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* RFC: Latency reducing TCP modifications for thin-stream interactive applications
@ 2008-11-27 13:39 ` Andreas Petlund
  0 siblings, 0 replies; 11+ messages in thread
From: Andreas Petlund @ 2008-11-27 13:39 UTC (permalink / raw)
  To: linux-net, linux-kernel, linux-rt-users
  Cc: mdavem, jgarzik, kohari, ilpo.jarvinen, peterz, jzemlin, mrexx,
	tytso, mingo, kristrev, griff, paalh

A wide range of Internet-based services that use reliable transport 
protocols display what we call thin-stream properties. This means 
that the application sends data with such a low rate that the 
retransmission mechanisms of the transport protocol are not fully 
effective. In time-dependent scenarios (like online games, control 
systems or some sensor networks) where the user experience depends 
on the data delivery latency, packet loss can be devastating for 
the service quality. Extreme latencies are caused by TCP's 
dependency on the arrival of new data from the application to trigger 
retransmissions effectively through fast retransmit instead of 
waiting for long timeouts. After analyzing a large number of 
time-dependent interactive applications, we have seen that they 
often produce thin streams (as described above) and also stay with 
this traffic pattern throughout its entire lifespan. The 
combination of time-dependency and the fact that the streams 
provoke high latencies when using TCP is unfortunate.

In order to reduce application-layer latency when packets are lost, 
we have implemented modifications to the TCP retransmission 
mechanisms in the Linux kernel. We have also implemented a 
bundling mechanisms that introduces redundancy in order to 
preempt the experience of packet loss. In short, if the kernel 
detects a thin stream, we trade a small amount of bandwidth for 
latency reduction and apply:

Removal of exponential backoff: To prevent an exponential increase 
in retransmission delay for a repeatedly lost packet, we remove 
the exponential factor.

FASTER Fast Retransmit: Instead of waiting for 3 duplicate 
acknowledgments before sending a fast retransmission, we retransmit 
after receiving only one.

Redundant Data Bundling: We copy (bundle) data from the 
unacknowledged packets in the send buffer into the next packet if
 space is available.

These enhancements are applied only if the stream is detected as
thin. 
This is accomplished by defining thresholds for packet size and 
packets in flight. Also, we consider the redundancy introduced 
by our mechanisms acceptable because the streams are so thin 
that normal congestion mechanisms do not come into effect.

We have implemented these changes in the Linux kernel (2.6.23.8), 
and have tested the modifications on a wide range of different 
thin-stream applications (Skype, BZFlag, SSH, ...) under varying 
network conditions. Our results show that applications which use 
TCP for interactive time-dependent traffic will experience a 
reduction in both maximum and average latency, giving the users 
quicker feedback to their interactions.

Availability of this kind of mechanisms will help provide 
customizability for interactive network services. The quickly 
growing market for Linux gaming may benefit from lowered latency. 
As an example, most of the large MMORPG's today use TCP (like World 
of Warcraft and Age of Conan) and several multimedia applications 
(like Skype) use TCP fallback if UDP is blocked.

The modifications are all TCP standard compliant and transparent
to the receiver. As such, a game server could implement the 
modifications and get a one-way latency benefit without touching 
any of the clients.

In the following papers, we discuss the benefits and tradeoffs of 
the decribed mechanisms:
"The Fun of using TCP for an MMORPG": 
http://simula.no/research/networks/publications/Griwodz.2006.1
"TCP Enhancements For Interactive Thin-Stream Applications": 
http://simula.no/research/networks/publications/Simula.ND.83
"Improving application layer latency for reliable thin-stream game
traffic": http://simula.no/research/networks/publications/Simula.ND.185
"TCP mechanisms for improving the user experience for time-dependent 
thin-stream applications": 
http://simula.no/research/networks/publications/Simula.ND.159
Our presentation from the 2008 Linux-Kongress can be found here: 
http://data.guug.de/slides/lk2008/lk-2008-Andreas-Petlund.pdf

We have included a patch for the 2.6.23.8 kernel which implements the 
modifications. The patch is not properly segmented and formatted, but 
attached as a reference. We are currently working on an updated patch 
set which we hopefully will be able to post in a couple of weeks. This 
will also give us time to integrate any ideas that may arise from the 
discussions here.

We are happy for all feedback regarding this:
Is something like this viable to introduce into the kernel? Is the 
scheme for thin-stream detection mechanism acceptable. Any viewpoints 
on the architecture and design? 


diff -Nur linux-2.6.23.8.vanilla/include/linux/sysctl.h linux-2.6.23.8-tcp-thin/include/linux/sysctl.h
--- linux-2.6.23.8.vanilla/include/linux/sysctl.h	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/linux/sysctl.h	2008-07-03 11:47:21.000000000 +0200
@@ -355,6 +355,11 @@
 	NET_IPV4_ROUTE=18,
 	NET_IPV4_FIB_HASH=19,
 	NET_IPV4_NETFILTER=20,
+	
+	NET_IPV4_TCP_FORCE_THIN_RDB=29,         /* Added @ Simula */
+	NET_IPV4_TCP_FORCE_THIN_RM_EXPB=30,     /* Added @ Simula */
+	NET_IPV4_TCP_FORCE_THIN_DUPACK=31,      /* Added @ Simula */
+	NET_IPV4_TCP_RDB_MAX_BUNDLE_BYTES=32,   /* Added @ Simula */
 
