This patchset speeds up mxfdec in various ways. The test file has been
generated with
ffmpeg -t 10000 -f lavfi -i testsrc -s 160x120 out2.mxf
Performance is measured with callgrind using the command
valgrind --tool=callgrind ./ffmpeg_g -loglevel quiet -i out2.mxf -codec copy
-f null -
The callgraph has then been inspected using kcachegrind. The results
are as follows:
task_wrapper
5 812 306 937 a577d31
5 669 552 343 Speed up klv_read_packet()
5 648 440 947 Add and use IS_KLV_KEY_FAST() in some places
5 633 846 074 Add and use mxf_is_encrypted_triplet_key()
3 667 721 703 Speed up mxf_edit_unit_absolute_offset()
3 587 869 726 Remove a call to avio_tell() in klv_read_packet()
mxf_read_packet (250 001 calls)
3 821 662 859 a577d31
3 665 058 265 Speed up klv_read_packet()
3 647 320 931 Add and use IS_KLV_KEY_FAST() in some places
3 624 081 036 Add and use mxf_is_encrypted_triplet_key()
1 660 495 552 Speed up mxf_edit_unit_absolute_offset()
1 592 469 709 Remove a call to avio_tell() in klv_read_packet()
The biggest difference is made by speeding up
mxf_edit_unit_absolute_offset(). Here's how many cycles it uses before
and after patch 4:
mxf_edit_unit_absolute_offset.constprop.31
2 076 774 255 Add and use mxf_is_encrypted_triplet_key()
105 047 878 Speed up mxf_edit_unit_absolute_offset()
Of the remaining cycles in mxf_edit_unit_absolute_offset(), 67 882 294
are spent in mxf_absolute_bodysid_offset() (272 cycles per call). Since
it already does a binary search it didn't seem worthwhile to mess with.
Patches 2 and 3 are somewhat dubious, but I've included them anyway to
get some feedback. We could both speed up the demuxer and cut down on
.text by omitting the first 4 bytes of every key.
/Tomas
From da4daac750955ccdf578c703fca7a90c93f7a1a8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomas=20H=C3=A4rdin?= <g...@haerdin.se>
Date: Sat, 14 Sep 2024 11:48:09 +0200
Subject: [PATCH 1/5] lavf/mxfdec: Speed up klv_read_packet()
---
libavformat/mxfdec.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/libavformat/mxfdec.c b/libavformat/mxfdec.c
index 24f4ed1c33..99bf352e00 100644
--- a/libavformat/mxfdec.c
+++ b/libavformat/mxfdec.c
@@ -458,10 +458,26 @@ static int mxf_read_sync(AVIOContext *pb, const uint8_t *key, unsigned size)
return i == size;
}
+// special case of mxf_read_sync for mxf_klv_key
+static int mxf_read_sync_klv(AVIOContext *pb)
+{
+ uint32_t key = avio_rb32(pb);
+ // key will never match mxf_klv_key on EOF
+ if (key == AV_RB32(mxf_klv_key))
+ return 1;
+
+ while (!avio_feof(pb)) {
+ key = (key << 8) | avio_r8(pb);
+ if (key == AV_RB32(mxf_klv_key))
+ return 1;
+ }
+ return 0;
+}
+
static int klv_read_packet(MXFContext *mxf, KLVPacket *klv, AVIOContext *pb)
{
int64_t length, pos;
- if (!mxf_read_sync(pb, mxf_klv_key, 4))
+ if (!mxf_read_sync_klv(pb))
return AVERROR_INVALIDDATA;
klv->offset = avio_tell(pb) - 4;
if (klv->offset < mxf->run_in)
@@ -3982,6 +3998,7 @@ static int mxf_read_packet(AVFormatContext *s, AVPacket *pkt)
ret = klv_read_packet(mxf, &klv, s->pb);
if (ret < 0)
break;
+ // klv.key[0..3] == mxf_klv_key from here forward
max_data_size = klv.length;
pos = klv.next_klv - klv.length;
PRINT_KEY(s, "read packet", klv.key);
--
2.39.2
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