[Qemu-devel] Re: QEMU-KVM and video performance

[Avi Kivity]

On 04/19/2010 10:14 PM, Gerhard Wiesinger wrote:
> Hello,
>
> Finally I got QEMU-KVM to work but video performance under DOS is very 
> low (QEMU 0.12.3 stable and QEMU GIT master branch is fast, QEMU KVM 
> is slow)
>
> I'm measuring 2 performance critical video performance parameters:
> 1.) INT 10h, function AX=4F05h (set same window/set window/get window)
> 2.) Memory performance to segment page A000h
>
> So BIOS performance (which might be port performance to VGA 
> index/value port) is about factor 5 slower, memory performance is 
> about factor 100 slower.
>
> QEMU 0.12.3 and QEMU GIT performance is the same (in the measurement 
> tolerance) and listed only once, QEMU KVM is much more slower (details 
> see below).
>
> Test programs can be provided, source code will be release soon.
>
> Any ideas why KVM is so slow? 

16-color vga is slow because kvm cannot map the framebuffer to the guest 
(writes are not interpreted as RAM writes).  256+-color vga should be 
fast, except when switching the vga window.  Note it's only fast on 
average, the first write into a page will be slow as kvm maps it in.

Which mode are you using?

> Any ideas for improvement?

Currently when the physical memory map changes (which is what happens 
when the vga window is updated), kvm drops the entire shadow cache.  
It's possible to do this only for vga memory, but not easy.

--
error compiling committee.c: too many arguments to function