[kinovea-dev] Asynchronous recording
- From: Joan <joan@xxxxxxxxxxx>
- To: kinovea-dev@xxxxxxxxxxxxx
- Date: Wed, 13 Jul 2011 17:36:12 +0200
Hi everyone,
A message (and questions) regarding threading in the Capture screen.
Yesterday I have comitted an important change in the Capture code to fix
bug250. The relevant svn blurb is (r479):
- On frame grab, switch back to main thread (=UI thread) as fast as
possible.
- Creation of a producer-consumer class for recording. Spawns a new
thread and push frames in a queue. The queue is then processed by this
new recording thread.
- Removed dropping and flickering protection as they don't fit in the
new paradigm. Will have to implement queue overflow protection.
In 0.8.15, there was only two threads for capture, the grabbing thread
on which we receive new frames (thread created within AForge library
code, we just hook the event handler) and the Main/UI thread.
We were doing too much work in the grabbing thread (i.e: copy the frame,
push it to buffer, read other part of buffer, create another copy for
saving, send to saving routine, wait for it to return, signal UI to
redraw). Doing all this may take longer than the frame grabbing
interval, and the grabbing thread was slowed down.
The consequence was frame grabs more further apart in time than the
actual framerate, and a resulting video with wrong timing compared to
what we put in the header.
Now there are 3 threads and we spend only the minimum time in the
grabbing handler.
tl;dr:
Under heavy load the grabbing thread is competing with the other threads
and the UI become unresponsive. The frames accumulates in the recording
queue.
We didn't have this behavior before because the recording operations
were synchronous with the grabbing. Grabbing was simply slowed by
exactly as much as the CPU needed.
Any comments, ideas, guidelines on how to tackle this issue ?
I experimented with lowering the grabbing thread priority (back to wrong
timing) or increasing main thread one (not much help for the recording
queue overflow)
Thanks
joan
PS: I'm leaving town tonight until monday, but I'll sure read all your
awsome ideas when I come back !
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- » [kinovea-dev] Asynchronous recording - Joan