 	NET_IPV4_TCP_TIMESTAMPS=33,
 	NET_IPV4_TCP_WINDOW_SCALING=34,
diff -Nur linux-2.6.23.8.vanilla/include/linux/tcp.h linux-2.6.23.8-tcp-thin/include/linux/tcp.h
--- linux-2.6.23.8.vanilla/include/linux/tcp.h	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/linux/tcp.h	2008-07-02 15:17:38.000000000 +0200
@@ -97,6 +97,10 @@
 #define TCP_CONGESTION		13	/* Congestion control algorithm */
 #define TCP_MD5SIG		14	/* TCP MD5 Signature (RFC2385) */
 
+#define TCP_THIN_RDB            15      /* Added @ Simula - Enable redundant data bundling  */
+#define TCP_THIN_RM_EXPB        16      /* Added @ Simula - Remove exponential backoff  */
+#define TCP_THIN_DUPACK         17      /* Added @ Simula - Reduce number of dupAcks needed */
+
 #define TCPI_OPT_TIMESTAMPS	1
 #define TCPI_OPT_SACK		2
 #define TCPI_OPT_WSCALE		4
@@ -296,6 +300,10 @@
 	u8	nonagle;	/* Disable Nagle algorithm?             */
 	u8	keepalive_probes; /* num of allowed keep alive probes	*/
 
+	u8      thin_rdb;       /* Enable RDB                           */
+	u8      thin_rm_expb;   /* Remove exp. backoff                  */
+	u8      thin_dupack;    /* Remove dupack                        */
+
 /* RTT measurement */
 	u32	srtt;		/* smoothed round trip time << 3	*/
 	u32	mdev;		/* medium deviation			*/
diff -Nur linux-2.6.23.8.vanilla/include/net/sock.h linux-2.6.23.8-tcp-thin/include/net/sock.h
--- linux-2.6.23.8.vanilla/include/net/sock.h	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/net/sock.h	2008-07-02 17:07:10.000000000 +0200
@@ -462,7 +462,10 @@
 
 static inline void sk_stream_free_skb(struct sock *sk, struct sk_buff *skb)
 {
-	skb_truesize_check(skb);
+	/* Modified @ Simula 
+	   skb_truesize_check creates unnecessary 
+	   noise when combined with RDB */
+	//skb_truesize_check(skb);
 	sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
 	sk->sk_wmem_queued   -= skb->truesize;
 	sk->sk_forward_alloc += skb->truesize;
diff -Nur linux-2.6.23.8.vanilla/include/net/tcp.h linux-2.6.23.8-tcp-thin/include/net/tcp.h
--- linux-2.6.23.8.vanilla/include/net/tcp.h	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/include/net/tcp.h	2008-07-03 11:48:54.000000000 +0200
@@ -188,9 +188,19 @@
 #define TCP_NAGLE_CORK		2	/* Socket is corked	    */
 #define TCP_NAGLE_PUSH		4	/* Cork is overridden for already queued data */
 
+/* Added @ Simula - Thin stream support */
+#define TCP_FORCE_THIN_RDB            0 /* Thin streams: exp. backoff   default off */
+#define TCP_FORCE_THIN_RM_EXPB        0 /* Thin streams: dynamic dupack default off */
+#define TCP_FORCE_THIN_DUPACK         0 /* Thin streams: smaller minRTO default off */
+#define TCP_RDB_MAX_BUNDLE_BYTES      0 /* Thin streams: Limit maximum bundled bytes */
+
 extern struct inet_timewait_death_row tcp_death_row;
 
 /* sysctl variables for tcp */
+extern int sysctl_tcp_force_thin_rdb;         /* Added @ Simula */
+extern int sysctl_tcp_force_thin_rm_expb;     /* Added @ Simula */
+extern int sysctl_tcp_force_thin_dupack;      /* Added @ Simula */
+extern int sysctl_tcp_rdb_max_bundle_bytes;   /* Added @ Simula */
 extern int sysctl_tcp_timestamps;
 extern int sysctl_tcp_window_scaling;
 extern int sysctl_tcp_sack;
@@ -723,6 +733,16 @@
 	return (tp->packets_out - tp->left_out + tp->retrans_out);
 }
 
+/* Added @ Simula
+ *
+ * To determine whether a stream is thin or not
+ * return 1 if thin, 0 othervice 
+ */
+static inline unsigned int tcp_stream_is_thin(const struct tcp_sock *tp)
+{
+	return (tp->packets_out < 4 ? 1 : 0);
+}
+
 /* If cwnd > ssthresh, we may raise ssthresh to be half-way to cwnd.
  * The exception is rate halving phase, when cwnd is decreasing towards
  * ssthresh.
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/sysctl_net_ipv4.c linux-2.6.23.8-tcp-thin/net/ipv4/sysctl_net_ipv4.c
--- linux-2.6.23.8.vanilla/net/ipv4/sysctl_net_ipv4.c	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/sysctl_net_ipv4.c	2008-07-03 11:49:59.000000000 +0200
@@ -187,6 +187,38 @@
 }
 
 ctl_table ipv4_table[] = {
+	{       /* Added @ Simula for thin streams */
+                .ctl_name       = NET_IPV4_TCP_FORCE_THIN_RDB,
+                .procname       = "tcp_force_thin_rdb",
+                .data           = &sysctl_tcp_force_thin_rdb,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec
+        },
+	{       /* Added @ Simula for thin streams */
+                .ctl_name       = NET_IPV4_TCP_FORCE_THIN_RM_EXPB,
+                .procname       = "tcp_force_thin_rm_expb",
+                .data           = &sysctl_tcp_force_thin_rm_expb,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec
+        },
+	{       /* Added @ Simula for thin streams */
+                .ctl_name       = NET_IPV4_TCP_FORCE_THIN_DUPACK,
+                .procname       = "tcp_force_thin_dupack",
+                .data           = &sysctl_tcp_force_thin_dupack,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec
+        },
+	{       /* Added @ Simula for thin streams */
+                .ctl_name       = NET_IPV4_TCP_RDB_MAX_BUNDLE_BYTES,
+                .procname       = "tcp_rdb_max_bundle_bytes",
+                .data           = &sysctl_tcp_rdb_max_bundle_bytes,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = &proc_dointvec
+        },
 	{
 		.ctl_name	= NET_IPV4_TCP_TIMESTAMPS,
 		.procname	= "tcp_timestamps",
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp.c	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp.c	2008-07-03 11:51:55.000000000 +0200
@@ -270,6 +270,10 @@
 
 int sysctl_tcp_fin_timeout __read_mostly = TCP_FIN_TIMEOUT;
 
+/* Added @ Simula */
+int sysctl_tcp_force_thin_rdb __read_mostly = TCP_FORCE_THIN_RDB;
+int sysctl_tcp_rdb_max_bundle_bytes __read_mostly = TCP_RDB_MAX_BUNDLE_BYTES;
+
 DEFINE_SNMP_STAT(struct tcp_mib, tcp_statistics) __read_mostly;
 
 atomic_t tcp_orphan_count = ATOMIC_INIT(0);
@@ -658,6 +662,167 @@
 	return tmp;
 }
 
+/* Added at Simula to support RDB */
+static int tcp_trans_merge_prev(struct sock *sk, struct sk_buff *skb, int mss_now)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	
+	/* Make sure that this isn't referenced by somebody else */
+	
+	if(!skb_cloned(skb)){
+		struct sk_buff *prev_skb = skb->prev;
+		int skb_size = skb->len;
+		int old_headlen = 0;
+		int ua_data = 0;
+		int uad_head = 0;
+		int uad_frags = 0;
+		int ua_nr_frags = 0;
+		int ua_frags_diff = 0;
+		
+		/* Since this technique currently does not support SACK, I
+		 * return -1 if the previous has been SACK'd. */
+		if(TCP_SKB_CB(prev_skb)->sacked & TCPCB_SACKED_ACKED){
+			return -1;
+		}
+		
+		/* Current skb is out of window. */
+		if (after(TCP_SKB_CB(skb)->end_seq, tp->snd_una+tp->snd_wnd)){
+			return -1;
+		}
+		
+		/*TODO: Optimize this part with regards to how the 
+		  variables are initialized */
+		
+		/*Calculates the ammount of unacked data that is available*/
+		ua_data = (TCP_SKB_CB(prev_skb)->end_seq - tp->snd_una > 
+			   prev_skb->len ? prev_skb->len : 
+			   TCP_SKB_CB(prev_skb)->end_seq - tp->snd_una);
+		ua_frags_diff = ua_data - prev_skb->data_len;
+		uad_frags = (ua_frags_diff > 0 ? prev_skb->data_len : ua_data);
+		uad_head = (ua_frags_diff > 0 ? ua_data - uad_frags : 0);
+
+		if(ua_data <= 0)
+			return -1;
+		
+		if(uad_frags > 0){
+			int i = 0;
+			int bytes_frags = 0;
+			
+			if(uad_frags == prev_skb->data_len){
+				ua_nr_frags = skb_shinfo(prev_skb)->nr_frags;
+			} else{
+				for(i=skb_shinfo(prev_skb)->nr_frags - 1; i>=0; i--){
+					if(skb_shinfo(prev_skb)->frags[i].size 
+					   + bytes_frags == uad_frags){
+						ua_nr_frags += 1;
+						break;
+					} 	  
+					ua_nr_frags += 1;
+					bytes_frags += skb_shinfo(prev_skb)->frags[i].size;
+				}
+			}
+		}
+		
+		/*
+		 * Do the diffrenet checks on size and content, and return if
+		 * something will not work.
+		 *
+		 * TODO: Support copying some bytes
+		 *
+		 * 1. Larger than MSS.
+		 * 2. Enough room for the stuff stored in the linear area
+		 * 3. Enoug room for the pages
+		 * 4. If both skbs have some data stored in the linear area, and prev_skb
+		 * also has some stored in the paged area, they cannot be merged easily.
+		 * 5. If prev_skb is linear, then this one has to be it as well.
+		 */
+		if ((sysctl_tcp_rdb_max_bundle_bytes == 0 && ((skb_size + ua_data) > mss_now))
+		    || (sysctl_tcp_rdb_max_bundle_bytes > 0 && ((skb_size + ua_data) >
+								sysctl_tcp_rdb_max_bundle_bytes))){
+			return -1;
+		}
+		
+		/* We need to know tailroom, even if it is nonlinear */
+		if(uad_head > (skb->end - skb->tail)){
+			return -1;
+		}
+		
+		if(skb_is_nonlinear(skb) && (uad_frags > 0)){
+			if((ua_nr_frags +
+			    skb_shinfo(skb)->nr_frags) > MAX_SKB_FRAGS){
+				return -1;
+			}
+			
+			if(skb_headlen(skb) > 0){
+				return -1;
+			}
+		}
+		
+		if((uad_frags > 0) && skb_headlen(skb) > 0){
+			return -1;
+		}
+		
+		/* To avoid duplicate copies (and copies
+		   where parts have been acked) */
+		if(TCP_SKB_CB(skb)->seq <= (TCP_SKB_CB(prev_skb)->end_seq - ua_data)){
+			return -1;
+		}
+		
+		/*SYN's are holy*/
+		if(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN || TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN){
+			return -1;
+		}
+		
+		/* Copy linear data */
+		if(uad_head > 0){
+			
+			/* Add required space to the header. Can't use put due to linearity */
+			old_headlen = skb_headlen(skb);
+			skb->tail += uad_head;
+			skb->len += uad_head;
+			
+			if(skb_headlen(skb) > 0){
+				memmove(skb->data + uad_head, skb->data, old_headlen);
+			}
+			
+			skb_copy_to_linear_data(skb, prev_skb->data + (skb_headlen(prev_skb) - uad_head), uad_head);
+		}
+		
+		/*Copy paged data*/
+		if(uad_frags > 0){
+			int i = 0;
+			/*Must move data backwards in the array.*/
+			if(skb_is_nonlinear(skb)){
+				memmove(skb_shinfo(skb)->frags + ua_nr_frags,
+					skb_shinfo(skb)->frags,
+					skb_shinfo(skb)->nr_frags*sizeof(skb_frag_t));
+			}
+			
+			/*Copy info and update pages*/
+			memcpy(skb_shinfo(skb)->frags,
+			       skb_shinfo(prev_skb)->frags + (skb_shinfo(prev_skb)->nr_frags - ua_nr_frags),
+			       ua_nr_frags*sizeof(skb_frag_t));
+			
+			for(i=0; i<ua_nr_frags;i++){
+				get_page(skb_shinfo(skb)->frags[i].page);
+			}
+			
+			skb_shinfo(skb)->nr_frags += ua_nr_frags;
+			skb->data_len += uad_frags;
+			skb->len += uad_frags;
+		}
+		
+		TCP_SKB_CB(skb)->seq = TCP_SKB_CB(prev_skb)->end_seq - ua_data;
+		
+		if(skb->ip_summed == CHECKSUM_PARTIAL)
+			skb->csum = CHECKSUM_PARTIAL;
+		else
+			skb->csum = skb_checksum(skb, 0, skb->len, 0);
+	}
+	
+	return 1;
+}
+
 int tcp_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
 		size_t size)
 {
@@ -825,6 +990,16 @@
 
 			from += copy;
 			copied += copy;
+			
+			/* Added at Simula to support RDB */
+			if(((tp->thin_rdb || sysctl_tcp_force_thin_rdb)) && skb->len < mss_now){
+				if(skb->prev != (struct sk_buff*) &(sk)->sk_write_queue
+				   && !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN)
+				   && !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN)){
+					tcp_trans_merge_prev(sk, skb, mss_now);
+				}
+			} /* End - Simula */
+			
 			if ((seglen -= copy) == 0 && iovlen == 0)
 				goto out;
 
@@ -1870,7 +2045,25 @@
 			tcp_push_pending_frames(sk);
 		}
 		break;
-
+		
+        /* Added @ Simula. Support for thin streams */
+	case TCP_THIN_RDB:
+		if(val)
+			tp->thin_rdb = 1;
+		break;
+		
+        /* Added @ Simula. Support for thin streams */
+	case TCP_THIN_RM_EXPB:
+		if(val)
+			tp->thin_rm_expb = 1;
+		break;
+		
+        /* Added @ Simula. Support for thin streams */
+	case TCP_THIN_DUPACK:
+		if(val)
+			tp->thin_dupack = 1;
+		break;
+		
 	case TCP_KEEPIDLE:
 		if (val < 1 || val > MAX_TCP_KEEPIDLE)
 			err = -EINVAL;
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp_input.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp_input.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp_input.c	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp_input.c	2008-07-03 11:57:08.000000000 +0200
@@ -89,6 +89,9 @@
 int sysctl_tcp_frto_response __read_mostly;
 int sysctl_tcp_nometrics_save __read_mostly;
 
+/* Added @ Simula */
+int sysctl_tcp_force_thin_dupack __read_mostly = TCP_FORCE_THIN_DUPACK;
+
 int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
 int sysctl_tcp_abc __read_mostly;
 
@@ -1709,6 +1712,12 @@
 		 */
 		return 1;
 	}
+	
+	/*Added at Simula to modify fast retransmit */
+	if ((tp->thin_dupack || sysctl_tcp_force_thin_dupack) &&
+	    tcp_fackets_out(tp) > 1 && tcp_stream_is_thin(tp)){
+	  return 1;
+	}
 
