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I'm certainly no expert, but I would guess that the context-switch time (where I believe the new overhead is added) in games is fairly small compared to the raw number-crunching in-process, so the effect would be minimal in any case

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Every draw call will need to transition to kernel space to send data over the PCIe bus to the GPU. Modern games execute something on the order of 1000+ draws per frame, so assuming 60fps that's going to be at least 60,000*2 context switches into the kernel and back per second, more if you're doing high refresh rates.

How big the impact I will be, I don't know - but I wouldn't be surprised if it was a couple percent (effectively ruining the single-threaded performance boost Intel has in gaming over AMD before accounting for overclocking).

I was also worried about network I/O too, which could be an absolute pain for games where you care about latency.

Network IO is negligible in game. The source engine for instance is hard limited to using 30 kB/s of bandwidth.

I would have thought that would have been a lot of context switching with graphics driver calls

(Or maybe they already do as much as possible in userspace and then batch kernel calls?)

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