On Sat, 2007-01-27 at 10:34 +0200, Avi Kivity wrote:
> In addition, quite a few performance optimizations are missing from kvm:
Hi Avi!
Just thought I'd share my experience with some of these optimizations
in lguest. I use virtbench (http://ozlabs.org/~rusty/virtbench) to try
to measure optimization results; it still needs more tests (and an
explicit kvm backend).
> - after modifying a pte, kvm doesn't preload the modified pte into
> shadow, but instead lets the guest fault it in
lguest doesn't either, but don't you still want the fault to update the
host accessed bit?
> - disk access is blocking instead of non-blocking. this will be fixed
> by merging qemu-cvs, which uses aio for disk access.
Do you plan on multiple guest I/Os outstanding, too? I would think that
I/O scheduling in the guest could be more effective than I/O scheduling
in the host if there is more than one guest sharing a host device. It
could potentially reduce guest<->host transitions too.
> - better heuristics for recycling page tables are needed
Definitely a whole area of research here...
> - prefaulting for common access patterns can help
Hmm, interesting idea. Any specific thoughts?
> - kvm currently saves the entire fpu state on every exit, even if it has
> not been modified
This was measurable for lguest, but our transition is slow so your %
improvement might be greater. With intelligent TS (2.13GHz Core Duo2):
Time for one context switch via pipe: 53514 nsec
Time for one Copy-on-Write fault: 14841 nsec
Time to exec client once: 1155102 nsec
Time for one fork/exit/wait: 764490 nsec
Time for two PTE updates: 23800 nsec
Removing it and restoring FPU every time:
Time for one context switch via pipe: 56229 nsec
Time for one Copy-on-Write fault: 15989 nsec
Time to exec client once: 1243624 nsec
Time for one fork/exit/wait: 824647 nsec
Time for two PTE updates: 25056 nsec
Cheers!
Rusty.
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