Re: [FFmpeg-devel] [PATCH 4/8] avformat/flvenc: Use array instead of linked list for index

["Tomas Härdin" <tjoppen@acc.umu.se>]

tis 2022-07-05 klockan 22:26 +0200 skrev Andreas Rheinhardt:
> Using a linked list had very much overhead (the pointer to the next
> entry increased the size of the index entry struct from 16 to 24
> bytes,
> not to mention the overhead of having separate allocations), so it is
> better to (re)allocate a continuous array for the index.
> av_fast_realloc_array() is used for this purpose, in order not to
> reallocate the array for each entry. It's feature of allowing to
> restrict the number of elements of the array is put to good use here:
> Each entry will lead to 18 bytes being written and the array is
> contained in an element whose size field has a length of three bytes,
> so that more than (2^24 - 1) / 18 entries make no sense.
> 

> +    /* The filepositions array is part of a metadata element whose
> size field
> +     * is three bytes long; so bound the number of filepositions in
> order not
> +     * to allocate more than could ever be written. */
> +    int ret = av_fast_realloc_array(&flv->filepositions,
> +                                    &flv->filepositions_allocated,
> +                                    flv->filepositions_count + 1,
> +                                    (((1 << 24) - 1) - 10) /

Why -10? I see it bumps max_nb down from 932067 to 932066, but it make
no difference keeping below 1<<24

Looks like the rest of the patch makes the code faster and easier to
read. Very nice.

/Tomas


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