High delay of video-streams

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Hello spice developers,

we are trying to develop an Open Source virtual desktop infrastructure to be deployed at multiple German universities as described, by my colleagues, in the paper which I have put in the attachment. The solution based on openstack, qemu, spice... Our plan is also to have VM instances with virtual GPUs (SR-IOV). Due to the resulting requirements, it is necessary to transmit the image data as a video stream. We have seen Vivek Kasireddy recent work on spice which solves exactly this problem. However, when we tested it, we noticed a very high input to display delay (400 ms+ but only if the image data is transferred as video-stream). However, the problem seems to be a more general spice problem or is there something wrong with our setup or are there special parameters that we are missing?

Our setup:

QEMU: https://gitlab.freedesktop.org/Vivek/qemu/-/commits/spice_gl_on_v2
Spice: https://gitlab.freedesktop.org/Vivek/spice/-/commits/encode_dmabuf_v6
Intel HW decoder/encoder (but same with sw)

I have looked into what is causing the delay and have noticed that encoding only takes about 3-4ms. In general, the image seems to reach the client in less than 15ms. The main problem seems to be that gstreamer gets a very high margin(https://gitlab.freedesktop.org/spice/spice-gtk/-/blob/master/src/channel-display.c?ref_type=heads#L1773) and therefore waits a long time before starting decoding. And the reason for the high margin seems to be the bad mm_time_offset https://gitlab.freedesktop.org/spice/spice-gtk/-/blob/master/src/spice-session.c?ref_type=heads#L2418 which is used to offset the server time to the client time (with some margin). And this variable is set by the spice server to initially 400 ms https://gitlab.freedesktop.org/spice/spice/-/blob/master/server/reds.cpp?ref_type=heads#L3062 and gets updated with the latency https://gitlab.freedesktop.org/spice/spice/-/blob/master/server/reds.cpp?ref_type=heads#L2614 (but only increased). I still need to see how this latency is calculated.

Am I missing something or is this design not intended for transmitting interactive content via video stream? Temporarily overwriting the margin and tweaking parameter settings on the msdkh264dec brought the delay to about 80-100ms, which is not yet optimal but usable. To see what is technical possible on my setup, I made a comparison using moonlight/sunshine which resulted in an delay of 20-40ms.

Our goal is to achieve some round trip time similar to the moonlight/sunshine scenario to achieve a properly usable desktop experience.


Attachment: Cloud__OpenStack_VDI-1.pdf
Description: Adobe PDF document

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