Re: KVM performance

[Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>]

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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