[vbox-dev] Monitor count limitations

Rūdolfs Bundulis rudolfs.bundulis at gmail.com
Wed Aug 27 16:38:05 UTC 2014

Hi Klaus,

Thanks again for the fast response. Well, this is a university research
project to build monitor walls without the underlying hardware, so I
guess its reasonable that I am breaking some limits that should be
normal in a normal use case.

Regarding 3D, when I installed guest additions I checked the Direct3D
box but I dont have the "Enable 3D acceleration" checkbox checked in
the VirtualBox manager. Should I reinstall the guest additions without

And more important, is it actually possible to fit 25 full hd
(1920x1080) framebuffers in the video ram if it is only 256mb inside
virtualbox? I could try to recompile from sources with lets say 1GB if
redefining the video memory macro does not break other stuff.

Best Regards,
Rudolfs Bundulis
From: Klaus Espenlaub
Sent: 2014.08.27. 19:05
To: vbox-dev at virtualbox.org
Subject: Re: [vbox-dev] Monitor count limitations
Hi Rūdolfs,

On 27.08.2014 16:49, Rūdolfs Bundulis wrote:
> Hi,
> I've run into some more issues with high monitor count on VirtualBox.
> Basically when selecting 25 displays it complains that at least 600
> something MB of VRAM is needed, but while the arrows in the display
> number box allow to select more than 8 monitors, seems that i can't go
> above 256 mb in VRAM. Is this a hard limitation? Since the COM API also
> does not allow me to set more than that.

Sounds like you have 3D enabled - then the GUI does a little too simple
maths, extrapolating the necessary space beyond sanity. It's only a
proposal, not a hard "must have". Don't think anyone tried to go to such
extremes, because it's just nonsense to expect even basic 3D able to
cope with so many screens.

If you disable 3D then the requirements should be a lot lower, in the
order of what's needed to represent the pixels.

Yes, there is currently a hard limit of 256 MiB VRAM, and we didn't find
cases where it was absolutely necessary to have more.

> Best Regards,
> Rudolfs Bundulis
> 2014-06-18 13:41 GMT+03:00 Rūdolfs Bundulis <rudolfs.bundulis at gmail.com
> <mailto:rudolfs.bundulis at gmail.com>>:
>     Hi Klaus,
>     Thanks a lot for the clarification. I thought the same that it was
>     just a sanity limit, but in my case this is really needed. Then I
>     guess will be able to do the same with the COM API, thanks a lot again.
>     2014-06-18 13:13 GMT+03:00 Klaus Espenlaub
>     <klaus.espenlaub at oracle.com <mailto:klaus.espenlaub at oracle.com>>:
>         Hi Rūdolfs,
>         On 18.06.2014 11:48, Rūdolfs Bundulis wrote:
>          > Hi,
>          >
>          > I'm developing a system that needs to run a headless VM with
>         a large
>          > number of attached monitors - currently the GUI has the limit
>         to set
>          > maximum monitor count to 8, I didn't check the COM API myself
>         but I
>          > suspect that it will not allow me to set the count greater
>         than 8, while
>          > browsing the VirtualBox sources show that the internal C macro is
>          > defined to 64:
>          >
>          > /* this should be in sync with monitorCount <xsd:maxInclusive
>          > value="8"/> in
>         src/VBox/Main/xml/VirtualBox-settings-common.xsd */
>          > #define VBOX_VIDEO_MAX_SCREENS 64
>         Looks to me that this comment is simply outdated. Nothing more,
>         nothing
>         less.
>         Many limits can be listed with "VBoxManage list
>         systemproperties", and
>         it shows 64.
>          > Is there really a need to limit the count to 8? If I change
>         the xml
>          > schema file and increase the count from 8 to 64 and recompile
>         would
>          > there be any actual limitations in the VirtualBox core?
>         There is a constant conflict in the GUI between sanity (i.e.
>         protecting
>         users against their stupidity - you can't believe how many people
>         believe that more is better, even if it's a giant waste of resources
>         instead) and allowing expert users to go to the extreme. There are
>         extremely few people out there who ever need to deal with more
>         than 8
>         monitors.
>         In my quick test I could use the GUI to select 20 monitors -
>         sure, the
>         slider doesn't allow that, but one can enter the number or use the
>         up/down arrows. This needs 256MB of VRAM, which this high number
>         automatically unlocks.
>         Klaus

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