Re: [Qemu-devel] [PATCH] [RFC] Variable video ram size option

[Trolle Selander <trolle.selander@eu.citrix.com>]

>
> Trolle Selander wrote:
> >/ Paul Brook wrote:/
> >/ >>Trying hard to think up a use for more than 16 Megs in the current case,/
> >/ >>    /
> >/ >/
> >/ >I'm pretty sure the current device allows the guest to do page flipping /
> >/ >(using an oversize virtual framebuffer)./
> >/ >/
> >/ >Paul/
> >/ >  /
> >/ I could have sworn I checked that at some point and found that it/
> >/ didn't, but looking at the vgabios code now it looks like it's all/
> >/ implemented. That definitely bumps the useable limit to 32 Megs even in/
> >/ the current case. Thanks for catching this, I'll add it to the fix list./
>
> For games _triple_ buffering is a common technique when there's enough RAM.
>
> With double buffering, after you flip away from screen 0 to screen 1, you
> have to pause until the next vsync before it's safe to draw a new
> image on screen 0.
>
> With triple buffering, this pause isn't required which makes games
> (etc.) able to run at a higher average frame rate.
>
> (More buffers can smooth the average further, which is more useful
> when playing video than games, because it adds latency but hides
> individual frame drawing time spikes.  But it's not that useful.
> Triple buffering is relatively common though.)
>
> So there's a use for 3 screens in the virtual framebuffer - 48 MB?
>
> -- Jamie
>   
While I have my doubts about there being any applications or games using 
VBE & triple buffering that will actually run at 2560x1600 with any kind 
of decent performance , I also see no reason to set this limit any lower 
than the theoretically usable max, so 48 Megs it is.

-- Trolle