Batch-decode a list of *.MKV videos into *.WEBM (VP9) with deinterlace filter, using multiple parallel instances of FFmpeg

decode.pl

#!/usr/bin/perl -w
#
# Batch-decode a list of *.MKV videos into *.WEBM (VP9) with deinterlace filter,
# using multiple parallel instances of FFmpeg.
#

use threads;

# XXX The number of parallel threads to use.
my($nthreads) = 8;

sub thread_func
{
	my($file) = shift;
	my($tid) = threads->tid();

	my($basename) = $file;
	$basename =~ s/\.mkv$//g;
	my($output) = "$basename.webm";

	print "Decoding $file pass 1\n";
	system("ffmpeg -y -i \"$file\" -c:v libvpx-vp9 -pass 1 -passlogfile $tid -b:v 1000K -threads 8 -speed 4 -tile-columns 6 -frame-parallel 1 -an -f webm /dev/null");
	print "Decoding $file pass 2\n";
	system("ffmpeg -i \"$file\" -vf yadif -c:v libvpx-vp9 -pass 2 -passlogfile $tid -b:v 1000K -threads 8 -speed 1 -tile-columns 6 -frame-parallel 1 -auto-alt-ref 1 -lag-in-frames 25 -c:a libopus -b:a 96k -f webm \"$output\"");
}

my(@files) = split("\n", `find . -name "*.mkv"`);

# Keep array of threads if you want to gracefully exit the
# app by waiting for them all to finish
my(@threads) = ();

foreach $file (@files)
{
	my($basename) = $file;
	$basename =~ s/\.mkv$//g;
	my($output) = "$basename.webm";

	if (-e "$output")
	{
		print "Skipping \"$file\" which is already done\n";		
		next;
	}

	push(@threads, threads->create(\&thread_func, $file));
	
	if (scalar(@threads) >= $nthreads)
	{
		my($blocked) = 1;
		while ($blocked)
		{
			foreach $thread (@threads)
			{
				if (not $thread->is_running())
				{
					$blocked = 0;
					$thread->join();
					@threads = grep { $_ ne $thread } @threads;
					last;
				}
			}
			
			sleep(10);
		}
	}
}

foreach (@threads)
{
	$_->join();
}