Thursday 4 December 2014

Software Switching GPU Connections to Monitors

Many thanks to Paul King from for the following blog entry.
Any red blooded Octane user would struggle to overlook the fact that with a multi-Nvidia GPU setup, they have a fairly decent SLI gaming rig on their hands.

The problem is that Octane performs best when all Nvidia GPUs are allocated exclusively to Octane, with the mundane task of driving monitors delegated to the generic GPU most motherboards have built in.

Should the need to smite a few bad guys in Battlefield 12 become pressing, an Octane user is faced with crawling around behind the computer, disconnecting monitors from the motherboard ports and connecting them to their Nvidia card ports.   And of course putting them back again afterwards.

Most monitors have a few port types available, as do the GPUs, and by using two of these at once it is nominally possible to leave both motherboard and NVidia GPUS connected to the same monitor even for multiple monitor setupsThe Windows display manager often struggles with this arrangement however; going into paroxysms as it keeps detecting and attempting to start unused connections, particularly while the user is attempting to switch multiple monitors one by one from one GPU type to the other, or to a hybrid combination  of both.  The only sure way to succeed involves physically disconnecting the cables from ports that are to be switched off.

A windows app called Display Fusion ( however solves this problem by use of stored profiles that can switch multiple monitors simultaneously from one combination of inputs to another – eliminating the flaky process of sequential manual switching via Display Manager.  Users can thus leave the cables in place connecting all GPUs to all monitors at the same time.

Users can also set up application specific display profile settings (triggered when app is launched) and manage a range of other display related tasks. There is a free version and a more fully featured paid version ($US25).

NOTE 1:  If you happen to ever use a hybrid of motherboard GPU and Nvidia cards simultaneously to drive a multi monitor setup, this won’t work well while Octane is active. It is important to ensure that BEFORE Octane becomes active there is no active connection between any Nvidia Card and ANY of the monitors (whether achieved by physically disconnecting , or deactivating connections via monitor profile switching).

Consequences otherwise are that long freezes and lags will occur while NVidia Cards are set as prioritised to Octane - even if the monitor in use by the ArchiCAD/Octane application window is being driven by the motherboard GPU and not an Nvidia card; when windows desktop is extended to span across monitors , the whole user interface system then has to wait for the already busy Nvidia card to complete every refresh of those parts of the user interface occurring on it’s monitor (desktop display, mouse pointer etc.).

NOTE 2: DVI-D (single) and HDMI ports are generally no good at driving displays beyond 1920x1080 resolution (without flaky driver hacks and compromised refresh rates). You will generally need a dual DVI-D or DisplayPort connection to drive monitor resolutions higher than this.  HDMI version 2 has recently been released and can in theory handle much higher resolutions, but the ports at both ends would need to meet the new standard, and while there are 4K capable TVs/monitors available with HDMI 2, so far very few GPUs use it.

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