Re: [gentoo-user] how to use two graphics cards with one display

[lee <lee@yagibdah.de>]

Andrew Savchenko <bircoph@gentoo.org> writes:

> Hi,
>
> On Sun, 05 Jun 2016 19:34:15 +0200 lee wrote:
>> Hi,
>> 
>> is there a way to reasonably use two graphics cards with a single
>> display?
>> 
>> SLI won't work because it's retarded in requiring the GPUs to be the
>> same, which they aren't --- not to mention that the cards would be too
>> far away from each other in the slots for a bridge to fit.
>> 
>> So what I'm thinking of is like using one card as a default and being
>> able to use the other one to play a video in some window on the same
>> display, preferably managed by the same fvwm, with the window optionally
>> being fullscreen in size.  I'd like to do that because the card I have
>> isn't powerful enough to play a video while an open gl application is
>> running at the same time.
>> 
>> I'll probably get a better card once prices come down a bit, but it
>> might have the same problem, and why would I want to waste an otherwise
>> perfectly good graphics card.
>
> Yes, but it depends on your hardware setup. What's yours and why
> you need such unusual thing: connect two video cards to a single
> monitor, or do you mean by display X display spawn over multiple
> monitors?

a single monitor

> In case of laptops such configuration is quite common: they may
> have two video cards with single switchable output: intel card is
> used for general work to save power and nvidia card is used for
> applications, requiring high GPU performance. Switching is done
> using sys-power/bbswitch. But looks like this is not your case,
> since you are talking about card replacement, since most laptop GPU
> cards are not replaceable.

Right, it's not a laptop, and I don't want to switch between different
cards.

> If you want a multihead setup using two cards, this is trivial using
> either xinerama or X screens depending on your taste.

That is only simple when you have multiple monitors.

> As far as I understand your e-mail, you are trying to mux video
> outputs of two GPU cards to a single monitor (excuse me if I'm
> wrong, but it is hard to understand what your hardware is), this is
> also doable if your monitor supports dual input (most modern
> monitors do). This way separate X screens may be used to achive
> your goal. (Xinerama setup is also possible, but GL acceleration
> will be limited to abilities of the weakest card).

Exactly, but I don't want to use the picture-in-picture feature of the
monitor, and I don't want separate X screens, and I don't have room to
fit another monitor on my desk.

I simply want to use one of the graphics cards to handle an application
that uses open gl and the other one to play a video.

> But honestly I don't get why you need this: if you have a powerful
> GPU and it is not a laptop, where power consumption is critical,
> why just don't use that card? Most cards have multiple outputs, so
> it is not a problem to setup multihead with a single card either.

The GPU isn't quite powerful enough for some of what I'm doing.
Otherwise, it's a perfectly good card.

So I need to get a better graphics card, and once I do, it would be a
pity to have the current one laying around uselessly.  I wouldn't get
much if I tried to sell it, so I rather keep it in case I need a spare.
Buying another one which is the same, to use SLI, won't help, either.

IIUC, it takes some processing power to decode a video, so why not use
one of the cards for just that?  Multiple cards should be able to work
together.