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
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