Re: [PATCH] kvm tools, ui: Optimize SDL updates

[Sasha Levin <levinsasha928@gmail.com>]

On Sat, 2011-06-04 at 15:48 +0200, Alexander Graf wrote:
> On 04.06.2011, at 14:04, Sasha Levin wrote:
> 
> > On Sat, 2011-06-04 at 13:53 +0200, Ingo Molnar wrote:
> >> * Alexander Graf <agraf@suse.de> wrote:
> >> 
> >>> Why would you need panning/scrolling for a fast FB? It's really an 
> >>> optimization that helps a lot with VNC, but on local machines or 
> >>> SDL you shouldn't see a major difference.
> >> 
> >> Qemu's fb console scrolling graphics is pretty slow to me even 
> >> locally so i assume that the dirty bitmap trick is not enough.
> >> 
> >> VirtualBox graphics is very fast, but it probably has its own console 
> >> abstraction and scrolling/2D/3D acceleration.
> >> 
> >> Also, since tools/kvm/ is really also about learning interesting 
> >> stuff, smooth scrolling was the historic first 'acceleration' usecase 
> >> that video graphics cards added - before they evolved more complex 2D 
> >> acceleration and then started doing 3D.
> >> 
> >> Walking that path would allow us to do a gradual approach, while 
> >> still having relevant functionality and enhancements at every step.
> >> 
> >>> Unless you use the FB as MMIO. Qemu just maps the FB as RAM and 
> >>> checks for dirty bitmap updates periodically. That way you don't 
> >>> constantly exit due to MMIO and are good on speed. The slowness you 
> >>> describe sounds a lot as if you don't do that trick.
> >> 
> >> Correct, and i assumed we already do the dirty-bitmap trick:
> >> 
> >> 	KVM_MEM_LOG_DIRTY_PAGES
> >> 	KVM_GET_DIRTY_LOG
> >> 
> >> But you are right, we do not actually do that!
> >> 
> >> Pekka, i think this should be the next step. We'll need scrolling 
> >> after that ...
> >> 
> >> In theory it would also be nice to tunnel the VGA text frame buffer 
> >> over to the KVM tool - as serial console is not supported by most 
> >> installers and default distro images. We could actually do a rather 
> >> good job of emulating it via Slang/Curses.
> > 
> > I doubt we could use dirty pages because unless guest VESA driver
> > supports panning, it will redraw the entire FB - which means that all
> > pages will be dirty.
> 
> Please recheck the math and compare 60 dirty bitmap checks+flushes per second to a few million MMIO exits for every single pixel :).

I might be missing something here, but if every single pixel changes due
to scrolling, doesn't it mean that all the pages will be marked as
'dirty' anyway?

> 
> If your concern is about whether dirty bitmapping makes sense, it does. If you check say 60 times a second for updates and you don't do anything during that second, no screen updates happen, so the dirty log is empty. Without dirty log, you end up refreshing/recomparing/resending the full screen multiple times a second.


-- 

Sasha.