On 30/12/21 6:20 pm, Michael Clark wrote:
Dear Mesa Developers,
I have been using GLFW for tiny cross-platform OpenGL demos for some
time but something that has really been bothering me are the visual
artifacts when resizing windows. Over the last year or so I have made
multiple attempts at solving this issue, digging progressively deeper
each time, until spending the last month researching compositor
synchronization protocols, reading compositor code, and writing this
demo as a prelude to figuring out how one might fix this issue in GLFW
or even Chrome.
I decided that first it might be a good idea to come up with the
simplest possible isolated example comprising of a near complete
solution without the unnecessary complexity of layering for all of the
cross-platform abstractions. It seems to me despite the ease this can be
solved with Wayland EGL, it is still useful, primarily for wider
compatibility, to be able to package X11 GLX applications, which is the
window system that I typically use when targeting Linux with GLFW.
That brings me to _glxsync_ which is an attempt at creating a minimally
correct implementation of explicit frame synchronization using X11, GLX,
XSync and the latest compositor synchronization protocols [1,2], tested
to work with mutter and GNOME on Xorg or Xwayland.
- https://github.com/michaeljclark/glxsync/
_glxsync_ is an X Windows OpenGL demo app using GLX and XSync extended
frame synchronization responding to synchronization requests from the
compositor in response to configuration changes for window resizes. The
demo updates extended synchronization counters before and after frames
to signal to the compositor that rendering is in progress so that
buffers read by the compositor are complete and matches the size in
configuration change events. It also has rudimentary congestion control.
_glxsync_ depends on the following X11 window system atoms:
- _NET_WM_SYNC_REQUEST
- _NET_WM_SYNC_REQUEST_COUNTER
- _NET_WM_FRAME_DRAWN
- _NET_WM_FRAME_TIMINGS
- _NET_WM_PING
_glxsync_ *does not* yet implement the following extensions:
- _NET_WM_SYNC_FENCES
- _NET_WM_MOVERESIZE
_glxsync_ depends on the following libraries: _X11, Xext, GLX, GL_.
I have to say there were numerous subtle issues that I found while
testing this code on Ubuntu 21.10 XWayland with an Intel Mesa graphics
stack and Ubuntu 20.04 LTS Xorg with the NVIDIA proprietary graphics
stack, so I have no idea how it will fly with other drivers and am very
interested in feedback. There really is not much sample code that I
could find that addresses this issue.
I found the Intel driver particularly finicky and there are some very
carefully placed XFlush calls *before* frame renders, and XSync calls
during congestion. There are also the beginnings of adaptive frame rate
using frame times and render timings stored in a circular buffer. That
said, there is no advanced adaptive frame rate logic beyond detecting
circumstances that can lead to tears with a back-off to the measured
short term average frame rate from statistics, and some logic to delay
frames when there are collisions with Expose events.
I would like to add these implementation notes to the README because
this is information one cannot easily find. It occurs to me that XFlush
before frames makes a lot more sense than after frames if one thinks
about Nagle and flow control combined with frame pacing. If we have
capacity to render at a constant frame rate with accurate scheduling for
the start of frames, then an XFlush(dpy) marker placed at the start of
the frame will occur at a constant rate, subject to variable render
times, whereas an XFlush(dpy) marker placed at the end of the frame
would have irregular timings needing stats for recovery. I am guessing
these are conversations that folks have already had because it seems to
work on my machine. An XSync(dpy, False) marker for congestion control
also seems to make sense to me because if we get frame drops we want to
resynchronize input and output. I am not sure under which conditions one
may wish to do XSync(dpy, True). Possibly some sort of watchdog or hang
check for IO when recovering from flooding.
Anyway I don't know where to go for this information so I am verbalizing
it to see if anyone can acknowledge it as being reasonable protocol.
There is also some rudimentary tracing infrastructure and some carefully
placed calls to poll, XEventsQueued(d, QueuedAlready), XEventsQueued(d,
QueuedAfterReading) to avoid blocking in XNextEvent at all costs. I
found it necessary to add a heuristic to avoid frame submission until
receiving frame timings from the compositor. Intuitively one might think
this makes the loop synchronous, but with the NVIDIA driver, it appears
the heuristic still allows multiple frames to be submitted in advance.
It is certainly finicky to debug. There is a --no-sync option to
simulate the absence of compositor synchronization as a testing aid.
There is very little back-pressure signaling to the client beyond the
ability to observe timings and serial numbers in frame drawn and frame
timing messages. It worries me that I need very careful placement of
XFlush and XSync to make the demo work so I would really appreciate
feedback if I am doing it wrong. There is some interesting potential for
control loops when using stats for adaptive frame rate, so I have not
yet attempted any sophisticated congestion control algorithm.
I have a feeling the delays I am introducing after collision alter the
frame time offset and this is not something I have added to that sample
to recover from after a flood of Expose events. Does one stutter or does
one warp time over some period to resynchronize back to the vertical
blank time offset. I implemented frame pacing but that sample does not
consider the vertical blank offset yet. Interesting problem.
It occurs that mixing implicit and explicit frame synchronization would
be a nightmare to debug. I am wondering if the use of XFlush (and maybe
XSync) markers as part of the frame sync protocol for OpenGL over the
GLX encapsulation is a good idea. The XFlush before each frame seemed
necessary in my testing, at least for interoperability between the Mesa
stack and the NVIDIA stack. nouveau and amdgpu are still unknowns.
In any case I am sharing this code with the hopes that folk can help
with testing. I was thinking to make a patch for GLFW but this was a
first step. I would really appreciate if folks could help test on
different drivers such as nouveau and amdgpu as I don't have access to
them. The code is currently released under the PLEASE LICENSE which is
practically public domain with one exception, but I am not disinclined
towards releasing it under an MIT license if it were found to be a
useful sample to add to the mesa demos.
Is there a place in mesa-demos for a frame synchronization demo? I see
glsync. Is there a compositor sync example that I may have missed? I can
imagine with the addition of WM_MOVERESIZE it could be used for tests.
This is pretty much version 0.0.1. i.e. is clean enough to release.
Regards,
Michael Clark
[1] https://fishsoup.net/misc/wm-spec-synchronization.html
[2] https://lwn.net/Articles/814587/
