Re: Test report: Migration from 4.1 to 4.2 works

[Ian Jackson <Ian.Jackson@eu.citrix.com>]

Ian Jackson writes ("Re: Test report: Migration from 4.1 to 4.2 works"):
>   xc: Saving memory: iter 1 (last sent 134144 skipped 1553): 0/1048576
>   0%xc: error: Failed to allocate memory for batch.!: Internal error

15:22 <ijc> It might be interesting to compare the actual memory usage
            of the same domain started with xend and xl on 4.1/4.2?

Started with xl create:

root@potato-beetle:~# xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   512     4     r-----     641.0
win.guest.osstest                           10   507     2     -b----     223.4
root@potato-beetle:~#

Then switched to xend:

root@potato-beetle:~# /etc/init.d/xend start
root@potato-beetle:~# xenstore-rm /local/domain/0/libxl/disable_udev

Started with xm create:

root@potato-beetle:~# xl list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0   512     4     r-----     720.9
win.guest.osstest                           11   515     2     ------     238.0
root@potato-beetle:~#

I think what is happening is this: xend accounts memory differently to
xl, giving the guest actually slightly more than is specified in the
config file.  When xl during incoming migration reads the guest config
file, it assumes that the config file's memory maximum is an accurate
representation of the guest's actual memory use.

I can reproduce this problem by ballooning a PV guest before migrating
it.  Eg,
   xl create /etc/xen/debian.guest.osstest.cfg
with memory=512, maxmem=1024.  Then
   xl migrate debian.guest.osstest localhost
works but
   xl mem-set debian.guest.osstest 1024
   xl migrate debian.guest.osstest localhost

I think we need to fix this in 4.2.1 somehow.

Ian.