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Performance input switching weirdness

I searched around and didn’t see anything about this, so apologies if this has already been addressed.

I’m running an Ubuntu 20.04 host and Windows 10 guest through virt-manager, and have looking glass B3 running pretty nicely. I’m running into an issue though where after I start up the vm, I get low performance for my passthrough card (radeon hd 6770). To pick a random benchmark for reference, it gets 44fps in Cinebench R11.5.

If I close looking glass and switch inputs on my monitor instead, Cinebench’s score jumps to 70fps. I figured this was just the hit on this old gpu. The weird thing is when I switch back to my host os and open looking glass again, the score stays at 70fps when I run the test.

The score only jumps when I close looking glass, switch inputs, then open looking glass again. If I leave looking glass open when switching inputs, it doesn’t work.

This happened consistently when I tried this a few more times. I also switched to the latest bleeding edge version from 2021-04-25, no change.

Is there something I’m doing wrong, or some way to troubleshoot this?

To be clear: you’re saying you get better performance plugging directly into the pass-through card with your monitor vs. looking glass?

This is expected. LG has an overhead due to the requirement for memory copies, etc.

IIRC this is CPU related - you need CPU power to do the memory copies required. What CPU are you running, and have you maybe starved your host for CPU cores by allocating them to the guest? It may simply be your CPU can’t keep up with the required memory copies.

i.e. - this isn’t a bug, it is an expected trade-off of having a discrete GPU doing passthrough back into your host window manager…

Like I mentioned, at first I thought this was the case, but then when I switched back to my host and opened looking glass again, performance remained exactly as high as switching.

I’m running a 5600x, I doubt that’s my limit here.