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DR configuration considerations with 2 graphics cards in the box

What is the best DR configuration for a box with 2 graphics cards?

There are a lot of specifics and details to these types of questions.  To really answer these (DR) questions, we really need to know what the goals are.  Keep in mind that there is also more than one way to achieve the same goal.

EnSight configuration - if you want to use the capabilities of EnSight DR, you probably want to configure each card as a separate X screen (first card :0.0, second card :0.1). Then you can specify these displays in the dconfig or prdist file for either replication mode or compositing. 

This works just fine, and using two cards for compositing in one machine works well - the performance is generally better than with two cards in two machines. You would want to make sure that the cards are identical for load-balancing. 

There are other ways, independent of EnSight, that you can configure such a system. You could use SLI-mode with nVidia cards, which makes multiple cards appear as one. You wouldn't have to use EnSight DR in this case, but I really don't have numbers to compare performance for you. 

For the replication case, this method can also be handled by the X11 solution of Xinerama mode. If you have a two monitor setup and two cards, you can have one card render one screen and the second card render the second screen. With Xinerama, this all appears as one screen (:0.0), so again, this is transparent to EnSight. 

For the replication case, let me make one other observation, and that is that DR replication gives you a detached display. On a single workstation with two cards, you would have the main graphics window on one of the screens, plus a detached display with one window on each screen. This is like the old SGI parallel rendering solution, and there are problems switching back and forth between the GUI window and the detached display. With Xinerama, you simply have the GUI, which moves freely across both screens. In full-screen mode, you will be using both cards. 

In general, I would suggest that DR replication is really for scalable solutions. Using replication on a single node probably defeats the purpose. You'll double the memory usage and introduce problems with multiple overlapping windows. DR composite mode is more compelling on a single system - it does perform well. In this case, make sure the xorg.conf configures two separate screens, and does not enable Xinerama to create a "virtual desktop" spanning the two screens.

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