Re: [FFmpeg-devel] [PATCH 1/2] lavc/speedhqdec: Add AV_CODEC_CAP_SLICE_THREADS

["Tomas Härdin" <git@haerdin.se>]

I forgot to mention it is possible to go even further with ||izing the
decoder by separating VLC decode from IDCT. This would affect serial
performance however, since all coefficients would likely not fit in the
innermost cache. For 4k yuv444p this is 51 MiB. Even 1080p yuv420p is
still 6 MiB. For comparison, at the moment only a single DCT block is
kept, which is just 128 bytes

Another possibility could be to have two threads per slice,
interleaving VLC decode and IDCT, using double buffering on more
modestly sized buffers. Two 16-line buffers per pair of threads or 768
KiB for 4k, 3 MiB across all four thread pairs. But, this would require
mutexes between pairs of threads, which execute2() isn't designed for

A final possibility is to do just enough upfront work to find the bit
boundary between 16-line blocks within each slice. That is, doing VLC
decode but not writing coefficients anywhere. Then each group of 16
lines could be made into its own job since we know the exact start and
end bit for all of them. This would degrade serial performance and
performance with 4/8 threads but probably a win for 4k and up with
large numbers of threads

/Tomas
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