 	return 0;
 }
@@ -2442,30 +2451,127 @@
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	struct sk_buff *skb;
+	struct sk_buff *skb = tcp_write_queue_head(sk);
+	struct sk_buff *next_skb;
+
 	__u32 now = tcp_time_stamp;
 	int acked = 0;
 	int prior_packets = tp->packets_out;
+	
+	/*Added at Simula for RDB support*/
+	__u8 done = 0;
+	int remove = 0;
+	int remove_head = 0;
+	int remove_frags = 0;
+	int no_frags;
+	int data_frags;
+	int i;
+		
 	__s32 seq_rtt = -1;
 	ktime_t last_ackt = net_invalid_timestamp();
-
-	while ((skb = tcp_write_queue_head(sk)) &&
-	       skb != tcp_send_head(sk)) {
+	
+	while (skb != NULL 
+	       && ((!(tp->thin_rdb || sysctl_tcp_force_thin_rdb) 
+		    && skb != tcp_send_head(sk) 
+		    && skb != (struct sk_buff *)&sk->sk_write_queue) 
+		   || ((tp->thin_rdb || sysctl_tcp_force_thin_rdb) 
+		       && skb != (struct sk_buff *)&sk->sk_write_queue))){
 		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
 		__u8 sacked = scb->sacked;
-
+		
+		if(skb == NULL){
+			break;
+		}
+		
+		if(skb == tcp_send_head(sk)){
+			break;
+		}
+		
+		if(skb == (struct sk_buff *)&sk->sk_write_queue){
+			break;
+		}
+		
 		/* If our packet is before the ack sequence we can
 		 * discard it as it's confirmed to have arrived at
 		 * the other end.
 		 */
 		if (after(scb->end_seq, tp->snd_una)) {
-			if (tcp_skb_pcount(skb) > 1 &&
-			    after(tp->snd_una, scb->seq))
-				acked |= tcp_tso_acked(sk, skb,
-						       now, &seq_rtt);
-			break;
+			if (tcp_skb_pcount(skb) > 1 && after(tp->snd_una, scb->seq))
+				acked |= tcp_tso_acked(sk, skb, now, &seq_rtt);
+			
+			done = 1;
+			
+			/* Added at Simula for RDB support*/
+			if ((tp->thin_rdb || sysctl_tcp_force_thin_rdb) && after(tp->snd_una, scb->seq)) {
+				if (!skb_cloned(skb) && !(scb->flags & TCPCB_FLAG_SYN)){
+					remove = tp->snd_una - scb->seq;
+					remove_head = (remove > skb_headlen(skb) ? 
+						       skb_headlen(skb) : remove);
+					remove_frags = (remove > skb_headlen(skb) ? 
+							remove - remove_head : 0);
+					
+					/* Has linear data */
+					if(skb_headlen(skb) > 0 && remove_head > 0){
+						memmove(skb->data,
+							skb->data + remove_head,
+							skb_headlen(skb) - remove_head);
+						
+						skb->tail -= remove_head;
+					}
+					
+					if(skb_is_nonlinear(skb) && remove_frags > 0){
+						no_frags = 0;
+						data_frags = 0;
+						
+						/*Remove unecessary pages*/
+						for(i=0; i<skb_shinfo(skb)->nr_frags; i++){
+							if(data_frags + skb_shinfo(skb)->frags[i].size 
+							   == remove_frags){
+								put_page(skb_shinfo(skb)->frags[i].page);
+								no_frags += 1;
+								break;
+							}
+							put_page(skb_shinfo(skb)->frags[i].page);
+							no_frags += 1;
+							data_frags += skb_shinfo(skb)->frags[i].size;
+						}
+						
+						if(skb_shinfo(skb)->nr_frags > no_frags)
+							memmove(skb_shinfo(skb)->frags,
+								skb_shinfo(skb)->frags + no_frags,
+								(skb_shinfo(skb)->nr_frags 
+								 - no_frags)*sizeof(skb_frag_t));
+						
+						skb->data_len -= remove_frags;
+						skb_shinfo(skb)->nr_frags -= no_frags;
+						
+					}
+					
+					scb->seq += remove;
+					skb->len -= remove;
+					
+					if(skb->ip_summed == CHECKSUM_PARTIAL)
+						skb->csum = CHECKSUM_PARTIAL;
+					else
+						skb->csum = skb_checksum(skb, 0, skb->len, 0);
+					
+				}
+				
+				/*Only move forward if data could be removed from this packet*/
+				done = 2;
+				
+			}
+			
+			if(done == 1 || tcp_skb_is_last(sk,skb)){
+				break;
+			} else if(done == 2){
+				skb = skb->next;
+				done = 1;
+				continue;
+			}
+			
 		}
-
+		
 		/* Initial outgoing SYN's get put onto the write_queue
 		 * just like anything else we transmit.  It is not
 		 * true data, and if we misinform our callers that
@@ -2479,14 +2585,14 @@
 			acked |= FLAG_SYN_ACKED;
 			tp->retrans_stamp = 0;
 		}
-
+		
 		/* MTU probing checks */
 		if (icsk->icsk_mtup.probe_size) {
 			if (!after(tp->mtu_probe.probe_seq_end, TCP_SKB_CB(skb)->end_seq)) {
 				tcp_mtup_probe_success(sk, skb);
 			}
 		}
-
+		
 		if (sacked) {
 			if (sacked & TCPCB_RETRANS) {
 				if (sacked & TCPCB_SACKED_RETRANS)
@@ -2510,24 +2616,32 @@
 			seq_rtt = now - scb->when;
 			last_ackt = skb->tstamp;
 		}
+		
+		if ((tp->thin_rdb || sysctl_tcp_force_thin_rdb) && skb == tcp_send_head(sk)) {
+			tcp_advance_send_head(sk, skb);
+		}
+		
 		tcp_dec_pcount_approx(&tp->fackets_out, skb);
 		tcp_packets_out_dec(tp, skb);
+		next_skb = skb->next;
 		tcp_unlink_write_queue(skb, sk);
 		sk_stream_free_skb(sk, skb);
 		clear_all_retrans_hints(tp);
+		/* Added at Simula to support RDB */
+		skb = next_skb;
 	}
-
+	
 	if (acked&FLAG_ACKED) {
 		u32 pkts_acked = prior_packets - tp->packets_out;
 		const struct tcp_congestion_ops *ca_ops
 			= inet_csk(sk)->icsk_ca_ops;
-
+		
 		tcp_ack_update_rtt(sk, acked, seq_rtt);
 		tcp_ack_packets_out(sk);
-
+		
 		if (ca_ops->pkts_acked) {
 			s32 rtt_us = -1;
-
+			
 			/* Is the ACK triggering packet unambiguous? */
 			if (!(acked & FLAG_RETRANS_DATA_ACKED)) {
 				/* High resolution needed and available? */
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp_output.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp_output.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp_output.c	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp_output.c	2008-07-03 11:55:45.000000000 +0200
@@ -1653,7 +1653,7 @@
 
 		BUG_ON(tcp_skb_pcount(skb) != 1 ||
 		       tcp_skb_pcount(next_skb) != 1);
-
+		
 		/* changing transmit queue under us so clear hints */
 		clear_all_retrans_hints(tp);
 
