Add row-mt option to libaom-av1

[Ewout]

Like libvpx libaom can use the row-mt option to allow row based multi-threading within tiles. FFmpeg has the row-mt option integrated with libvpx, it would be great if it could also be added to libaom-av1.

[Hendrik]

row-mt is enabled by default in libaom. An option should not be needed.

[Ewout]

In the latest build of aomenc 1.0.0-1009-g​2d7eb8bce it's not enabled by default:
--row-mt=<arg> Enable row based multi-threading (0: off (default), 1: on)

Also, when running ffmpeg (20181205-2c64a6b) CPU usage never spikes above 35%, averaging around 27% on a quad-core machine.

[James]

You should use either the -tiles or -tile-columns and -tile-rows options to encode a file with more than one tile if you want any multi threading from libaom, in any case.

[Ewout]

I agree that encoding with tiles is still preferred for decoding performance ([data ​https://docs.google.com/spreadsheets/d/1AO3lDZnpC8pNJffOknY1rIxXwLog_ISwHhO_sv3Xlhg/edit#gid=1957344704]). But with -tile-columns 1 the CPU spiked at 32% usage, while with -tile-columns 2 and -tiles 2x2 the max CPU usage was 53%. More than 4 tiles is not necessary to reach great decode performance, so row-mt could still provide a big speedup in encoding performance.

Edit: Excuse me, didn't mean to revert the properties of the ticket.

[James]

Fixed in d12d4d45153e45e5b91ee079045f8fcde3537b33.