Don't forget the additional costs of double the number of uniprocessor boxes in terms of power, air conditioning, and maintenance. All of these make CPU density more important again. On small processor farms, this might not be so important, but once you get into the realm of thousands of CPUs it makes an enormous difference. Tim.
True. But to extend my original example, the Dual Athlon boxes take almost as much juice as 2 single boxes, and the MTBF on contemporary PC parts is such that you wouldn't expect significantly more failures on double the number of parts. This snippet is from http://www.theinquirer.net/11050104.htm...
"That 460W power budget results from the use of two 1.8GHz Athlon Processors (180 Watts), motherboard with 4GB memory (75 Watts), AGP Pro Card (50 Watts), 2 SCSI HDD (75 Watts), CD-ROM & Floppy (20 Watts), 3 PCI Cards (45W) plus keyboard, mouse and USB devices (10W)."
Sure, us clusterers will be able to build this a little more efficiently but you see my point. If your goal is a good price / performance ratio, then I maintain the SMP solutions haven't broken in yet, and they are the leading candidate for increasing density. Because it is difficult to calculate performance before you buy the equipment, I have just been using overall budget to overall cycles (or some other aggregate number) as a predictive ratio. So you don't cross your price break point until you are building a very large cluster.