Hyper-V affects passed-through PCIe performance?

[testdasi]

I was doing some testing last night and noticed that with Hyper-V turned off, CrystalDiskMark on my passed-through Samsung 970 NVMe was:

• about 60% slower with random I/O!
• sequential (read) is capped at 2.6GB/s (Hyper-V On is 3.5GB/s)

 

Especially considering how much the random I/O dropped, I would hypothesize that it has something to do with latency. I didn't have time to run any GPU benchmark but given the above, presumably that would affect GPU performance too.

 

So perhaps, anyone who thinks their GPU isn't performing up to par might want to turn Hyper-V on (and perhaps try resolve the error code 43, if affected, via some other tweaks - I don't have any issue with this for both my 1070 and 710).

Edited November 27, 2019 by testdasi

[bastl]

@testdasi On default there is a cache option set to writeback for the drive.

<driver name='qemu' type='raw' cache='writeback'/>

Not 100% sure but I think caching is dissabled if you turn off Hyper-V at least it feels like it is. You can test it by changing cache to "none" and benchmark if you see the same drops with Hyper-V ON.

[testdasi]

24 minutes ago, bastl said:

@testdasi On default there is a cache option set to writeback for the drive.

<driver name='qemu' type='raw' cache='writeback'/>

Not 100% sure but I think caching is dissabled if you turn off Hyper-V at least it feels like it is. You can test it by changing cache to "none" and benchmark if you see the same drops with Hyper-V ON.

I think that would only affect disk device.

My 970 is passed through via PCIe (like a GPU) so it theoretically shouldn't be affected by that.

[bastl]

2 minutes ago, testdasi said:

I think that would only affect disk device.

Sry, my fault. This affects drives passed through by-id you are right. PCIe devices shouldn't be affected by this. 👍

[bastl]

@testdasi Quick question, in one of the 6.8 RC builds I noticed that if you switch the Hyper-V setting in the ui and hit save it doesn't have an effect. The XML stays the same and editing the VM the next time the Hyper-V is reverted back. Did you noticed something like this? I've already opened a bug report. I'am still on RC6 and hadn't the time yet to upgrade and retest with RC7.

[testdasi]

39 minutes ago, bastl said:

@testdasi Quick question, in one of the 6.8 RC builds I noticed that if you switch the Hyper-V setting in the ui and hit save it doesn't have an effect. The XML stays the same and editing the VM the next time the Hyper-V is reverted back. Did you noticed something like this? I've already opened a bug report. I'am still on RC6 and hadn't the time yet to upgrade and retest with RC7.

Yes still on RC7 - I had to start new templates for my testing.

Also, it has been a bug since I can remember so must have traced back to even before 6.8