[vlc-devel] [PATCH 2/2] srt: Read several chunks per blocking read call

[Olivier Crête]

From: Roman Diouskine <rdiouskine at haivision.com>

libsrt input is asynchonously buffered internally and it makes sense
to empty those receive buffers as much as possible on every signaled
receive event from epoll. Doing so reduces context
switching/re-scheduling and improves performance.
---
 modules/access/srt.c | 62 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 53 insertions(+), 9 deletions(-)

diff --git a/modules/access/srt.c b/modules/access/srt.c
index 5b7734310b..c95123c738 100644
--- a/modules/access/srt.c
+++ b/modules/access/srt.c
@@ -40,6 +40,9 @@
 /* libsrt defines default packet size as 1316 internally
  * so srt module takes same value. */
 #define SRT_DEFAULT_CHUNK_SIZE 1316
+/* Minimum/Maximum chunks to allow reading at a time from libsrt */
+#define SRT_MIN_CHUNKS_TRYREAD 10
+#define SRT_MAX_CHUNKS_TRYREAD 100
 /* The default timeout is -1 (infinite) */
 #define SRT_DEFAULT_POLL_TIMEOUT -1
 /* The default latency is 125
@@ -64,6 +67,7 @@ typedef struct
     bool        b_interrupted;
     char       *psz_host;
     int         i_port;
+    int         i_chunks; /* Number of chunks to allocate in the next read */
 } stream_sys_t;
 
 static void srt_wait_interrupted(void *p_data)
@@ -193,6 +197,11 @@ static bool srt_schedule_reconnect(stream_t *p_stream)
         failed = true;
     }
 
+    /* Reset the number of chunks to allocate as the bitrate of
+     * the stream may have changed.
+     */
+    p_sys->i_chunks = SRT_MIN_CHUNKS_TRYREAD;
+
 out:
     if (failed && p_sys->sock != SRT_INVALID_SOCK)
     {
@@ -221,7 +230,13 @@ static block_t *BlockSRT(stream_t *p_stream, bool *restrict eof)
         return NULL;
     }
 
-    block_t *pkt = block_Alloc( i_chunk_size );
+    if ( p_sys->i_chunks == 0 )
+        p_sys->i_chunks = SRT_MIN_CHUNKS_TRYREAD;
+
+    size_t i_chunk_size_actual = ( i_chunk_size > 0 )
+        ? i_chunk_size : SRT_DEFAULT_CHUNK_SIZE;
+    size_t bufsize = i_chunk_size_actual * p_sys->i_chunks;
+    block_t *pkt = block_Alloc( bufsize );
     if ( unlikely( pkt == NULL ) )
     {
         return NULL;
@@ -259,20 +274,49 @@ static block_t *BlockSRT(stream_t *p_stream, bool *restrict eof)
                 continue;
         }
 
-        int stat = srt_recvmsg( p_sys->sock,
-            (char *)pkt->p_buffer, i_chunk_size );
-        if ( stat > 0 )
+        /* Try to get as much data as possible out of the lib, if there
+         * is still some left, increase the number of chunks to read so that
+         * it will read faster on the next iteration. This way the buffer will
+         * grow until it reads fast enough to keep the library empty after
+         * each iteration.
+         */
+        pkt->i_buffer = 0;
+        while ( ( bufsize - pkt->i_buffer ) >= i_chunk_size_actual )
+        {
+            int stat = srt_recvmsg( p_sys->sock,
+                (char *)( pkt->p_buffer + pkt->i_buffer ),
+                bufsize - pkt->i_buffer );
+            if ( stat <= 0 )
+            {
+                break;
+            }
+            pkt->i_buffer += (size_t)stat;
+        }
+
+        msg_Dbg ( p_stream, "Read %zu bytes out of a max of %zu"
+            " (%d chunks of %zu bytes)", pkt->i_buffer,
+            p_sys->i_chunks * i_chunk_size_actual, p_sys->i_chunks,
+                 i_chunk_size_actual );
+
+
+        /* Gradually adjust number of chunks we read at a time
+        * up to a predefined maximum. The actual number we might
+        * settle on depends on stream's bit rate.
+        */
+        size_t rem = bufsize - pkt->i_buffer;
+        if ( rem < i_chunk_size_actual )
         {
-            pkt->i_buffer = stat;
-            goto out;
+            if ( p_sys->i_chunks < SRT_MAX_CHUNKS_TRYREAD )
+            {
+                p_sys->i_chunks++;
+            }
         }
 
         goto out;
     }
 
-    /* if the poll reports errors for any reason at all
-     * including a timeout, or there is a read error,
-     * we skip the turn.
+    /* if the poll reports errors for any reason at all,
+     * including a timeout, we skip the turn.
      */
 
     block_Release(pkt);
-- 
2.17.1