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

From: Jamie Lokier
Subject: Re: [Qemu-devel] Re: QEMU-KVM and video performance
Date: Wed, 21 Apr 2010 19:33:57 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

Gerhard Wiesinger wrote:
> I'm using VESA mode 0x101 (640x480 256 colors), but performance is 
> there very low (~1MB/s). Test is also WITHOUT any vga window change, so 
> there isn't any page switching overhead involved in this test case.
> >>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.
> I don't think changing VGA window is a problem because there are 
> 500.000-1Mio changes/s possible.

1MB/s, 500k-1M changes/s.... Coincidence?  Is it taking a page fault
or trap on every write?

> Would it be possible to handle these writes through QEMU directly (without 
> KVM), because performance is there very well (looking at the code there 
> is some pointer arithmetic and some memory write done)?

I've noticed extremely slow VGA performance too, when installing OSes.
It makes the difference between installing in a few minutes, and
installing taking hours - just because of the slow VGA.

So generally I use qemu for installing old versions of Windows, then
change to KVM to run them after installing.

Switching between KVM and qemu automatically based on guest code
behaviour, and making both memory models and device models compatible
at run time, is a difficult thing.  I guess it's not worth the
difficulty just to speed up VGA.

-- Jamie

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