auto-ballooning crashing Dom0?

[Andre Przywara <andre.przywara@amd.com>]

Hi,

during some experiments with many guests I get crashing Dom0s because of 
too less memory. Actually the OOM killer goes 'round and kills random 
things, preferably qemu-dm's ;-)
The box in question has 128GB of memory, I start with dom0_mem=8192M (or 
16384M, doesn't matter). I also used "dom0_mem=8192M,min:1536M", but 
that didn't make any difference. Xen is c/s 25688.

Then I start some guests with 2GB each. This works fine until about 55 
guests, then I get some denies from xl when starting guests (which would 
be OK). But sometimes the guest start works (even after having failed 
before), but it has obviously ripped off precious memory from Dom0. With 
around 55 guests Dom0 has about 500MB in use.
The whole Dom0 is in trouble then, I get "fork: cannot allocate memory" 
messages for a simple "ls" and have to reboot the box.
This is with xl.conf:autoballooning=1 (= the commented default)
Setting it to 0 works, but is obviously not a real option as a default.

I found the hardcoded 128MB limit in libxl_internal.h, I guess this is 
way too small for this type of machine.

Either we change this to something higher (768 MB worked for me) or we 
make this a config option in xl.conf (like it was in xend-config.sxp)

Another option would be to make it dynamic, by looking at the actual 
memory currently used in Dom0 and don't balloon down to 110% or so of it.

Sadly (well..) I am about to leave for vacation, so no patch this time, 
I leave this as an exercise to the tool buffs ;-)

In any case we should do something still for Xen 4.2, as I guess people 
dislike crashing Dom0, tearing down all the domains with it...

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany