From: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Date: Mon, 17 Dec 2007 12:12:25 +0000 (+0100)
Subject: 01_DCCP_shutdown-unnecessary.diff
X-Git-Tag: v0.3.0~48
X-Git-Url: http://git.tue.mpg.de/?a=commitdiff_plain;h=73f02f72cb49baebff3e0f6946750a5bc0693321;p=paraslash.git

01_DCCP_shutdown-unnecessary.diff

This calls shutdown(2) for unused communication paths in the DCCP sender/receiver:
 * the sender does not read data from the client and so calls shutdown(SHUT_RD);
 * the client does not send data to the server and so calls shutdown(SHUT_WR).
The advantage that this buys is a performance optimisation: using shutdown(2) in
DCCP means disabling the corresponding CCID kernel modules, which reduces the
processing costs, i.e.
 * when using SHUT_RD, the receiver congestion-control module is de-activated;
 * when using SHUT_WR, the sender congestion-control module is de-activated.

More information can be found on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/shutdown/
---

diff --git a/dccp_recv.c b/dccp_recv.c
index eeb7c57a..c0618602 100644
--- a/dccp_recv.c
+++ b/dccp_recv.c
@@ -61,6 +61,13 @@ static int dccp_recv_open(struct receiver_node *rn)
 
 	if (ret < 0)
 		return ret;
+	/*
+	 * Disable unused CCIDs: the receiver does not send any application data to the
+	 * server. By shutting down this unused path we reduce internal processing costs,
+	 * as the unused CCIDs (in the kernel) are then bypassed.
+	 */
+	if (shutdown(ret, SHUT_WR) < 0)
+		return -ERRNO_TO_PARA_ERROR(errno);
 
 	rn->buf = para_calloc(DCCP_BUFSIZE);
 	rn->private_data = pdd = para_calloc(sizeof(struct private_dccp_recv_data));
diff --git a/dccp_send.c b/dccp_send.c
index 5d8e6593..f3b9120b 100644
--- a/dccp_send.c
+++ b/dccp_send.c
@@ -72,6 +72,15 @@ static void dccp_post_select(fd_set *rfds, __a_unused fd_set *wfds)
 		PARA_ERROR_LOG("%s\n", PARA_STRERROR(-ret));
 		return;
 	}
+	/*
+	 * Bypass unused CCID paths: the sender does not receive application data
+	 * from the client; by shutting down this unused communication path we can
+	 * reduce processing costs a bit. See analogous comment in dccp_recv.c.
+	 */
+	if (shutdown(ret, SHUT_RD) < 0) {
+		PARA_ERROR_LOG("shutdown(SHUT_RD): %s\n", strerror(errno));
+		return;
+	}
 	dc = para_calloc(sizeof(struct dccp_client));
 	dc->fd = ret;
 	dc->name = make_message("%s", remote_name(dc->fd));