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Re: [Xen-users] Xen Performance


Apologies for my first incomplete reply.

Here's more context. The VMs weren't page scanning. They did show non- trivial %steal (where non-trivial is > 1%) These VMs are commercially hosted on five quad core hosts with approx 14 VMs per host and just under 1GB RAM per VM. Thats not a lot of memory, but then the workload of one nginx and three mongrels per VM is comfortably under 512MB of RSS.

I have heard numerous mentions of similar behavior from users of other utility platforms . There is a recent (Feb 2009) report by IBM that also describes this behavior once #domU exceeds six.

My point, however, is that Xen performance is not well understood in general, and there are situations where virtualization doesnt perform well.


On May 30, 2009, at 4:11 PM, Peter Booth wrote:

In this example, swapping/page scanning was not an issue.

On May 30, 2009, at 4:05 PM, Luke S Crawford wrote:

Peter Booth <peter_booth@xxxxxxx> writes:
One real world example:
native Linux:           page response times of (  400ms/150ms)
[mean/standard  deviation]
Xen VM:                 page response times of (  700ms/3.5s)
[mean/standard  deviation]

In this scenario, we have mean response times that are almost 100%
worse, and the 90th percentile is 1000% worse.

Yeah, but if your average native times are 400ms, you are very likely
already hitting swap and experiencing what I would call unacceptable
performance. It's no suprise that swap on a shared device is going to be
slower than a non-shared swap.

Hm. unless that xen vm is on a xen box doing nothing else, in which case the results whould suprise me quite a lot (unless you were using HVM mode
or had less ram in the xen vm than in the native box)

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