance"
<br>between observed memory bandwidth and peak flops.<br><br>the stream benchmark is a great way to do this, and has traditionally<br>promulgated the "balance" argument. here's an example:<br><br><a href="http://www.cs.virginia.edu/stream/stream_mail/2007/0001.html">
http://www.cs.virginia.edu/stream/stream_mail/2007/0001.html</a><br><br>basically, 13 GB/s for a 2x2 opteron/2.8 system (peak flops would<br>be 2*2*2*2.8=22.4, so you need 1.7 flops per byte to be happy.<br><br>I don't have a report handy for core2, but iirc, people report hitting
<br>a wall of around 9 GB/s for any dual-FSB core2 system. assuming dual-core<br>parts like the paper, peak theoretical flops is 37 GFlops, for a balance<br>of just over 4. that ratio should really be called "imbalance" ;)
<br>quad-core would be worse, of course.<br></blockquote></div><br>
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