From: Andre Noll <maan@systemlinux.org>
Date: Mon, 26 Sep 2011 06:27:18 +0000 (+0200)
Subject: vss: Mmap audio files using MAP_POPULATE.
X-Git-Tag: v0.4.9~15^2
X-Git-Url: http://git.tuebingen.mpg.de/?p=paraslash.git;a=commitdiff_plain;h=7bba6232

vss: Mmap audio files using MAP_POPULATE.

This fixes buffer underruns on an old laptop containing a
slow IDE disk. The problem was that getting the next chunk
from the map sometimes hit the disk and took more than 300ms.

This patch adds MAP_POPULATE to the flags for mmap() to
turn on read-ahead for the mapping. This almost fixed the
problem, but some buffer underruns remained. Moreover,
MAP_POPULATE is only available on Linux. To fix also the
remaining cases, we now read one byte from each of the next
few pages in the map after a chunk has been sent. This way
the next chunk should already be cached when it is needed.
---

diff --git a/vss.c b/vss.c
index b9afc8ed..e833543e 100644
--- a/vss.c
+++ b/vss.c
@@ -952,6 +952,10 @@ static int recv_afs_msg(int afs_socket, int *fd, uint32_t *code, uint32_t *data)
 	return 1;
 }
 
+#ifndef MAP_POPULATE
+#define MAP_POPULATE 0
+#endif
+
 static void recv_afs_result(struct vss_task *vsst, fd_set *rfds)
 {
 	int ret, passed_fd, shmid;
@@ -986,8 +990,8 @@ static void recv_afs_result(struct vss_task *vsst, fd_set *rfds)
 	}
 	mmd->size = statbuf.st_size;
 	mmd->mtime = statbuf.st_mtime;
-	ret = para_mmap(mmd->size, PROT_READ, MAP_PRIVATE, passed_fd,
-		0, &vsst->map);
+	ret = para_mmap(mmd->size, PROT_READ, MAP_PRIVATE | MAP_POPULATE,
+		passed_fd, 0, &vsst->map);
 	if (ret < 0)
 		goto err;
 	close(passed_fd);
@@ -1076,6 +1080,23 @@ static void vss_send(struct vss_task *vsst)
 		}
 		mmd->chunks_sent++;
 		mmd->current_chunk++;
+		/*
+		 * Prefault next chunk(s)
+		 *
+		 * If the backing device of the memory-mapped audio file is
+		 * slow and read-ahead is turned off or prevented for some
+		 * reason, e.g. due to memory pressure, it may take much longer
+		 * than the chunk interval to get the next chunk on the wire,
+		 * causing buffer underruns on the client side. Mapping the
+		 * file with MAP_POPULATE seems to help a bit, but it does not
+		 * eliminate the delays completely. Moreover, it is supported
+		 * only on Linux. So we do our own read-ahead here.
+		 */
+		buf += len;
+		for (i = 0; i < 5 && buf < vsst->map + mmd->size; i++) {
+			__a_unused volatile char x = *buf;
+			buf += 4096;
+		}
 	}
 }