[Libav-user] Why does sws_scale runtime depend on calling frequency?

[lukas]

I just checked the CPU frequency with lscpu and indeed when running my
code with the additional sleep, the clock stays at 800 MHz whereas
without sleeps it bursts up to 4.7 GHz. I disabled the clock stepdown
and now the code runs with constant runtime regardless of an additional
sleep.

So, you guys were perfectly right!! Thanks a lot for the hint. This
saved me from dumping this project as a dead end.

My actual application does not use a sleep of course, but instead
captures multiple streams within different threads which wait on a
condition variable. But the problem and solution are the same.

Best Regards,

Lukas


On 3/6/19 2:59 PM, Omar Álvarez wrote:
> It is probably related to how turbo boost works. You need to tune your
> OS for CPU latency (governor performance on Debian/Ubuntu), it has to
> do with the delay changing between low power states and performance
> states. When you sleep continuously, you cannot reach peak CPU clock
> speed. When you don't use the sleep, you are using peak clock speed
> all the time.
>
> Cheers,
>
> Omar.
>
> El sáb., 1 jun. 2019 a las 19:33, Paul Sanderson
> (<nannapie at hotmail.co.uk <mailto:nannapie at hotmail.co.uk>>) escribió:
>
>     I think Carl’s point is interesting. It could be something funky
>     with sleep which if I remember right is a Linux/ Debian based
>     call. Maybe try a different method of making your program hang and
>     see if the problem persists.
>
>     All my best,
>
>     Paul
>
>     Sent from the top a castle using a moderately sized European swallow
>
>     On 1 Jun 2019, at 16:17, Lukas Bommes <lukas.bommes at gmx.de
>     <mailto:lukas.bommes at gmx.de>> wrote:
>
>>     I'm just buffled because OpenCV uses the same code  (in fact I
>>     copied the retrieveFrame funtion from the videoio module 1:1) and
>>     it does not show this weird behaviour. There, the runtime of
>>     sws_scale is constant regardless of the calling frequency and
>>     retrieving one frame takes always around 1.3 ms.
>>     However, my OpenCV build (version 4.1) uses FFMPEG 3.4.6, not
>>     4.1. So, I'm gonna try building my code with FFMPEG 3.4.6 and see
>>     if the problem persists.
>>     Alternatively, I add a function to the OpenCV videoio module
>>     which retrieves frame + motion vectors. However, altering the
>>     OpenCV API brings a lot of other challenges.
>>
>>     But maybe someone else has noticed a similar issue with sws_scale
>>     in FFMPEG 4.1 before.
>>
>>     Lukas
>>
>>
>>     On 1 Jun 2019 7:04 pm, Carl Eugen Hoyos <ceffmpeg at gmail.com
>>     <mailto:ceffmpeg at gmail.com>> wrote:
>>
>>         Am Do., 30. Mai 2019 um 11:59 Uhr schrieb lukas
>>         <lukas.bommes at gmx.de <mailto:lukas.bommes at gmx.de>>:
>>
>>         > I wrote a program which extracts motion vectors from a
>>         video stream and
>>         > encountered the problem that the runtime of sws_scale
>>         changes depending
>>         > on whether I put a sleep command in the main loop of the
>>         caller.
>>
>>         Shouldn't you ask this your CPU provider?
>>
>>         Carl Eugen
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