memtest86, the quintessential memory tester app, requires physical console access and a reboot. In the meantime, I've run some tests on geoffrin from within Linux with a program called 'memtester' which was recommended to me:

http://www.qcc.ca/~charlesc/software/memtester/

It only seemed willing to lock about 2 gigs at a time, so I ran two memtester processes, which between them covered most of the available memory. I didn't tell it to test _all_ memory as I was afraid Linux might start killing sshd processes for daring to ask for more memory to send me the results. ;)

Well, it's spewing out errors right and left; 19 failures caught between all the tests on the first pass. At this point we can't be sure if these are actual physical RAM defects, or artifacts of kernel bugs or other problems (or even bugs in memtester, which might not have been thoroughly tested on amd64), but it's not a good sign.

I'll post the logs once it's run a while longer.

-- brion vibber (brion @ pobox.com)