Saturday, 21 March 2009

Two more nodes

I've added two additional nodes to my cluster. The new nodes have Asus rather than Abit motherboards and, unfortunately, CentOS 5.2 doesn't support the Nvidia nForce 630a chipset. So I was left with a choice of either trying to build the necessary drivers, or finding an alternative. In the end I decided to give CAOS Linux another try. There seems to be no link on the website anymore to download a VNFS image, but if you Google you can find the FTP site easily enough. I used version 1.0 rather than the RC1 version I had tried previously. It is still a "bare bones" distribution - it doesn't even come with vi - but it worked. I even managed to get Python installed from source using "make DESTDIR", which didn't happen before. There are a few quirks: I don't seem to be able to specify which eth device gets which MAC address, which means I can't use it on all the nodes, but for the new nodes it's fine.

I wasn't going to go on about performance results anymore, they are just a bit of fun after all. However, having added the two new nodes they had to be tested :-) The new nodes have Athlon 64 X2 5200+ (2700MHz) processors instead of 4200 (2200MHz) processors, and 4Gb RAM instead of 2Gb RAM. So a big increase in performance right? Er... no. There is an increase in performance - from 13.3 to 14.25 Gflops on a two node test, but this modest increase in output is offset by a big drop in computational efficiency: down from 75.6% to 66%. What's going on?

On the original nodes, with 2Gb RAM, pretty much all I had to do to find the maximum performance was keep increasing the problem size (N) until xhpl ran out of memory and crashed. The bigger the problem size, the better the measured performance. With the nodes with 4Gb RAM, the measured performance peaks before (way before) we run out of memory. What we see is an initial steep increase in performance as the problem size increases, leading to a peak, and then a slow decline.

It's as if the processors can't make effective use of all that extra memory. Or perhaps we've hit the limit of what the bonded Gigabit Ethernet interconnect can deliver.

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