Xen users - Dec 2011 - using maxmem

Hi,

I am using xen 4.0.1 from debian Squeeze repositories and dom0 has 
kernel 2.6.32-5-xen-amd64(also stock debian).
I wanted to set a high value to maxmem in my xmdomain.cfg vm config 
files so that I can increase memory as needed without rebooting.
Unfortunately as soon as I set this paramater I see really wiered things.
For example, I am trying to install a windows HVM guest. When maxmem is 
not set, the first screen of the installer appear after less than 30. If 
I set maxmem to 12288 (12GB, i guess), the installer screen takes more 
than 3min30sec to splash...
I have also tried to decrease maxmem to 8GB then 4GB (while 
memory=2048)... but that doesn''t help... Well I have to say that the 
lower maxmem is set, the faster the VM are.
But when set the VM are generally slow and unstable, as I did not 
succeded to complete any installl because of guests crashing.
Not sure if it''s related but, few times ago, I had an issue while doing
live migration of a PV guest (migration seemed OK but VM was frozen) 
that also had maxmem set to such a value...  I never experienced livemig 
problem like this before...

Is there any options that may be incompatible with maxmem? Or am just 
doing bad use of maxmem?

Did anybody experienced similar behaviour?

Regards
-- 
<http://www.horoa.net>

Alexandre Chapellon

Ingénierie des systèmes open sources et réseaux.
Follow me on twitter: @alxgomz <http://www.twitter.com/alxgomz>

> 
> Made some tests to illustrate what I said in my previous mail.
> I counting time to reach the splash screen of windows installer using
> different value of maxmem (nothing else changed):
> 
> - no maxmem / mem= 2048
> 33 sec.
> 
> - maxmem=2048 / mem =2048
> 33 sec
> 
> - maxmem=4096 / mem =2048
> 1 min 57 sec
> 
> - maxme=8192 / mem =2048
> 4 min 32 sec
> 
> nobody has a clue?
> 
I haven''t been following this thread but it sounds awfully familiar to
something I looked at a while ago. What version of Xen are you using?

Have a look at this thread -
http://xen.1045712.n5.nabble.com/slow-xp-hibernation-revisited-td4451615.html -
where I posted an initial patch which was massaged by the xen devs into
something more correct and I believe it was applied but I don''t know at
what version.

James

> 
> Hi,
> 
> I am using xen 4.0.1 from debian Squeeze repositories and dom0 has kernel
> 2.6.32-5-xen-amd64(also stock debian).
> I wanted to set a high value to maxmem in my xmdomain.cfg vm config files
> so that I can increase memory as needed without rebooting.
> Unfortunately as soon as I set this paramater I see really wiered things.
> For example, I am trying to install a windows HVM guest. When maxmem is
> not set, the first screen of the installer appear after less than 30. If I
set
> maxmem to 12288 (12GB, i guess), the installer screen takes more than
> 3min30sec to splash...
> I have also tried to decrease maxmem to 8GB then 4GB (while
> memory=2048)... but that doesn''t help... Well I have to say that
the lower
> maxmem is set, the faster the VM are.
> But when set the VM are generally slow and unstable, as I did not succeded
> to complete any installl because of guests crashing.
> Not sure if it''s related but, few times ago, I had an issue while
doing live
> migration of a PV guest (migration seemed OK but VM was frozen) that also
> had maxmem set to such a value...  I never experienced livemig problem like
> this before...
> 
> Is there any options that may be incompatible with maxmem? Or am just
> doing bad use of maxmem?
> 
> Did anybody experienced similar behaviour?
> 
Okay I just read this too. For Windows HVM to work with PoD memory
you''ll need PV drivers installed I think so that they allocate the
extra memory before Windows gets a chance to touch it. This is a bit tricky
during an install but possible during actual use once you have the PV drivers
loaded.

The way the PoD memory works (which I don''t even know if 4.0.1 supports
properly), is that the DomU is given "maxmem" amount of memory, but
xen only populates "memory" amount of pages with actual memory. When
the DomU touches a page of memory that isn''t populated, Xen find a
spare page and populates it. If there aren''t any spare pages because
"memory" pages are already populated, Xen scans for pages which are
all 0''s and uses one of them, leaving a hole behind. There was a bug
causing really bad performance when this happened (see previous email) but even
with that fixed, as soon as DomU uses more than "memory" amount of
actual pages, you are going to crash.

So don''t set "maxmem" > "memory" until you have
PV drivers loaded.

James