glxsync - explicit frame synchronization sample implementation

Michael Clark michaeljclark at
Thu Dec 30 11:19:05 UTC 2021

On 12/30/21 11:19 PM, Eero Tamminen wrote:
> Hi,
> Different window managers do resizes differently.  See for example:
> On which window managers did you test this?

I did mention that. I tested with the mutter compositor inside 
gnome-shell, along with whichever decorator extensions are loaded from 
the toolkits present in GNOME on Ubuntu 20.04 LTS and Ubuntu 21.10.

; ( for i in $(pgrep gnome-shell); do sudo lsof -p $i ; done ) | grep 
mutter | awk '{ print $9; }'


One would hope folks implement standards in such a way that clients 
using the documented window manager sync protocols are interoperable.

This is in fact the reasoning that spurred this email. Is there a 
reference implementation for frame synchronization for X11 GLX apps?

I guess I am primarily satisfied if I can solve the artifacts on the 
window manager that I use but if I am distributing an app more widely 
then I might like something well-tested and a little more robust.

Does KDE implement _NET_WM_SYNC_REQUEST et al?

>      - Eero
> On 30.12.2021 7.20, 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.

^^ here

>> -
>> _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:
>> _glxsync_ *does not* yet implement the following extensions:
>> _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.
>> 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.
>> 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]
>> [2]

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