Re: [FFmpeg-devel] [PATCH] slicethread: Limit the automatic number of threads to 16

[Lukas Fellechner <Lukas.Fellechner@gmx.net>]

Gesendet: Dienstag, 06. September 2022 um 23:11 Uhr
Von: "Andreas Rheinhardt" <andreas.rheinhardt@outlook.com>
An: ffmpeg-devel@ffmpeg.org
Betreff: Re: [FFmpeg-devel] [PATCH] slicethread: Limit the automatic number of threads to 16
Lukas Fellechner:
>> 1. Running out of address space in 32-bit processes
>>
>> It probably makes sense to limit auto threads to 16, but it should only
>> be done in 32-bit processes. A 64-bit process should never run out of
>> address space. We should not cripple high end machines running
>> 64-bit applications.
>>
>>
>> Sidenotes about "it does not make sense to have more than 16 slices":
>>
>> On 8K video, when using 32 threads, each thread will process 256 lines
>> or about 1MP (> FullHD!). Sure makes sense to me. But even for sw decoding
>> 4K video, having more than 16 threads on a powerful machine makes sense.
>>
>> Intel's next desktop CPUs will have up to 24 physical cores. The
>> proposed change would limit them to use only 16 cores, even on 64-bit.
>
> This part is completely wrong: You can always set the number of threads
> manually.
> (Btw: 1. 8K is the horizontal resolution; the vertical resolution of it
> is 4360 (when using 16:9), so every thread processes 135 lines which
> have as many pixels as 540 lines of FullHD. 2. FullHD has about 2MP.)
> 
> - Andreas

You are right. What I ment was: When someone does not explicitly set
threads, then only 16 of his 24 cores will be used. I know that it is
always possible to manually override the auto value without limits.

And indeed I somehow confused the resolutions. Sill each thread would
process 1MP of pixel data, which is a lot of data.

- Lukas

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