@@ -1702,6 +1702,166 @@
 	}
 }
 
+/* Added at Simula. Variation of the regular collapse,
+   adapted to support RDB  */
+static void tcp_retrans_merge_redundant(struct sock *sk,
+					struct sk_buff *skb,
+					int mss_now)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct sk_buff *next_skb = skb->next;
+	int skb_size = skb->len;
+	int new_data = 0;
+	int new_data_head = 0;
+	int new_data_frags = 0;
+	int new_frags = 0;
+	int old_headlen = 0;
+	
+	int i;
+	int data_frags = 0;
+	
+	/* Loop through as many packets as possible
+	 * (will create a lot of redundant data, but WHATEVER).
+	 * The only packet this MIGHT be critical for is
+	 * if this packet is the last in the retrans-queue.
+	 *
+	 * Make sure that the first skb isnt already in
+	 * use by somebody else. */
+	
+	if (!skb_cloned(skb)) {
+		/* Iterate through the retransmit queue */
+		for (; (next_skb != (sk)->sk_send_head) && 
+			     (next_skb != (struct sk_buff *) &(sk)->sk_write_queue); 
+		     next_skb = next_skb->next) {
+			
+			/* Reset variables */
+			new_frags = 0;
+			data_frags = 0;
+			new_data = TCP_SKB_CB(next_skb)->end_seq - TCP_SKB_CB(skb)->end_seq;
+			
+			/* New data will be stored at skb->start_add + some_offset, 
+			   in other words the last N bytes */
+			new_data_frags = (new_data > next_skb->data_len ? 
+					  next_skb->data_len : new_data);
+			new_data_head = (new_data > next_skb->data_len ? 
+					 new_data - skb->data_len : 0);
+			
+			/*
+			 * 1. Contains the same data
+			 * 2. Size
+			 * 3. Sack
+			 * 4. Window
+			 * 5. Cannot merge with a later packet that has linear data
+			 * 6. The new number of frags will exceed the limit
+			 * 7. Enough tailroom
+			 */
+			
+			if(new_data <= 0){
+				return;
+			}
+			
+			if ((sysctl_tcp_rdb_max_bundle_bytes == 0 && ((skb_size + new_data) > mss_now))
+			    || (sysctl_tcp_rdb_max_bundle_bytes > 0 && ((skb_size + new_data) >
+									sysctl_tcp_rdb_max_bundle_bytes))){
+				return;
+			}
+			
+			if(TCP_SKB_CB(next_skb)->flags & TCPCB_FLAG_FIN){
+				return;
+			}
+			
+			if((TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
+			   (TCP_SKB_CB(next_skb)->sacked & TCPCB_SACKED_ACKED)){
+				return;
+			}
+			
+			if(after(TCP_SKB_CB(skb)->end_seq + new_data, tp->snd_una + tp->snd_wnd)){
+				return;
+			}
+			
+			if(skb_shinfo(skb)->frag_list || skb_shinfo(skb)->frag_list){
+				return;
+			}
+			
+			/* Calculate number of new fragments. Any new data will be 
+			   stored in the back. */
+			if(skb_is_nonlinear(next_skb)){
+				i = (skb_shinfo(next_skb)->nr_frags == 0 ? 
+				     0 : skb_shinfo(next_skb)->nr_frags - 1);
+				for( ; i>=0;i--){
+					if(data_frags + skb_shinfo(next_skb)->frags[i].size == 
+					   new_data_frags){
+						new_frags += 1;
+						break;
+					}
+					
+					data_frags += skb_shinfo(next_skb)->frags[i].size;
+					new_frags += 1;
+				}
+			}
+			
+			/* If dealing with a fragmented skb, only merge 
+			   with an skb that ONLY contain frags */
+			if(skb_is_nonlinear(skb)){
+				
+				/*Due to the way packets are processed, no later data*/
+				if(skb_headlen(next_skb) && new_data_head > 0){
+					return;
+				}
+				
+				if(skb_is_nonlinear(next_skb) && (new_data_frags > 0) && 
+				   ((skb_shinfo(skb)->nr_frags + new_frags) > MAX_SKB_FRAGS)){
+					return;
+				}
+				
+			} else {
+				if(skb_headlen(next_skb) && (new_data_head > (skb->end - skb->tail))){
+					return;
+				}
+			}
+			
+			/*Copy linear data. This will only occur if both are linear, 
+			  or only A is linear*/
+			if(skb_headlen(next_skb) && (new_data_head > 0)){
+				old_headlen = skb_headlen(skb);
+				skb->tail += new_data_head;
+				skb->len += new_data_head;
+				
+				/* The new data starts in the linear area, 
+				   and the correct offset will then be given by 
+				   removing new_data ammount of bytes from length. */
+				skb_copy_to_linear_data_offset(skb, old_headlen, next_skb->tail - 
+							       new_data_head, new_data_head);
+			}
+			
+			if(skb_is_nonlinear(next_skb) && (new_data_frags > 0)){
+				memcpy(skb_shinfo(skb)->frags + skb_shinfo(skb)->nr_frags, 
+				       skb_shinfo(next_skb)->frags + 
+				       (skb_shinfo(next_skb)->nr_frags - new_frags), 
+				       new_frags*sizeof(skb_frag_t));
+				
+				for(i=skb_shinfo(skb)->nr_frags; 
+				    i < skb_shinfo(skb)->nr_frags + new_frags; i++)
+					get_page(skb_shinfo(skb)->frags[i].page);
+				
+				skb_shinfo(skb)->nr_frags += new_frags;
+				skb->data_len += new_data_frags;
+				skb->len += new_data_frags;
+			}
+			
+			TCP_SKB_CB(skb)->end_seq += new_data;
+						
+			if(skb->ip_summed == CHECKSUM_PARTIAL)
+				skb->csum = CHECKSUM_PARTIAL;
+			else
+				skb->csum = skb_checksum(skb, 0, skb->len, 0);
+			
+			skb_size = skb->len;
+		}
+		
+	}
+}
+
 /* Do a simple retransmit without using the backoff mechanisms in
  * tcp_timer. This is used for path mtu discovery.
  * The socket is already locked here.
@@ -1756,6 +1916,8 @@
 /* This retransmits one SKB.  Policy decisions and retransmit queue
  * state updates are done by the caller.  Returns non-zero if an
  * error occurred which prevented the send.
+ * Modified at Simula to support thin stream optimizations
+ * TODO: Update to use new helpers (like tcp_write_queue_next())
  */
 int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
 {
@@ -1802,10 +1964,21 @@
 	    (skb->len < (cur_mss >> 1)) &&
 	    (tcp_write_queue_next(sk, skb) != tcp_send_head(sk)) &&
 	    (!tcp_skb_is_last(sk, skb)) &&
-	    (skb_shinfo(skb)->nr_frags == 0 && skb_shinfo(tcp_write_queue_next(sk, skb))->nr_frags == 0) &&
-	    (tcp_skb_pcount(skb) == 1 && tcp_skb_pcount(tcp_write_queue_next(sk, skb)) == 1) &&
-	    (sysctl_tcp_retrans_collapse != 0))
+	    (skb_shinfo(skb)->nr_frags == 0
+	     && skb_shinfo(tcp_write_queue_next(sk, skb))->nr_frags == 0)
+	    && (tcp_skb_pcount(skb) == 1
+		&& tcp_skb_pcount(tcp_write_queue_next(sk, skb)) == 1)
+	    && (sysctl_tcp_retrans_collapse != 0)
+	    && !((tp->thin_rdb || sysctl_tcp_force_thin_rdb))) {
 		tcp_retrans_try_collapse(sk, skb, cur_mss);
+       	} else if ((tp->thin_rdb || sysctl_tcp_force_thin_rdb)) {
+		if (!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN) &&
+		    !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN) &&
+		    (skb->next != tcp_send_head(sk)) &&
+		    (skb->next != (struct sk_buff *) &sk->sk_write_queue)) {
+			tcp_retrans_merge_redundant(sk, skb, cur_mss);
+		}
+	}
 
