[FFmpeg-devel] [PATCH] lavf/mux: add the flush_packets option.

[Nicolas George]

Signed-off-by: Nicolas George <nicolas.george at normalesup.org>
---
 doc/ffmpeg-formats.texi     |    5 +++++
 libavformat/avformat.h      |    7 +++++++
 libavformat/mux.c           |    2 ++
 libavformat/options_table.h |    1 +
 4 files changed, 15 insertions(+)


Reimar Döffinger wrote:
> I see we are doing this in a lot of encoders, but this seems just bad to
> me.
> While it reduces latency, it also reduces performance in some use-cases.

I believe the cases where the performance hit would be non-negligible are
very particular; enough to make flushing the default behaviour.

> In addition, it is just silly code duplication to have this inside
> (almost) every single muxer.
> Extracting it in some way and possibly making it configurable seems
> like it would be a good thing to do long-term.

I believe you are right. New patch attached.

Regards,

-- 
  Nicolas George


diff --git a/doc/ffmpeg-formats.texi b/doc/ffmpeg-formats.texi
index 30cf415..ea61725 100644
--- a/doc/ffmpeg-formats.texi
+++ b/doc/ffmpeg-formats.texi
@@ -142,6 +142,11 @@ Use wallclock as timestamps.
 @item avoid_negative_ts @var{integer} (@emph{output})
 Shift timestamps to make them positive. 1 enables, 0 disables, default
 of -1 enables when required by target format.
+
+ at item flush_packets @var{integer} (@emph{output})
+Flush the underlying I/O stream after each packet. Default 1 enables it, and
+has the effect of reducing the latency; 0 disables it and may slightly
+increase performance in some cases.
 @end table
 
 @c man end FORMAT OPTIONS
diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 0fc71aa..ce4b8af 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -1217,6 +1217,13 @@ typedef struct AVFormatContext {
      */
     int seek2any;
 
+    /**
+     * Flush the I/O context after each packet.
+     * - encoding: Set by user via AVOPtions (NO direct access)
+     * - decoding: unused
+     */
+    int flush_packets;
+
     /*****************************************************************
      * All fields below this line are not part of the public API. They
      * may not be used outside of libavformat and can be changed and
diff --git a/libavformat/mux.c b/libavformat/mux.c
index 0d74d96..46684cc 100644
--- a/libavformat/mux.c
+++ b/libavformat/mux.c
@@ -494,6 +494,8 @@ static inline int split_write_packet(AVFormatContext *s, AVPacket *pkt)
 
     did_split = av_packet_split_side_data(pkt);
     ret = s->oformat->write_packet(s, pkt);
+    if (s->flush_packets && s->pb && s->pb->error >= 0)
+        avio_flush(s->pb);
     if (did_split)
         av_packet_merge_side_data(pkt);
     return ret;
diff --git a/libavformat/options_table.h b/libavformat/options_table.h
index 6750050..9f86137 100644
--- a/libavformat/options_table.h
+++ b/libavformat/options_table.h
@@ -75,6 +75,7 @@ static const AVOption options[]={
 {"avoid_negative_ts", "shift timestamps to make them positive. 1 enables, 0 disables, default of -1 enables when required by target format.", OFFSET(avoid_negative_ts), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 1, E},
 {"skip_initial_bytes", "skip initial bytes", OFFSET(skip_initial_bytes), AV_OPT_TYPE_INT, {.i64 = 0}, 0, INT_MAX-1, D},
 {"correct_ts_overflow", "correct single timestamp overflows", OFFSET(correct_ts_overflow), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, D},
+{"flush_packets", "enable flushing of the I/O context after each packet", OFFSET(flush_packets), AV_OPT_TYPE_INT, {.i64 = 1}, 0, 1, E},
 {NULL},
 };
 
-- 
1.7.10.4