Comments on: Xen and Swap

By: etbe

etbe — Fri, 23 May 2008 10:24:04 +0000

Devin: Interesting program, however it appears to have no support for rate limiting of memory access. I want to be able to allocate X amount of memory and then access Y amount per second with Z amount of that access being writes.

Alex: When I had the idea I thought it would be a silver bullet, but the evidence suggests that isn’t the case. I look forward to someone telling me about another way to do it or pointing out a flaw in my testing.

By: Alex Malinovich

Alex Malinovich — Thu, 22 May 2008 16:55:53 +0000

When I read the first paragraph here, it sounded like the silver bullet I’ve been looking for: a way to allow a DomU to use more RAM than it should for brief periods. The big balancing act of administering Dom0′s (at least in my experience), comes from having a large number (20+) of DomU’s on a single Dom0. Here the goal is to have each domu have as little RAM allocated to it as possible, but to allow a way for it to temporarily grow without a reboot. Or, more specifically, to have just enough RAM to handle day-to-day operations, but to allow for more during special operations.

I’d suspect that the numbers you’re seeing have something to do with the way the swapping functionality in the kernel is implemented, though I really am not quite familiar enough with the kernel internals to make that judgement.

If you do throw together any Xen testing tools, I’ll definitely make use of them. I’m (un-)fortunate enough to get paid to work with Xen all day, and if I can get so much as a 1% improvement that can be quite important.

By: Devin Carraway

Devin Carraway — Thu, 22 May 2008 01:16:51 +0000

> I am idly considering writing a program to exercise virtual memory for the purpose of benchmarking swap on virtual machines.

I wrote such a thing (http://devin.com/lookbusy/) a ways back, as part of a somewhat silly app to make fake system load of various sorts. Might be useful to you, might not.