[Qemu-devel] linux guests and ksm performance

[Peter Lieven]

Hi,

i have recently been playing with an old idea (originally in grsecurity
for security reasons) to change
the policy from zero on allocate to zero after free in the linux page
allocator. My concern is that linux
leaves a lot of waste in the physical memory unlike Windows which per
default zeros pages after
they are freed.

I have run some tests and I can confirm some old results that a hardware
Linux machine
is approximately 2-3% slower with zero after free on big compilation jobs.
This might be due
to either the fact that pages are only zeroed on allocate if GFP_ZERO is
set or due to caching
benefits.

However, in a virtual machine I have not observed the above slow down to
that extend
while the benefit of zero after free in a virtualisation environment is
obvious:

1) zero pages can easily be merged by ksm or other technique.
2) zero (dup) pages are a lot faster to transfer in case of migration.

Therefore I would like to hear your thoughts if it would be a good idea to
change
the strategy in the Linux kernel from zero on allocate to zero after free
automatically
if the 'hypervisor' cpu feature is set? Or even have another technique to
tell a linux
guest that ksm is running on the host.

If this is not feasible can someone think of a kernel module / userspace
program that
zeroes out unused pages periodically.

Peter