> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Trolle Selander
> Sent: 04 June 2007 10:58
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Does faster 2D Host video card help
> HVM guest?
>
> 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.
Yes, the "slowness" in graphics is more about update frequency than to
do with the actual "read/write" performance of the "graphics adapter".
The way it works (for those inclined to wanting to know these things) is
that the frame-buffer is shared between qemu-dm and the guest. So data
can be read/written with normal RAM-speed, as Trolle says. To preserve
CPU-performance on qemu-dm, updates are rate-limited, because the method
to update the screen is that we read the frame-buffer, and compare it
with an "old" frame buffer [I believe it's actually a check-sum, rather
than byte by byte compare (ok, technically, it's also using SSE
instructions to get as good a throughput as possible)]. Doing this at
50-100Hz would consume a lot more than the current CPU consumption of
qemu-dm.
--
Mats
>
> /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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