Am 29.07.21 um 13:00 schrieb Pekka Paalanen:
On Thu, 29 Jul 2021 12:14:18 +0200
Christian König <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
Am 29.07.21 um 11:15 schrieb Pekka Paalanen:
If the app happens to be frozen (e.g. some weird bug in fence handling
to make it never ready, or maybe it's just bugged itself and never
drawing again), then the app is frozen, and all the rest of the desktop
continues running normally without a glitch.
But that is in contradict to what you told me before.
See when the window should move but fails to draw it's new content what
happens?
Are the other windows which would be affected by the move not drawn as well?
No, all the other windows will continue behaving normally just like
they always did. It's just that one frozen window there that won't
update; it won't resize, so there is no reason to move that other
window either.
Everything continues as if the frozen window never even sent anything
to the compositor after its last good update.
We have a principle in Wayland: the compositor cannot afford to wait
for clients, the desktop as a whole must remain responsive. So there is
always a backup plan even for cases where the compositor expects the
client to change something. For resizes, in a floating-window manager
it's easy: just let things continue as they are; in a tiling window
manager they may have a timeout after which... whatever is appropriate.
Another example: If a compositor decides to make a window maximized, it
tells the client the new size and state it must have. Until the client
acks that specific state change, the compositor will continue managing
that window as if nothing changed. Given the asynchronous nature of
Wayland, the client might even continue submitting updates
non-maximized for a while, and that will go through as if the
compositor didn't ask for maximized. But at some point the client acks
the window state change, and from that point on if it doesn't behave
like maximized state requires, it will get a protocol error and be
disconnected.
Yeah and all of this totally makes sense.
The problem is that not forwarding the state changes to the hardware
adds a CPU round trip which is rather bad for the driver design,
especially power management.
E.g. when you submit the work only after everybody becomes available the
GPU becomes idle in between and might think it is a good idea to reduce
clocks etc...
How about doing this instead:
1. As soon as at least one window has new committed state you submit the
rendering.
As far as I understand it that is already the case anyway.
2. Before starting rendering the hardware driver waits with a timeout
for all the window content to become ready.
The timeout is picked in a way so that we at least reach a
reasonable fps. Making that depending on the maximum refresh rate of the
display device sounds reasonable to me.
3a. If all windows become ready on time we draw the frame as expected.
3b. If a timeout occurs the compositor is noted of this and goes on a
fallback path rendering only the content known to be ready.
4. Repeat.
This way we should be able to handle all use cases gracefully, e.g. the
hanging client won't cause the server to block and when everything
becomes ready on time we just render as expected.
Regards,
Christian.
Thanks,
pq
