Re: [FFmpeg-devel] [PATCH 2/2] aviobuf: Avoid clearing the whole buffer in fill_buffer

[Marton Balint <cus@passwd.hu>]

On Fri, 24 Mar 2023, Martin Storsjö wrote:

> On Fri, 24 Mar 2023, Marton Balint wrote:
>
>>  I am uneasy about complicating an already complicated and hard-to-follow
>>  AVIO layer with heuristics which activate on magic behaviour. And we all
>>  know how long temporary solutions last :)
>>
>>  I guess we could add some new parameter to AVIOContext end enable this
>>  data-shifting behaviour explicitly when you reconfigure the buffer size
>>  for index in the MOV demuxer. But is it worth it? How significant is the
>>  "improvement" this patch provides over the previous one in the series?
>
> With the 2.6 GB, 40 minute mov file I'm looking at, originally, due to the 
> issue fixed in patch 1/2, the buffer size was never increased from the 
> original 32 KB, so when reading the file linearly, we would do many tens of 
> thousands of seek requests, giving absolutely abysmal performance. (I saw a 
> server side log number saying 120 000 requests.)
>
> With patch 1/2 applied, while reading the bulk of the file, it does ~170 
> seeks. So nothing terrible, but it still feels unnecessarily inefficient to 
> do >4 seeks per minute due to the fact that the aviobuf layer is throwing 
> away good data that it already had buffered.
>
> In this case, it used a buffer size of 16 MB, and calculating 2.6 GB / 16 MB 
> ends up very near 170. So every time the 16 MB aviobuf buffer gets full and 
> aviobuf clears it, we end up doing a seek backwards.

Thanks for the details. Patch/1 already made the significant improvement, 
so yeah, I am not sure about Patch/2 knowing it is not the "right" way.

Regards,
Marton


>
> With patch 2/2 applied, we no longer do any seeks while reading the bulk of 
> the file (at the start/end of the file there are still a bunch of scattered 
> seeks though).
>
> // Martin
>
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