Re: [Xen-users] Does faster 2D Host video card help HVM guest?

["Trolle Selander" <trolle.selander@xxxxxxxxx>]

I recently measured graphics performance on an OS/2 guest (which is as "unaccellerated" as it can get) and discovered, much to my surprise, that graphics performance is nowhere near as bad as one might assume, and in fact pretty good. I believe the explanation to be that though unacellerated, the virtual graphics card is sitting on a "bus" that actually has the full speed of RAM, which on a fast dual channel DDR2 system is actually faster than even PCIe 16x. Moroever, due to the fact that the cirrus device model uses buffered mmio, in a multi-core system, a certain amount of the "work" can get handed off to dom0 and the other CPU core.
It would seems to me that the main reason 2D & gui ops feel "slow" in guests right now is the fact that even with SDL, the guest VM's screen is only updated  at 30 Hz.

/Trolle

On 6/4/07, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:

> Because of how the qemu-dm graphics card emulation works, I believe host
> graphics card speed has pretty much no impact whatsoever with the current
> device model. The important factors are host CPU & RAM speed.

Right now, the guest's graphics performance is going to be fairly slow
whatever you're running in the host.  It's not worth trying to upgrade the
host graphics card, because it's not really going to make any difference to
the guest.

Guest display improvements will probably come from future optimisations in
future releases of Xen.

Cheers,
Mark

--
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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