On 9/1/06, John Summerfied <[EMAIL PROTECTED]> wrote:
is a dormant Linux guest entirely dormant? I expect not, I think there are always daemons checking for timeouts, bits of the kernel running round checking whether it can reassign unused memory, whether there are buffers to flush, whether any hardware (including power supply) has failed yet.
You're right, there remains a certain amount of background noise that you could call overhead. Once you have the 100 Hz timer off, you only go back for requested wake-up calls by kernel threads and processes. I did some work on the 2.4 kernel to identify and minimize those. I could get down to some 50 timer interrupts per minute, but the neat thing is that many of those aligned to the wall clock time, so you could have periods of almost 5 seconds without timer interrupts (iirc the 5 second was bdflush). I was a bit surprised to find different applications implement their own timer again based on a steady interrupt. When I looked, nscd was doing several of 1 and 2 seconds and apache used a 1 second rithm. Sure, it's not as bad as 10 ms but it does limit scalability in the end. And we also found some middleware had implemented a 50 ms (!) timer in a Java class to run their scheduling :-( The developer I talked to did not even understand my concerns about that. In this light, the 20-minute mark does not bother me a lot from impacting the dormant guest. But there's another effect due to the continuous writing to disk. Not only will it cause subsequent activity to flush dirty pages, it also causes the "hot spot" to slowly migrate through the large virtual machine and cause associated paging in VM. It really gets bad with active agents for monitoring or performance data collection. I once measured that the "idle" agents of a security compliance monitor (just sitting there waiting for the server to request a scan) used 10 times what the entire idle server used without them. A data gatherer using "only" 1-2% of a CPU means that 50 idle servers take up an entire CPU (plus whatever other resources when they keep the archive on the individual servers). That made some folks only monitor the production servers. This however is a bad suggestion because you would have no way to explain excessive usage in your development servers. For a large installation it is not unrealistic that you would find more than 25% of your capacity used by servers doing "nothing" or other things that could be avoided. That's sad when you realize that you could have used that capacity to run your business applications. Rob -- Rob van der Heij Velocity Software, Inc http://velocitysoftware.com/ ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
