Commit f54b3111 authored Dec 05, 2013 by Eric Dumazet Committed by David S. Miller Dec 06, 2013

tcp: auto corking

With the introduction of TCP Small Queues, TSO auto sizing, and TCP
pacing, we can implement Automatic Corking in the kernel, to help
applications doing small write()/sendmsg() to TCP sockets.

Idea is to change tcp_push() to check if the current skb payload is
under skb optimal size (a multiple of MSS bytes)

If under 'size_goal', and at least one packet is still in Qdisc or
NIC TX queues, set the TCP Small Queue Throttled bit, so that the push
will be delayed up to TX completion time.

This delay might allow the application to coalesce more bytes
in the skb in following write()/sendmsg()/sendfile() system calls.

The exact duration of the delay is depending on the dynamics
of the system, and might be zero if no packet for this flow
is actually held in Qdisc or NIC TX ring.

Using FQ/pacing is a way to increase the probability of
autocorking being triggered.

Add a new sysctl (/proc/sys/net/ipv4/tcp_autocorking) to control
this feature and default it to 1 (enabled)

Add a new SNMP counter : nstat -a | grep TcpExtTCPAutoCorking
This counter is incremented every time we detected skb was under used
and its flush was deferred.

Tested:

Interesting effects when using line buffered commands under ssh.

Excellent performance results in term of cpu usage and total throughput.

lpq83:~# echo 1 >/proc/sys/net/ipv4/tcp_autocorking
lpq83:~# perf stat ./super_netperf 4 -t TCP_STREAM -H lpq84 -- -m 128
9410.39

Performance counter stats for './super_netperf 4 -t TCP_STREAM -H lpq84 -- -m 128':

35209.439626 task-clock # 2.901 CPUs utilized
2,294 context-switches # 0.065 K/sec
101 CPU-migrations # 0.003 K/sec
4,079 page-faults # 0.116 K/sec
97,923,241,298 cycles # 2.781 GHz [83.31%]
51,832,908,236 stalled-cycles-frontend # 52.93% frontend cycles idle [83.30%]
25,697,986,603 stalled-cycles-backend # 26.24% backend cycles idle [66.70%]
102,225,978,536 instructions # 1.04 insns per cycle
# 0.51 stalled cycles per insn [83.38%]
18,657,696,819 branches # 529.906 M/sec [83.29%]
91,679,646 branch-misses # 0.49% of all branches [83.40%]

12.136204899 seconds time elapsed

lpq83:~# echo 0 >/proc/sys/net/ipv4/tcp_autocorking
lpq83:~# perf stat ./super_netperf 4 -t TCP_STREAM -H lpq84 -- -m 128
6624.89

Performance counter stats for './super_netperf 4 -t TCP_STREAM -H lpq84 -- -m 128':
40045.864494 task-clock # 3.301 CPUs utilized
171 context-switches # 0.004 K/sec
53 CPU-migrations # 0.001 K/sec
4,080 page-faults # 0.102 K/sec
111,340,458,645 cycles # 2.780 GHz [83.34%]
61,778,039,277 stalled-cycles-frontend # 55.49% frontend cycles idle [83.31%]
29,295,522,759 stalled-cycles-backend # 26.31% backend cycles idle [66.67%]
108,654,349,355 instructions # 0.98 insns per cycle
# 0.57 stalled cycles per insn [83.34%]
19,552,170,748 branches # 488.244 M/sec [83.34%]
157,875,417 branch-misses # 0.81% of all branches [83.34%]

12.130267788 seconds time elapsed

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent d8535a0a

Documentation/networking/ip-sysctl.txt

+10 −0

Original line number	Diff line number	Diff line
		@@ -156,6 +156,16 @@ tcp_app_win - INTEGER
		buffer. Value 0 is special, it means that nothing is reserved.
		Default: 31

		tcp_autocorking - BOOLEAN
		Enable TCP auto corking :
		When applications do consecutive small write()/sendmsg() system calls,
		we try to coalesce these small writes as much as possible, to lower
		total amount of sent packets. This is done if at least one prior
		packet for the flow is waiting in Qdisc queues or device transmit
		queue. Applications can still use TCP_CORK for optimal behavior
		when they know how/when to uncork their sockets.
		Default : 1

		tcp_available_congestion_control - STRING
		Shows the available congestion control choices that are registered.
		More congestion control algorithms may be available as modules,

include/net/tcp.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -282,6 +282,7 @@ extern int sysctl_tcp_limit_output_bytes;
		extern int sysctl_tcp_challenge_ack_limit;
		extern unsigned int sysctl_tcp_notsent_lowat;
		extern int sysctl_tcp_min_tso_segs;
		extern int sysctl_tcp_autocorking;

		extern atomic_long_t tcp_memory_allocated;
		extern struct percpu_counter tcp_sockets_allocated;

include/uapi/linux/snmp.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -258,6 +258,7 @@ enum
		LINUX_MIB_TCPFASTOPENCOOKIEREQD, /* TCPFastOpenCookieReqd */
		LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES, /* TCPSpuriousRtxHostQueues */
		LINUX_MIB_BUSYPOLLRXPACKETS, /* BusyPollRxPackets */
		LINUX_MIB_TCPAUTOCORKING, /* TCPAutoCorking */
		__LINUX_MIB_MAX
		};

net/ipv4/proc.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -279,6 +279,7 @@ static const struct snmp_mib snmp4_net_list[] = {
		SNMP_MIB_ITEM("TCPFastOpenCookieReqd", LINUX_MIB_TCPFASTOPENCOOKIEREQD),
		SNMP_MIB_ITEM("TCPSpuriousRtxHostQueues", LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES),
		SNMP_MIB_ITEM("BusyPollRxPackets", LINUX_MIB_BUSYPOLLRXPACKETS),
		SNMP_MIB_ITEM("TCPAutoCorking", LINUX_MIB_TCPAUTOCORKING),
		SNMP_MIB_SENTINEL
		};

net/ipv4/sysctl_net_ipv4.c

+9 −0

Original line number	Diff line number	Diff line
		@@ -732,6 +732,15 @@ static struct ctl_table ipv4_table[] = {
		.extra1 = &zero,
		.extra2 = &gso_max_segs,
		},
		{
		.procname = "tcp_autocorking",
		.data = &sysctl_tcp_autocorking,
		.maxlen = sizeof(int),
		.mode = 0644,
		.proc_handler = proc_dointvec_minmax,
		.extra1 = &zero,
		.extra2 = &one,
		},
		{
		.procname = "udp_mem",
		.data = &sysctl_udp_mem,

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