Xen devel - Apr 2008 - [PATCH] add self-ballooning to balloon driver

The attached patch adds a "self-ballooning" feature to the balloon
driver.  Currently it only works when the balloon driver is
compiled into the kernel (e.g. not as a pv-on-hvm driver) because,
alas, the key kernel global variable is not exported.

It can be turned on with "echo 1 > /proc/xen/balloon" and
off again with "echo 0 > /proc/xen/balloon".  It defaults off.

I've tested it with a program that randomly eats memory and it
seems to work pretty well balancing two domains and surrendering
lots of memory (but not too much) when a domain is idle.  The
only deleterious effect I've seen is that performance suffers
for a second or so when a big memory app is launched on a
previously ballooned-down system (due to swapping).  But this
is a small price to pay in some real world environments.

It obviously shouldn't be used on a domain that doesn't have
a (virtual) swap disk configured.  If you foresee other issues,
please let me know.

I've got lots of ideas for building on this (for example rate-limiting
memory surrendering) but thought I'd submit the basic patch
for others to try/test.

Dan

==================================If Xen could save time in a bottle / then
clocks wouldn't virtually skew /
It would save every tick / for VMs that aren't quick /
and Xen then would send them anew
(with apologies to the late great Jim Croce)

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