[vbox-dev] Monitor count limitations

Vitali Pelenjow Vitali.Pelenjow at oracle.com
Wed Sep 3 23:00:14 GMT 2014

Actually VBoxHeadless sets framebuffers for each guest monitor (in VBox 
4.3 or older).
Where exactly did you see that VBoxHeadless uses one IFramebuffer instance?

If no Framebuffer is assigned then graphics output from the VM does not 

I wonder why monitors in your guest resize to 1280x720.
Did you allow all modes in IFramebuffer::VideoModeSupported?
Can you run 1 monitor at 1920x1200?
How do you resize the guest monitors: from the guest or using VBoxManage 
controlvm setvideomodehint?

BTW, you can send me source code of the framebuffer implementation and 
code which sets framebuffers fro the VM.
I could take a look.

When I tested Win7 32 bit guest with VBoxHeadless and 24 monitors, I used
VBoxManage controlvm setvideomodehint to resize and enable them.


Rūdolfs Bundulis wrote:
> Hi,
> >I just tested a VBoxHeadless VM with 24 monitors at 1920x1200 and 256 MB
> >of guest VRAM and it seems to work fine.
> I fixed errors on my side, and still got the same (which is still much 
> better than with the WDDM driver) but it seems that you actually 
> cannot run that many monitors on full hd. I tried 25,24,20,16 and all 
> these numbers resized to 1280x720, didn't go much lower, but I will 
> find the boundary. I looked at VBoxHeadless, since it was mentioned 
> that it did work, and I saw that VBoxHeadless actually injects only 
> one IFramebuffer instance. So can that actually affect the behaivor? 
> What happens if no IFramebuffer implementation is provided for the a 
> display? Does that count as a disconnected video output or what?
> 2014-09-03 12:01 GMT+03:00 Rūdolfs Bundulis 
> <rudolfs.bundulis at gmail.com <mailto:rudolfs.bundulis at gmail.com>>:
>     Hi,
>     I made a new Windows 7 machine and installed the guest additions
>     without the 3D support, verified that the XPDM files were
>     installed and now the machine booted to 1280x720 at all 25
>     monitors and my software acutally run out of memory for the
>     NV12/H264 buffers so I guess if I fix that I should be able to go
>     to 1920x1200, thanks for the pointers on XPDM, I'll fix everything
>     on my side with the memory and try again.
>     2014-09-02 17:07 GMT+03:00 Rūdolfs Bundulis
>     <rudolfs.bundulis at gmail.com <mailto:rudolfs.bundulis at gmail.com>>:
>             Yes. However HGSMI is "The Interface" between the vitual
>             graphics card
>             and the host and
>             therefore can not be disabled.
>             Moreover HGSMI does not affect the memory allocation much.
>             The monitor count is limited by the screen surface memory.
>         Thanks a lot for the explanation.
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