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

[Omar Álvarez]

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>)
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> 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> wrote:
>
> Am Do., 30. Mai 2019 um 11:59 Uhr schrieb lukas <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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