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

From: Gerhard Wiesinger
Subject: Re: [Qemu-devel] Re: QEMU-KVM and video performance
Date: Wed, 21 Apr 2010 20:50:28 +0200 (CEST)
User-agent: Alpine 2.00 (LFD 1167 2008-08-23)

On Wed, 21 Apr 2010, Jamie Lokier wrote:

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?

To clarify:
Memory Performance writing to segmen A000 is about 1MB/st.
Calling INT 10 set/get window function with different windows (e.g. toggling between window page 0 and 1) is about 500.000 to 1Mio function calls per second.

To get real good VGA performance both parameters should be:
About >50MB/s for writes to segment A000
~500.000 bank switches per second.

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.

I think this is very easy to distingish:
1.) VGA Segment A000 is legacy and should be handled through QEMU and not through KVM (because it is much more faster). Also 16 color modes should be fast enough there. 2.) All other flat PCI memory accesses should be handled through KVM (there is a specialized driver loaded for that PCI device in the non legacy OS).

Is that easily possible?



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