Multiple display set up - how does GPU perform compared to a single screen?

[GundamF91]

I've finally gone dual-screen now, and I do like the setup. I run the desktop extended to the other side. One thing that bothered me is when I play a video file, and when I drag and drop it over to the other screen, it stutters for a second. Keep in mind this is with HD5870 with 1GB of GDDR5, and Quadcore Q9450 with 8GB of DDR2, and I'm only running 1680x1050 for each screen.

It makes me wonder what's causing this and how exactly multiple displays are powered by GPU. One plausible answer is that GPU treat 2 screens as 2 images, and allocates a portion of its memory to each screen, so that when the media player location shifts from one screen to another, the GPU physically has to flush its data, and reload into another portion of its memory, causing additional work and slight stutter. I wonder if the GPU could be more efficient at the extended desktop by treating the entire stretched screen as one display area of 3360x1050? Does the Eyefinity work differently for multi screens, to make it more efficient for multiple displays?

 

[SanDiegoPC]

I run two monitors here and don't have that issue on my build (see sig). These are 19" Samsung Flat Panel monitors. But I'm only running 1280x1124 in each screen so my setup has a whole lot less pixels than yours.

 

[lifeblood]

I've ran two or more monitors for years and have seen the issue your talking about. I am not sure if my current rig does it, but my previous one certainly did. I have seen this happen whether I had each monitor plugged into different video cards and both plugged into the same video card.

I think the video card treats each monitor as a separate image as you suggest. Also, when you are playing movies your video card is in a slightly different mode, kinda like 3D versus 2D mode. I suspect that's the reason for the pause. What I can say for sure is it's gotten a lot better these past few years. Used to be I would blue screen XP if I dragged movies from one monitor to the other. Now all we get is a slight pause in the movie.

I'm not sure how Eyefinity handles it but I suspect it treats it as one big display.

 

[KingFatty]

Turning off video overlay may eliminate the issue, but your CPU will be doing more work (I think, not at home to test)