Re: [Qemu-devel] X support for QXL and SPICE

[Izik Eidus]

On Sat, 12 Dec 2009 16:43:43 +0100
Alexander Graf <address@hidden> wrote:
> 
> 
> I'm always having a hard time understanding why VNC is slow. I've
> seriously always been wondering. And I'm still puzzled as to why RDP
> is so much superior performance-wise. After all, it basically only
> implements framebuffer updates too.

Not really, RDP does have commands in its protocol.
The latest RDP 7 is even more advance in that but lack the abilaty to
enjoy it when not using "same codecs / dx" support.

> 
> So now you bring in SPICE to that equation. I don't know _why_ SPICE
> is faster. I know that it is and I hear all those awesome great
> features. But I still don't have a feeling on which part of it really
> is the performance booster. As with the example above, I would never
> have guessed that doing synchronous updates is the performance
> killer. I'm still trying to figure out what is going wrong in VNC :-).
> 
> So don't take this as an offense. It's about learning from you guys.
> You're the ones that measured what makes things fast - and we want to
> know! Even if we're not building code on it, but using SPICE, it'd
> still be very valuable to know why exactly it is performing better.
> And best case also by how much each single feature saves us.


From my point of view, Effective commands transfer, Smart Depth tree
usage, Effective Cache, (the soon to be merged offscreens), Effective
Compression, and Effective Video streaming, this what need to have for
making things fast...

But It is not that easy as it sound, beacuse spice is really not just
about sending simple "draw_line command".

it is system that was desgiened to send commands from day one, and to
be used by kvm to reduce host cpu usage, The QXL driver, the QXL
Device, the Spice server, The spice Client, all of this peice of
software togather bring spice into what it is.

(As you saw above, you said it yourself, in the middle of making such
 system there are alot of performence questions, some times the answers
 are not absolute, And I sure VNC beat spice in some cases, But this is
 the whole point, allowing the user to choce what ever protocol he
 want)

Thanks.

> 
> 
> Alex
> 
> PS: Let me explain my background on those questions:
> 
> I'm stuck with VNC for SUSE Studio for now. While I can't make
> fundamental changes to the protocol, I can easily modify the guest
> and I can also easily modify the VNC viewer to add a few new
> commands. If I can get a throughput reduction of 50% by using 2 out
> of 10 features SPICE has as VNC protocol extensions, I'd gladly
> implement them. That'd just make user experience on our side a lot
> better. If that means using QXL inside the guest, so be it :-).
>