• Ben Avison's avatar
    vc-1: Optimise parser (with special attention to ARM) · a0d7f9ec
    Ben Avison authored
    The previous implementation of the parser made four passes over each input
    buffer (reduced to two if the container format already guaranteed the input
    buffer corresponded to frames, such as with MKV). But these buffers are
    often 200K in size, certainly enough to flush the data out of L1 cache, and
    for many CPUs, all the way out to main memory. The passes were:
    
    1) locate frame boundaries (not needed for MKV etc)
    2) copy the data into a contiguous block (not needed for MKV etc)
    3) locate the start codes within each frame
    4) unescape the data between start codes
    
    After this, the unescaped data was parsed to extract certain header fields,
    but because the unescape operation was so large, this was usually also
    effectively operating on uncached memory. Most of the unescaped data was
    simply thrown away and never processed further. Only step 2 - because it
    used memcpy - was using prefetch, making things even worse.
    
    This patch reorganises these steps so that, aside from the copying, the
    operations are performed in parallel, maximising cache utilisation. No more
    than the worst-case number of bytes needed for header parsing is unescaped.
    Most of the data is, in practice, only read in order to search for a start
    code, for which optimised implementations already existed in the H264 codec
    (notably the ARM version uses prefetch, so we end up doing both remaining
    passes at maximum speed). For MKV files, we know when we've found the last
    start code of interest in a given frame, so we are able to avoid doing even
    that one remaining pass for most of the buffer.
    
    In some use-cases (such as the Raspberry Pi) video decode is handled by the
    GPU, but the entire elementary stream is still fed through the parser to
    pick out certain elements of the header which are necessary to manage the
    decode process. As you might expect, in these cases, the performance of the
    parser is significant.
    
    To measure parser performance, I used the same VC-1 elementary stream in
    either an MPEG-2 transport stream or a MKV file, and fed it through ffmpeg
    with -c:v copy -c:a copy -f null. These are the gperftools counts for
    those streams, both filtered to only include vc1_parse() and its callees,
    and unfiltered (to include the whole binary). Lower numbers are better:
    
                    Before          After
    File  Filtered  Mean   StdDev   Mean   StdDev  Confidence  Change
    M2TS  No        861.7  8.2      650.5  8.1     100.0%      +32.5%
    MKV   No        868.9  7.4      731.7  9.0     100.0%      +18.8%
    M2TS  Yes       250.0  11.2     27.2   3.4     100.0%      +817.9%
    MKV   Yes       149.0  12.8     1.7    0.8     100.0%      +8526.3%
    
    Yes, that last case shows vc1_parse() running 86 times faster! The M2TS
    case does show a larger absolute improvement though, since it was worse
    to begin with.
    
    This patch has been tested with the FATE suite (albeit on x86 for speed).
    Signed-off-by: 's avatarMichael Niedermayer <michaelni@gmx.at>
    a0d7f9ec
vc1_parser.c 9.52 KB