On Tue, Apr 22, 2014 at 8:52 PM, Ben Avison <[email protected]> wrote: > 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 avconv > 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). > --- > libavcodec/vc1_parser.c | 276 > ++++++++++++++++++++++++++++++----------------- > 1 files changed, 175 insertions(+), 101 deletions(-) > > diff --git a/libavcodec/vc1_parser.c b/libavcodec/vc1_parser.c > index 1bedd98..713ffff 100644 > --- a/libavcodec/vc1_parser.c > +++ b/libavcodec/vc1_parser.c > @@ -30,117 +30,84 @@ > #include "vc1.h" > #include "get_bits.h" > > +/** The maximum number of bytes of a sequence, entry point or > + * frame header whose values we pay any attention to */ > +#define UNESCAPED_THRESHOLD 37 > + > +/** The maximum number of bytes of a sequence, entry point or > + * frame header which must be valid memory (because they are > + * used to update the bitstream cache in skip_bits() calls) > + */ > +#define UNESCAPED_LIMIT 144 > + > +typedef enum { > + NO_MATCH, > + ONE_ZERO, > + TWO_ZEROS, > + ONE > +} VC1ParseSearchState; > + > typedef struct { > ParseContext pc; > VC1Context v; > + uint8_t prev_start_code; > + size_t bytes_to_skip; > + uint8_t unesc_buffer[UNESCAPED_LIMIT]; > + size_t unesc_index; > + VC1ParseSearchState search_state; > } VC1ParseContext; > > -static void vc1_extract_headers(AVCodecParserContext *s, AVCodecContext > *avctx, > - const uint8_t *buf, int buf_size) > +static void vc1_extract_header(AVCodecParserContext *s, AVCodecContext > *avctx, > + const uint8_t *buf, int buf_size) > { > + /* Parse the header we just finished unescaping */ > VC1ParseContext *vpc = s->priv_data; > GetBitContext gb; > - const uint8_t *start, *end, *next; > - uint8_t *buf2 = av_mallocz(buf_size + FF_INPUT_BUFFER_PADDING_SIZE); > - > vpc->v.s.avctx = avctx; > vpc->v.parse_only = 1; > - next = buf; > - s->repeat_pict = 0; > - > - for(start = buf, end = buf + buf_size; next < end; start = next){ > - int buf2_size, size; > - > - next = find_next_marker(start + 4, end); > - size = next - start - 4; > - buf2_size = vc1_unescape_buffer(start + 4, size, buf2); > - init_get_bits(&gb, buf2, buf2_size * 8); > - if(size <= 0) continue; > - switch(AV_RB32(start)){ > - case VC1_CODE_SEQHDR: > - ff_vc1_decode_sequence_header(avctx, &vpc->v, &gb); > - break; > - case VC1_CODE_ENTRYPOINT: > - ff_vc1_decode_entry_point(avctx, &vpc->v, &gb); > - break; > - case VC1_CODE_FRAME: > - if(vpc->v.profile < PROFILE_ADVANCED) > - ff_vc1_parse_frame_header (&vpc->v, &gb); > - else > - ff_vc1_parse_frame_header_adv(&vpc->v, &gb); > - > - /* keep AV_PICTURE_TYPE_BI internal to VC1 */ > - if (vpc->v.s.pict_type == AV_PICTURE_TYPE_BI) > - s->pict_type = AV_PICTURE_TYPE_B; > - else > - s->pict_type = vpc->v.s.pict_type; > - > - if (avctx->ticks_per_frame > 1){ > - // process pulldown flags > - s->repeat_pict = 1; > - // Pulldown flags are only valid when 'broadcast' has been > set. > - // So ticks_per_frame will be 2 > - if (vpc->v.rff){ > - // repeat field > - s->repeat_pict = 2; > - }else if (vpc->v.rptfrm){ > - // repeat frames > - s->repeat_pict = vpc->v.rptfrm * 2 + 1; > - } > - } > - > - if (vpc->v.broadcast && vpc->v.interlace && !vpc->v.psf) > - s->field_order = vpc->v.tff ? AV_FIELD_TT : AV_FIELD_BB; > - else > - s->field_order = AV_FIELD_PROGRESSIVE; > - > - break; > - } > - } > + init_get_bits(&gb, buf, buf_size * 8); > + switch (vpc->prev_start_code) { > + case VC1_CODE_SEQHDR & 0xFF: > + ff_vc1_decode_sequence_header(avctx, &vpc->v, &gb); > + break; > + case VC1_CODE_ENTRYPOINT & 0xFF: > + ff_vc1_decode_entry_point(avctx, &vpc->v, &gb); > + break; > + case VC1_CODE_FRAME & 0xFF: > + if(vpc->v.profile < PROFILE_ADVANCED) > + ff_vc1_parse_frame_header (&vpc->v, &gb); > + else > + ff_vc1_parse_frame_header_adv(&vpc->v, &gb); > > - av_free(buf2); > -} > + /* keep AV_PICTURE_TYPE_BI internal to VC1 */ > + if (vpc->v.s.pict_type == AV_PICTURE_TYPE_BI) > + s->pict_type = AV_PICTURE_TYPE_B; > + else > + s->pict_type = vpc->v.s.pict_type; > > -/** > - * Find the end of the current frame in the bitstream. > - * @return the position of the first byte of the next frame, or -1 > - */ > -static int vc1_find_frame_end(ParseContext *pc, const uint8_t *buf, > - int buf_size) { > - int pic_found, i; > - uint32_t state; > - > - pic_found= pc->frame_start_found; > - state= pc->state; > - > - i=0; > - if(!pic_found){ > - for(i=0; i<buf_size; i++){ > - state= (state<<8) | buf[i]; > - if(state == VC1_CODE_FRAME || state == VC1_CODE_FIELD){ > - i++; > - pic_found=1; > - break; > + if (avctx->ticks_per_frame > 1){ > + // process pulldown flags > + s->repeat_pict = 1; > + // Pulldown flags are only valid when 'broadcast' has been set. > + // So ticks_per_frame will be 2 > + if (vpc->v.rff){ > + // repeat field > + s->repeat_pict = 2; > + }else if (vpc->v.rptfrm){ > + // repeat frames > + s->repeat_pict = vpc->v.rptfrm * 2 + 1; > } > + }else{ > + s->repeat_pict = 0; > } > - } > > - if(pic_found){ > - /* EOF considered as end of frame */ > - if (buf_size == 0) > - return 0; > - for(; i<buf_size; i++){ > - state= (state<<8) | buf[i]; > - if(IS_MARKER(state) && state != VC1_CODE_FIELD && state != > VC1_CODE_SLICE){ > - pc->frame_start_found=0; > - pc->state=-1; > - return i-3; > - } > - } > + if (vpc->v.broadcast && vpc->v.interlace && !vpc->v.psf) > + s->field_order = vpc->v.tff ? AV_FIELD_TT : AV_FIELD_BB; > + else > + s->field_order = AV_FIELD_PROGRESSIVE; > + > + break; > } > - pc->frame_start_found= pic_found; > - pc->state= state; > - return END_NOT_FOUND; > } > > static int vc1_parse(AVCodecParserContext *s, > @@ -148,22 +115,125 @@ static int vc1_parse(AVCodecParserContext *s, > const uint8_t **poutbuf, int *poutbuf_size, > const uint8_t *buf, int buf_size) > { > + /* Here we do the searching for frame boundaries and headers at > + * the same time. Only a minimal amount at the start of each > + * header is unescaped. */ > VC1ParseContext *vpc = s->priv_data; > - int next; > + int pic_found = vpc->pc.frame_start_found; > + uint8_t *unesc_buffer = vpc->unesc_buffer; > + size_t unesc_index = vpc->unesc_index; > + VC1ParseSearchState search_state = vpc->search_state; > + int next = END_NOT_FOUND; > + int i = vpc->bytes_to_skip; > + > + if (pic_found && buf_size == 0) { > + /* EOF considered as end of frame */ > + memset(unesc_buffer + unesc_index, 0, UNESCAPED_THRESHOLD - > unesc_index); > + vc1_extract_header(s, avctx, unesc_buffer, unesc_index); > + next = 0; > + } > + while (i < buf_size) { > + int start_code_found = 0; > + uint8_t b; > + while (i < buf_size && unesc_index < UNESCAPED_THRESHOLD) { > + b = buf[i++]; > + unesc_buffer[unesc_index++] = b; > + if (search_state <= ONE_ZERO) > + search_state = b ? NO_MATCH : search_state + 1; > + else if (search_state == TWO_ZEROS) { > + if (b == 1) > + search_state = ONE; > + else if (b > 1) { > + if (b == 3) > + unesc_index--; // swallow emulation prevention