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Re: [Xen-users] live migration time increases when using "mem-set"

To: Tim Wood <twwood@xxxxxxxxx>
Subject: Re: [Xen-users] live migration time increases when using "mem-set"
From: Tim Post <tim.post@xxxxxxxxxxxxxxx>
Date: Tue, 05 Sep 2006 08:29:20 +0800
Cc: Xen Users <xen-users@xxxxxxxxxxxxxxxxxxx>
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Tim -

What services are running on the dom-u's , and did you take any measures
from within the dom-u's to clear idle child daemons out of cache?

In other words lets use lighttpd with php as fastcgi for example.
Lighttpd starts xx # of php children upon startup, and scales to the max
# allowed during use.

So if you had .. 30 children @ 15 MB each, and each one malloc()'ing an
additional 10 MB of elbow room, then reduce ram suddenly and migrate, I
would think those children would start paging dirty .. which would
increase i/o wait and could explain it.

If you did restart daemons that start idle children so the cache was
freed prior to reducing memory allocated .. then I'm baffled too.

- Tim

On Mon, 2006-09-04 at 16:48 -0400, Tim Wood wrote:
> Hi,
> I'm noticing very strange behavior when I create a VM, reduce its
> memory allocation using "xm mem-set" and then live migrate it.
> I would expect reducing the memory allocation would make the migration
> occur faster, but instead I am seeing the opposite.
> For example, if I create a VM with 900MB of ram initially, these are
> the times I measure for different reduced memory allocations:
> RAM (MB) - Migration length (sec)
> 900 - 11
> 768 - 20
> 512 - 33
> 450 - 38
> 256 - 51
> 128 - 55
> Just the opposite of what I would expect!
> As reference, if a VM is created with 256 MB of ram and migrated, that
> takes about 4 seconds.
> The VM is idle and so is the gigabit network.
> Looking at the xend.log files it looks like I am getting hundreds of
> lines like this:
> ERROR (XendCheckpoint:227) netbuf race: iter 4, pfn 28f78. mfn ffffffff
> I am using xen-testing... I'm very curious if this still occurs in
> xen-unstable but can't test that myself right now.
> -Tim
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