DDA capture slow framerate on Windows 10 vs Windows 8.1 VMs

[weary-adventurer]

Hi, I've made a simple program that uses DXGI Desktop Duplication API to test the capture performance.
It's just a while(1) loop that calls AcquireNextFrame, then CopyResource, Map and Unmap to simulate reading the framebuffer.
Source is available here: https://gist.github.com/weary-adventurer/241eef5a6205403422a3573ce50f8672

I'm running it on QEMU VMs without GPUs and I'm getting very different performance on Windows 8.1 and Windows 10 VMs.

On Windows 8.1, I'm getting about 64 fps, which is the same as the refresh rate of the QEMU virtual display (64hz):

The capture takes minimal time and happens at the same consistent rate as vsync:

But on Windows 10, running the same program on the same host I'm getting low 20 to 30 fps while constantly redrawing the screen (dragging windows, playing animation or video):

It seems like vsyncs happen slower and are inconsistent, often skipping or taking longer:

I've also tried Windows.Graphics.Capture with similar results.

The only clue I have is: If I set DWMFRAMEINTERVAL dword value in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\WinStations (from https://learn.microsoft.com/en-us/troubleshoot/windows-server/remote/frame-rate-limited-to-30-fps), enable RDP, then connect to an RDP session, then the capture program runs at more than 64 fps!

Is there any way to get DDA capture to capture at a faster rate in a regular session?

[weary-adventurer]

Another clue: When I call DwmGetCompositionTimingInfo on Windows 10 I'm getting these numbers:

rateRefresh = 64/1 = 64.00
rateCompose = 64/1 = 64.00
qpcRefreshPeriod = 1562500 = 15.62ms

So DWM supposedly also thinks it's running at 64hz. But it looks like that's not true.

[weary-adventurer]

Windows 10 DDA capture inside RDP session with DWMFRAMEINTERVAL registry hack shows consistent vsyncs and the dwm.exe bars don't stretch until vsync lines:

Compared with Windows 10 DDA capture inside a regular session:

[robmikh]

I'll need to take a closer look, but my immediate reaction is that this sounds like either a QEMU issue or something lower level than the system compositor. We can't influence the vsync in a way that would make it inconsistent like that. Even more telling that you can make it behave using RDP, as RDP uses an indirect display driver and likely circumvents the QEMU display driver.

Thanks for the report!

[weary-adventurer]

It also happens with basic VGA (basicdisplay.sys), QXL drivers (qxldod.sys) and no display at all (-vga none), but only since Windows 10.

On Windows 8 and 8.1 it gets consistent 64 fps as seen in the first screenshot with QXL drivers.

[weary-adventurer]

I've profiled the same Windows 10 with standard VGA just now (-vga std or -device vga) with this setup:

The test was running the DDA capture program, GPUView log and UFO Test in a browser.

As usual, connecting to RDP session and using DWMFRAMEINTERVAL hack guarantees more than 64fps:

In a normal session, UFO Test measure 64fps and 64hz but capture shows only about ~30fps.
It seems like maybe the browser is able to render at 64fps but DWM only renders at 30?

GPUView shows a similar picture but with a few differences. Vsync is still a bit inconsistent but the frames don't actually come on every vsync timing and are sometimes delayed. Also there a few abnormal peaks:

Peak zoomed in:

After the peaks there are some delays after which I see these chunks (edited image):

[weary-adventurer]

Another profile from same Windows 10, this time with VirtIO VGA (-device virtio-vga) and viogpudo.sys driver (https://github.com/virtio-win/kvm-guest-drivers-windows/tree/master/viogpu/viogpudo)

Active signal resolution -1 x -1?

RDP session with DWMFRAMEINTERVAL hack as usual, 64fps+

Regular session is about ~20-30 fps (same results)

Slightly different GPUView, no more peaks in DDA capture program but instead some peaks in dwm.exe:

Zoomed in:

[weary-adventurer]

Is there anything I can try on my end to get this solved? I could use a different driver or compile a sample WDDM driver but I'm not sure what to change as both QXL and VirtIO drivers seem to be based on WDDM examples. Any advice would be greatly appreciated as it's quite difficult to figure out how to approach this problem. Thank you!