byte > + search_state = NO_MATCH; > + } > + } > + else { // search_state == ONE > + // Header unescaping terminates early due to detection of > next start code > + search_state = NO_MATCH; > + start_code_found = 1; > + break; > + } > + } > + if ((s->flags & PARSER_FLAG_COMPLETE_FRAMES) && > + unesc_index >= UNESCAPED_THRESHOLD && > + vpc->prev_start_code == (VC1_CODE_FRAME & 0xFF)) > + { > + // No need to keep scanning the rest of the buffer for > + // start codes if we know it contains a complete frame and > + // we've already unescaped all we need of the frame header > + vc1_extract_header(s, avctx, unesc_buffer, unesc_index); > + break; > + } > + if (unesc_index >= UNESCAPED_THRESHOLD && !start_code_found) { > + while (i < buf_size) { > + if (search_state == NO_MATCH) { > + i += vpc->v.vc1dsp.vc1_find_start_code_candidate(buf + > i, buf_size - i); > + if (i < buf_size) { > + search_state = ONE_ZERO; > + } > + i++; > + } else { > + b = buf[i++]; > + if (search_state == ONE_ZERO) > + search_state = b ? NO_MATCH : TWO_ZEROS; > + else if (search_state == TWO_ZEROS) { > + if (b >= 1) > + search_state = b == 1 ? ONE : NO_MATCH; > + } > + else { // search_state == ONE > + search_state = NO_MATCH; > + start_code_found = 1; > + break; > + } > + } > + } > + } > + if (start_code_found) { > + vc1_extract_header(s, avctx, unesc_buffer, unesc_index); > + > + vpc->prev_start_code = b; > + unesc_index = 0; > + > + if (!(s->flags & PARSER_FLAG_COMPLETE_FRAMES)) { > + if (!pic_found && (b == (VC1_CODE_FRAME & 0xFF) || b == > (VC1_CODE_FIELD & 0xFF))) { > + pic_found = 1; > + } > + else if (pic_found && b != (VC1_CODE_FIELD & 0xFF) && b != > (VC1_CODE_SLICE & 0xFF)) { > + next = i - 4; > + pic_found = b == (VC1_CODE_FRAME & 0xFF); > + break; > + } > + } > + } > + } > > - if(s->flags & PARSER_FLAG_COMPLETE_FRAMES){ > - next= buf_size; > - }else{ > - next= vc1_find_frame_end(&vpc->pc, buf, buf_size); > + vpc->pc.frame_start_found = pic_found; > + vpc->unesc_index = unesc_index; > + vpc->search_state = search_state; > > + if (s->flags & PARSER_FLAG_COMPLETE_FRAMES) { > + next = buf_size; > + } else { > if (ff_combine_frame(&vpc->pc, next, &buf, &buf_size) < 0) { > + vpc->bytes_to_skip = 0; > *poutbuf = NULL; > *poutbuf_size = 0; > return buf_size; > } > } > > - vc1_extract_headers(s, avctx, buf, buf_size); > + /* If we return with a valid pointer to a combined frame buffer > + * then on the next call then we'll have been unhelpfully rewound > + * by up to 4 bytes (depending upon whether the start code > + * overlapped the input buffer, and if so by how much). We don't > + * want this: it will either cause spurious second detections of > + * the start code we've already seen, or cause extra bytes to be > + * inserted at the start of the unescaped buffer. */ > + vpc->bytes_to_skip = 4; > + if (next < 0) > + vpc->bytes_to_skip += next; > > *poutbuf = buf; > *poutbuf_size = buf_size; > @@ -194,6 +264,10 @@ static av_cold int vc1_parse_init(AVCodecParserContext > *s) > { > VC1ParseContext *vpc = s->priv_data; > vpc->v.s.slice_context_count = 1; > + vpc->prev_start_code = 0; > + vpc->bytes_to_skip = 0; > + vpc->unesc_index = 0; > + vpc->search_state = NO_MATCH; > return ff_vc1_init_common(&vpc->v); > }
So since the results are good and fate passes, can we merge it? -- Vittorio _______________________________________________ libav-devel mailing list [email protected] https://lists.libav.org/mailman/listinfo/libav-devel
