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Re: [Xen-devel] Live migration fails when available memory exactly equal

To: "Graham, Simon" <Simon.Graham@xxxxxxxxxxx>
Subject: Re: [Xen-devel] Live migration fails when available memory exactly equal to required memory on target system
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Mon, 17 Jul 2006 20:33:43 +0100
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On 17 Jul 2006, at 20:17, Graham, Simon wrote:

This time, we can see that there was only 264MB free so we had to kick
the balloon driver to free up 1MB - once this was done (and we had
exactly 265MB free again), we were able to increase the reservation for
the target DomU to the requested amount...

The above is fairly reproducible but I'm not sure where to go next to
figure out where the issue really is (or, indeed, if there really is an
issue -- maybe this is just one of those inherently racy things;
however, I find it odd that it only seems to happen when the initial
free is exactly the same as the desired; I have plenty of other cases
where there is way more and way less memory available all of which seem
to work just fine).

Maybe there's a rounding error in the dom0 ballooning code. Your best bet is to get tracing out of Xen, which can tell you exactly how many pages are on Xen's free lists and how many are requested for the new domain being created -- a debug build of Xen would be a good start as that will turn on the DPRINTK tracing in Xen. Also bear in mind that the amount of free memory is always fluctuating in a live system -- for example the Xen network drivers are continually freeing and allocating memory to/from Xen. So if the dom0 ballooning logic is only freeing *exactly* what is required then that is probably a bit stupid: maybe some slack needs to be added (or the amount of slack increased).

 -- Keir

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