I have a dual Xeon 3.2GHz XDMCP server serving out about 50 Gnome desktops to thin clients. I currently am running 32 bit with 4GB of memory. Performance is fine, normally, and the machine runs with about 3GB in swap. However, if a user app suddenly sucks up the maximum allowed rss and virtual or 256MB/process as specified by ulimit, the resulting swap drags the machine down to the point that people think it is locked up.
I have another 4GB of memory on order. But I suspect that part of the problem may be running out of memory in zone-normal, which I know will cause swap storms. (An oops in the log file suggests that is where memory was tight.) I had low memory protection set to 150MB, and have increased that to 256MB. (I had to increase it to at least 135MB previously, else dd if=/dev/zero of=/tempfile.tmp would start a swap storm that would bring the machine to its knees.)
More users will be added to this machine, and I am worried about having an ever higher percentage of this (16GB maximum) machine in zone high.
So I am considering moving it from 32 bit CentOS4 to 64 bit CentOS5 or 64 bit Fedora 7. But a cursory test of the 32 and 64 bit versions of firefox suggest to me that, just opening them up and going to google.com, the 64 bit version uses about 50% more memory than the 32 bit version.
Could someone please comment on how much of an additional memory hit I might expect? The VM risks and benefits that I might be taking? Really, *any* commentary about this scenario would be appreciated.
Thanks, Steve Bergman