User error. And truth is stranger than fiction as always...
Unity8 was freezing every 5 seconds because that's the ext3/ext4 journal
commit interval.
Unity8 was not doing the IO directly, but since it's being bypassed
through USC, is very time-sensitive. Fullscreen bypass holds buffers
back from the client for the duration of a frame. So the freeze was USC
periodically failing to return Unity8 a buffer in time. The problem with
USC as shown above was in its Mir compositor thread. I had configured it
to use --compositor-report=log, which although it writes very little
text, is subject to the ext3/ext4 journaling rules which force a
commit/sync every 5 seconds by default.
So the delay was USC's compositor thread writing to its log, which only
once every 5 seconds is a slow operation. Whenever the hiccup occurred,
that held back a bypass buffer from Unity8 a bit too long.
To add to the problem, Mir's Mesa-KMS driver does not allow more than
one frame to be pre-rendered so as to keep visual lag down. So it
sometimes didn't have a fresh U8 frame ready in time and would skip a
whole frame. There's no catch-up in the Mir Mesa-KMS driver, although
I've coincidentally been thinking about implementing it. But doesn't
matter; as soon as I turned off --compositor-report=log the bug was
solved.
** Changed in: canonical-devices-system-image
Status: Confirmed => Invalid
** Changed in: unity-system-compositor
Status: Confirmed => Invalid
** Changed in: unity-system-compositor (Ubuntu)
Status: Confirmed => Invalid
** Changed in: unity8 (Ubuntu)
Status: Confirmed => Invalid
** Changed in: qtmir (Ubuntu)
Status: Confirmed => Invalid
** Changed in: canonical-devices-system-image
Milestone: u8c-1 => None
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https://bugs.launchpad.net/bugs/1664150
Title:
[performance] Unity8 rendering hiccups/stutters exactly every 5
seconds in unison with disk activity
Status in Canonical System Image:
Invalid
Status in Mir:
Invalid
Status in Unity System Compositor:
Invalid
Status in qtmir package in Ubuntu:
Invalid
Status in unity-system-compositor package in Ubuntu:
Invalid
Status in unity8 package in Ubuntu:
Invalid
Bug description:
Using Mir 0.26.1, Unity8 runs better than ever. But when displaying a
large continuously rendering window (e.g. eglplasma or egltriangle),
the Unity8 shell hiccups and visually stutters exactly every 5
seconds.
According to MIR_CLIENT_PERF_REPORT it's quite major, missing
somewhere around 9 frames every 5 seconds. This matches what I see on
screen - a large number of frames skipped once every five seconds...
[2017-02-13 16:16:26.761602] perf: : 60.00 FPS, render time 0.41ms, buffer
lag 66.31ms (4 buffers)
[2017-02-13 16:16:27.762435] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:28.763269] perf: : 51.00 FPS, render time 0.39ms, buffer
lag 78.10ms (4 buffers)
[2017-02-13 16:16:29.764068] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:30.764939] perf: : 60.00 FPS, render time 0.38ms, buffer
lag 66.33ms (4 buffers)
[2017-02-13 16:16:31.765773] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:32.766603] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:33.767437] perf: : 51.00 FPS, render time 0.38ms, buffer
lag 78.10ms (4 buffers)
[2017-02-13 16:16:34.768274] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:35.769109] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:36.769943] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:37.770780] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:38.771629] perf: : 51.00 FPS, render time 0.40ms, buffer
lag 78.09ms (4 buffers)
[2017-02-13 16:16:39.772446] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:40.773279] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:41.774111] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:42.774944] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:43.775796] perf: : 51.00 FPS, render time 0.38ms, buffer
lag 78.10ms (4 buffers)
[2017-02-13 16:16:44.776615] perf: : 60.00 FPS, render time 0.36ms, buffer
lag 66.35ms (4 buffers)
[2017-02-13 16:16:45.777448] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.33ms (4 buffers)
[2017-02-13 16:16:46.778282] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.33ms (4 buffers)
[2017-02-13 16:16:47.779135] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:48.779969] perf: : 51.00 FPS, render time 0.39ms, buffer
lag 78.09ms (4 buffers)
[2017-02-13 16:16:49.780801] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
[2017-02-13 16:16:50.781639] perf: : 60.00 FPS, render time 0.39ms, buffer
lag 66.32ms (4 buffers)
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