 	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
 		return -EHOSTUNREACH; /* Routing failure or similar. */
diff -Nur linux-2.6.23.8.vanilla/net/ipv4/tcp_timer.c linux-2.6.23.8-tcp-thin/net/ipv4/tcp_timer.c
--- linux-2.6.23.8.vanilla/net/ipv4/tcp_timer.c	2007-11-16 19:14:27.000000000 +0100
+++ linux-2.6.23.8-tcp-thin/net/ipv4/tcp_timer.c	2008-07-02 15:17:38.000000000 +0200
@@ -32,6 +32,9 @@
 int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
 int sysctl_tcp_orphan_retries __read_mostly;
 
+/* Added @ Simula */
+int sysctl_tcp_force_thin_rm_expb __read_mostly = TCP_FORCE_THIN_RM_EXPB;
+
 static void tcp_write_timer(unsigned long);
 static void tcp_delack_timer(unsigned long);
 static void tcp_keepalive_timer (unsigned long data);
@@ -368,13 +371,26 @@
 	 */
 	icsk->icsk_backoff++;
 	icsk->icsk_retransmits++;
-
+	
 out_reset_timer:
-	icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
+	/* Added @ Simula removal of exponential backoff for thin streams */
+	if ((tp->thin_rm_expb || sysctl_tcp_force_thin_rm_expb) && tcp_stream_is_thin(tp)) {
+		/* Since 'icsk_backoff' is used to reset timer, set to 0
+		 * Recalculate 'icsk_rto' as this might be increased if stream oscillates
+		 * between thin and thick, thus the old value might already be too high
+		 * compared to the value set by 'tcp_set_rto' in tcp_input.c which resets
+		 * the rto without backoff. */
+		icsk->icsk_backoff = 0;
+		icsk->icsk_rto = min(((tp->srtt >> 3) + tp->rttvar), TCP_RTO_MAX);
+	} else {
+		/* Use normal backoff */
+		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
+	}
+	/* End Simula*/
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
 	if (icsk->icsk_retransmits > sysctl_tcp_retries1)
 		__sk_dst_reset(sk);

^ permalink raw reply	[flat|nested] 11+ messages in thread
* Re: RFC: Latency reducing TCP modifications for thin-stream interactive applications
@ 2009-01-12 14:54 Andreas Petlund
  2009-01-14 15:32 ` Ilpo Järvinen
  0 siblings, 1 reply; 11+ messages in thread
From: Andreas Petlund @ 2009-01-12 14:54 UTC (permalink / raw)
  To: Ilpo Järvinen
  Cc: linux-net, LKML, linux-rt-users, mdavem, jgarzik, kohari, peterz,
	jzemlin, mrexx, tytso, mingo, kristrev, griff, paalh, Netdev,
	shemminger

Thank you for comprehensive feedback. We're sorry it took so long to reply.
We will begin the work on a patch set against the net-next tree now and 
try to follow your formatting and functional advice.

Ilpo Järvinen wrote:
> linux-net is users list, use netdev instead on matters relating
> development...

Bad case of typo :)

>
> Please make sure that it's based on net-next tree next time since there
> are lots of changes already in the areas you're touching.

We will do that for the updated patch set.

>> This will also give us time to integrate any ideas that may arise from
>> the discussions here.
>>
>> We are happy for all feedback regarding this:
>> Is something like this viable to introduce into the kernel?
>>
> No idea in general. But it has to be (nearly) minimal and clean if such
> thing is ever to be considered. The one below is definately not even close
> minimal in many areas...

We will clean up the noise, redo the formatting and divide it into logical 
segments for the new patch set.

