stdout slow when piped

[skiqh]

When playing a video from the console with the --stats parameter, omxplayer sends its status information quite frequently (around 4 to 5 times a second). However, when issueing the same command but to be piped (to a file or named pipe), the intervals between status outputs become rather large (about 6 seconds).
Also, time to first output (except for the clear-screen) increases significantly (from around 2 to 8 seconds).

Testcase:

# unpiped start
omxplayer --stats videofile

# takes ca. 2 seconds
> Video codec omx-h264 width 1280 height 720 profile 100 fps 23.976025
> Audio codec aac channels 2 samplerate 44100 bitspersample 16
> Subtitle count: 0, state: off, index: 1, delay: 0
> V:PortSettingsChanged: 1280x720@23.98 interlace:0 deinterlace:0v:     0k Ca:     0k
# this line refreshes quite frequently
> M: 4045785 V:  6.38s   4720k/  4800k A:  0.95   4.46s/  2.00s Cv: 10222k Ca:   851k



# piped start
omxplayer --stats videofile | cat

# takes ca. 8 seconds
> Video codec omx-h264 width 1280 height 720 profile 100 fps 23.976025
> Audio codec aac channels 2 samplerate 44100 bitspersample 16
> Subtitle count: 0, state: off, index: 1, delay: 0
> V:PortSettingsChanged: 1280x720@23.98 interlace:0 deinterlace:0v:     0k Ca:     0k
# this line refreshes merely every 6 seconds
> M: 5551415 V:  5.42s   4800k/  4800k A:  0.93   4.18s/  2.00s Cv: 10234k Ca:   821k

[popcornmix]

stdout is line buffered.
Piping will use larger (e.g. 4K) buffers. This is standard linux stuff and not really to do with omxplayer.

[skgsergio]

Dont know what do you want to do but maybe this works for you:

script -qc "omxplayer --stats videofile" /dev/null | cat

[skiqh]

Thank you both for your fast response!

My intention was to spawn omxplayer from nodejs, channel commands to its stdin and read from its output to fetch the current status*. (Maybe a duplicate of #10 ?)

@popcornmix 4K seems quite right; cat's output (using -v) is 4049 bytes for each of the 6-second-bursts.
@skgsergio It does. The ouput's speed is quite the same as without piping. However, I cannot use this from node, as the script process terminates immediately after launch.

Any idea how to go about this, as the 4K-buffer flushes come to infrequent?

Thanks again for your time, I am very grateful for the effort you put into omxplayer!

*) I looked into following the logfile, but I cannot use that in its current state, as the logfile grows quite fast, and it is hardcoded to go into a newly created ./omxplayer.log file. Maybe I should file an issue for this?

[popcornmix]

I think this is another instance where connecting to omxplayer through dbus is the best solution.
Look at what this code does:
https://github.com/popcornmix/omxplayer/blob/master/Keyboard.cpp

You should be able to control omxplayer and read back status using that interface.
If there is information missing, it should be possible to add.

[skiqh]

Okay, I'll look into that. Thanks.