[plug] benefits of SMP on an LTSP server

[Craig Ringer]

Hi folks

I've just run into something today that I think is worth mentioning, 
given that there's been the odd bit of discussion about terminal servers 
and such here.

Our dual Xeon LTSP server (it serves the boot images, NFS root, and 
displays) was running at a load average of between 1.2 and 1.5 for most 
of today - unusually high. I didn't even notice until the later 
afternoon, since I (for once) wasn't running gkrellm. A user process had 
gone runaway, attempting to start another process - resulting in about 
200 forks/second and totally maxing out one CPU. I was able to quietly 
kill it after the users had finished for the day. It promptly dropped 
back to it's usual 0.1 (it doesn't go above 0.2 much even when all users 
are quite active and there's a fair bit of mail flowing).

I would've had to leap to the rescue if this was a single processor box 
(trust me - I had to do it a few times on the 1.5GHz Athlon we used 
during testing)... which would've sucked if I wasn't here at the time.

The point is that if you're building any server for LTSP, no matter how 
small the user base, I think you're likely to benefit a LOT from going 
to at least a basic SMP system. It doesn't cost that much once you 
factor in other hardware costs like RAID, and I've just had it bought 
home to me today how much it benefits the system's ability to handle 
high load and runaway processes. The responsiveness improvement is 
amazing, too.

So ... if anybody has been considering whether dual processor is really 
worthwhile in their servers, you have my strong advice to go for it.

Craig Ringer