>> Is the scheme for thin-stream detection mechanism acceptable.
>
> Does reduncancy happen in the initial slow start as well (depends on
> write pattern)? Why it is so in case the stream is to become thick
> right after initial rtts?
>

If the window is halved, (but still not at less than 4 segment), the mechanisms
will be turned off (due to the < 4 packets in flight limit). If the stream backs off
to a minimal window, the mechanisms will be turned on.

The bundling mechanism (if activated) will stay active since it relies on
the packet sizes and interarrival time, not on number of packets in flight.

> ...In general this function should be split, having a separate skb
> (non-TCP) function(s) and tcp-side would be ideal.
>
> It's misnamed as well, it won't merge but duplicates?
>
> Also, this approach is extremely intrusive and adding non-linear seqno
> things into write queue will require _you_ to do _full audit_ over every
> single place to verify that seqno leaps backwards won't break anything
> (and you'll still probably miss some cases). I wonder if you realize
> how easily this kind of change manifests itself as a silent data
> corruption on stream level and have taken appropriate actions to validate
> that not a single one of scenario leads to data coming as different out as
> was sent in (every single byte, it's not enough to declare that
> application worked which could well happen with corrupted data too). TCP
> is very coreish and such bugs will definately hit people hard.
>

We have to admit we don't quite see the problem. Since a page can never be removed
before all owners have decleared that they no longer needs it, all data will be correctly 
present. Also, since the packets are placed linearly on the queue and we don't support 
when SACK is enabled, no gaps will occur. But maybe we have misunderstood your 
comment, please let us know if that is the case.


>> + if(TCP_SKB_CB(skb)->seq <= (TCP_SKB_CB(prev_skb)->end_seq - ua_data)){
>
> So ua_data must be prev_skb->len or this fails? Why ua_data needs such
> complex setup above?
>

>
> Does this thing of youes depend on skb being not cloned?
>

We will look into making the calculation ua_data easier and also the cloning requirement. The check 
is there to avoid duplicate data in the case when a previous packet has been bundled with later ones. 
However, when we think about it, this might not be neccessary since bundling on retransmission is not 
allowed to bundle with packets that are yet to be sent. Thank you for pointing this out.

>> +
>> + if ((tp->thin_rdb || sysctl_tcp_force_thin_rdb) && skb == tcp_send_head(sk)) {
>> + tcp_advance_send_head(sk, skb);
>> + }
>> +
>
> I kind of missed the point of this change, can you please justify it if
> it's still needed? I think it must either be bug in your code causing this
> to happen or unnecessary.
>

Thank you very much, this was placed there for a part of the development that we found out
was unecessary.

>> + * Make sure that the first skb isnt already in
>> + * use by somebody else. */
>> +
>> + if (!skb_cloned(skb)) {
>
> Relying on skb's not being cloned will make your change to work in a
> minority of the cases on many hardwares that have tx reclaims happening
> late enough. I recently got some numbers about clones after rtt and can
> claim this to happen for sure!
>

We were not aware of this and will look into the whole cloned-thin in this patch.

>> +
>> + if(skb_shinfo(skb)->frag_list || skb_shinfo(skb)->frag_list){
>
> If it wasn't cloned, why you do this check?
>

Good question, oversight by us, thank you for pointing it out.

> I suppose this function was effectively:
>
> {
> tcp_write_queue_walk_safe(...) {
> ...checks...
> if (tcp_trim_head(...))
> break;
> tcp_collapse_retrans(...);
> }
> }

Correct, except we dont collapse but bundle, will be modified and simplified in the next version of the patch.

> It's sort of counter-intuitive that first you use redundancy but now in
> here when the path _has shown problems_ you now remove the redundancy if
> I understand you correctly? The logic is...?
>

Even though the path has problems (as shown by the need for retransmission), we still bundle as many 
packets as possible. If you are still not sure, please let us know what part of the code that is confusing.

> It seems very intrusive solution in general. I doubt you succeed in
> pulling it off as is without breaking something. To me it seems rather
> fragile approach to make write queue seqno backleaps you're proposing. It
> also leads to troubles in the truesize as you have noticed. Why not just
> building those redundancy containing segments at the write time in case
> the stream is thin, then all other parts would not have to bother about
> dealing these things? Number of sysctls should be minimized, if they're
> to be added at all. Skb work functions should be separated from tcp layer
> things.
>
> If you depend on non-changing sysctl value to select right branch, you're
> asking for trouble as the userspave is allowed to change it during the
> flow as well and even during the ack processing.
>

As far as we understand this comment, you want us to do it on the application layer instead? Do you mean as a 
middleware, application-specific solution or something similar? Also, we believe doing it on the application layer 
will lead to the same delays that we try to prevent, since sent data will be delayed on the transport layer in case 
of loss.

-Andreas Petlund & Kristian Evensen

^ permalink raw reply	[flat|nested] 11+ messages in thread
end of thread, other threads:[~2009-01-22 14:13 UTC | newest]

Thread overview: 11+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2008-11-27 13:39 RFC: Latency reducing TCP modifications for thin-stream interactive applications Andreas Petlund
2008-11-27 13:39 ` Andreas Petlund
2008-11-28 12:25 ` Ilpo Järvinen
2008-12-04 12:15 ` Sven-Thorsten Dietrich
  -- strict thread matches above, loose matches on Subject: below --
2009-01-12 14:54 Andreas Petlund
2009-01-14 15:32 ` Ilpo Järvinen
2009-01-16 10:13   ` kristrev
2009-01-16 10:13     ` kristrev
2009-01-20 15:45     ` Ilpo Järvinen
2009-01-21 13:50       ` kristrev
2009-01-22 14:13         ` Ilpo Järvinen

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