make socket writing buffered

the primary objective is reducing the number of small SSL packets (which
are always padded), but fewer syscalls in the non-SSL case should be
good as well.

src/socket.c

@@ -281,6 +281,7 @@ static void start_tls_p3( conn_t *conn, int ok )
 #endif /* HAVE_LIBSSL */
 
 static void socket_fd_cb( int, void * );
+static void socket_fake_cb( void * );
 
 static void socket_connect_one( conn_t * );
 static void socket_connect_failed( conn_t * );
@@ -293,12 +294,14 @@ socket_open_internal( conn_t *sock, int fd )
 	sock->fd = fd;
 	fcntl( fd, F_SETFL, O_NONBLOCK );
 	init_notifier( &sock->notify, fd, socket_fd_cb, sock );
+	init_wakeup( &sock->fd_fake, socket_fake_cb, sock );
 }
 
 static void
 socket_close_internal( conn_t *sock )
 {
 	wipe_notifier( &sock->notify );
+	wipe_wakeup( &sock->fd_fake );
 	close( sock->fd );
 	sock->fd = -1;
 }
@@ -500,6 +503,8 @@ socket_close( conn_t *sock )
 #endif
 	while (sock->write_buf)
 		dispose_chunk( sock );
+	free( sock->append_buf );
+	sock->append_buf = 0;
 }
 
 static void
@@ -609,8 +614,6 @@ dispose_chunk( conn_t *conn )
 	buff_chunk_t *bc = conn->write_buf;
 	if (!(conn->write_buf = bc->next))
 		conn->write_buf_append = &conn->write_buf;
-	if (bc->data != bc->buf)
-		free( bc->data );
 	free( bc );
 }
 
@@ -641,48 +644,69 @@ do_queued_write( conn_t *conn )
 }
 
 static void
-do_append( conn_t *conn, char *buf, int len, ownership_t takeOwn )
+do_append( conn_t *conn, buff_chunk_t *bc )
 {
-	buff_chunk_t *bc;
-
-	if (takeOwn == GiveOwn) {
-		bc = nfmalloc( offsetof(buff_chunk_t, buf) );
-		bc->data = buf;
-	} else {
-		bc = nfmalloc( offsetof(buff_chunk_t, buf) + len );
-		bc->data = bc->buf;
-		memcpy( bc->data, buf, len );
-	}
-	bc->len = len;
 	bc->next = 0;
 	*conn->write_buf_append = bc;
 	conn->write_buf_append = &bc->next;
 }
 
+/* This is big enough to avoid excessive chunking, but is
+ * sufficiently small to keep SSL latency low with a slow uplink. */
+#define WRITE_CHUNK_SIZE 1024
+
 int
 socket_write( conn_t *conn, conn_iovec_t *iov, int iovcnt )
 {
-	for (; iovcnt; iovcnt--, iov++) {
+	int i, buf_avail, len, offset = 0, total = 0;
-		if (conn->write_buf) {
+	buff_chunk_t *bc, *exwb = conn->write_buf;
-			do_append( conn, iov->buf, iov->len, iov->takeOwn );
+
+	for (i = 0; i < iovcnt; i++)
+		total += iov[i].len;
+	bc = conn->append_buf;
+	if (bc && total >= WRITE_CHUNK_SIZE) {
+		/* If the new data is too big, queue the pending buffer to avoid latency. */
+		do_append( conn, bc );
+		bc  = 0;
+	}
+	while (total) {
+		if (!bc) {
+			buf_avail = total > WRITE_CHUNK_SIZE ? total : WRITE_CHUNK_SIZE;
+			bc = nfmalloc( offsetof(buff_chunk_t, data) + buf_avail );
+			bc->len = 0;
 		} else {
-			int n = do_write( conn, iov->buf, iov->len );
+			/* A pending buffer will always be of standard size - over-sized
-			if (n < 0) {
+			 * buffers are immediately filled and queued. */
-				do {
+			buf_avail = WRITE_CHUNK_SIZE - bc->len;
-					if (iov->takeOwn == GiveOwn)
+		}
-						free( iov->buf );
+		while (total) {
-					iovcnt--, iov++;
+			len = iov->len - offset;
-				} while (iovcnt);
+			if (len > buf_avail)
-				return -1;
+				len = buf_avail;
+			memcpy( bc->data + bc->len, iov->buf + offset, len );
+			bc->len += len;
+			buf_avail -= len;
+			offset += len;
+			total -= len;
+			if (offset == iov->len) {
+				if (iov->takeOwn == GiveOwn)
+					free( iov->buf );
+				iov++;
+				offset = 0;
 			}
-			if (n != iov->len) {
+			if (!buf_avail) {
-				conn->write_offset = n;
+				do_append( conn, bc );
-				do_append( conn, iov->buf, iov->len, iov->takeOwn );
+				bc = 0;
-			} else if (iov->takeOwn == GiveOwn) {
+				break;
-				free( iov->buf );
 			}
 		}
 	}
+	conn->append_buf = bc;
+	/* Queue the pending write once the main loop goes idle. */
+	conf_wakeup( &conn->fd_fake, bc ? 0 : -1 );
+	/* If no writes were queued before, ensure that flushing commences. */
+	if (!exwb)
+		return do_queued_write( conn );
 	return 0;
 }
 
@@ -733,6 +757,19 @@ socket_fd_cb( int events, void *aux )
 		socket_fill( conn );
 }
 
+static void
+socket_fake_cb( void *aux )
+{
+	conn_t *conn = (conn_t *)aux;
+
+	buff_chunk_t *exwb = conn->write_buf;
+	do_append( conn, conn->append_buf );
+	conn->append_buf = 0;
+	/* If no writes were queued before, ensure that flushing commences. */
+	if (!exwb)
+		do_queued_write( conn );
+}
+
 #ifdef HAVE_LIBSSL
 static void
 ssl_fake_cb( void *aux )

src/socket.h

@@ -57,9 +57,8 @@ typedef struct server_conf {
 
 typedef struct buff_chunk {
 	struct buff_chunk *next;
-	char *data;
 	int len;
-	char buf[1];
+	char data[1];
 } buff_chunk_t;
 
 typedef struct {
@@ -88,8 +87,10 @@ typedef struct {
 	void *callback_aux;
 
 	notifier_t notify;
+	wakeup_t fd_fake;
 
 	/* writing */
+	buff_chunk_t *append_buf; /* accumulating buffer */
 	buff_chunk_t *write_buf, **write_buf_append; /* buffer head & tail */
 	int write_offset; /* offset into buffer head */