Eric Dumazet a74f0fa082 tcp: reduce POLLOUT events caused by TCP_NOTSENT_LOWAT ... TCP_NOTSENT_LOWAT socket option or sysctl was added in linux-3.12 as a step to enable bigger tcp sndbuf limits. It works reasonably well, but the following happens : Once the limit is reached, TCP stack generates an [E]POLLOUT event for every incoming ACK packet. This causes a high number of context switches. This patch implements the strategy David Miller added in sock_def_write_space() : - If TCP socket has a notsent_lowat constraint of X bytes, allow sendmsg() to fill up to X bytes, but send [E]POLLOUT only if number of notsent bytes is below X/2 This considerably reduces TCP_NOTSENT_LOWAT overhead, while allowing to keep the pipe full. Tested: 100 ms RTT netem testbed between A and B, 100 concurrent TCP_STREAM A:/# cat /proc/sys/net/ipv4/tcp_wmem 4096 262144 64000000 A:/# super_netperf 100 -H B -l 1000 -- -K bbr & A:/# grep TCP /proc/net/sockstat TCP: inuse 203 orphan 0 tw 19 alloc 414 mem 1364904 # This is about 54 MB of memory per flow :/ A:/# vmstat 5 5 procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 0 0 0 256220672 13532 694976 0 0 10 0 28 14 0 1 99 0 0 2 0 0 256320016 13532 698480 0 0 512 0 715901 5927 0 10 90 0 0 0 0 0 256197232 13532 700992 0 0 735 13 771161 5849 0 11 89 0 0 1 0 0 256233824 13532 703320 0 0 512 23 719650 6635 0 11 89 0 0 2 0 0 256226880 13532 705780 0 0 642 4 775650 6009 0 12 88 0 0 A:/# echo 2097152 >/proc/sys/net/ipv4/tcp_notsent_lowat A:/# grep TCP /proc/net/sockstat TCP: inuse 203 orphan 0 tw 19 alloc 414 mem 86411 # 3.5 MB per flow A:/# vmstat 5 5 # check that context switches have not inflated too much. procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 2 0 0 260386512 13592 662148 0 0 10 0 17 14 0 1 99 0 0 0 0 0 260519680 13592 604184 0 0 512 13 726843 12424 0 10 90 0 0 1 1 0 260435424 13592 598360 0 0 512 25 764645 12925 0 10 90 0 0 1 0 0 260855392 13592 578380 0 0 512 7 722943 13624 0 11 88 0 0 1 0 0 260445008 13592 601176 0 0 614 34 772288 14317 0 10 90 0 0 Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>		2018-12-04 21:21:18 -08:00
..
acpi	pci-v4.20-changes	2018-10-25 06:50:48 -07:00
asm-generic	s390 updates for 4.20-rc2	2018-11-09 06:30:44 -06:00
clocksource
crypto	KEYS: asym_tpm: extract key size & public key [ver #2]	2018-10-26 09:30:46 +01:00
drm	drm, i915, amdgpu, bridge + core quirk	2018-11-02 10:58:20 -07:00
dt-bindings	This time it looks like a quieter release cycle in the clk tree. I guess that's	2018-10-31 11:08:30 -07:00
keys	KEYS: Move trusted.h to include/keys [ver #2]	2018-10-26 09:30:47 +01:00
kvm	KVM: arm/arm64: vgic-v3: Add core support for Group0 SGIs	2018-08-12 12:06:34 +01:00
linux	skbuff: Rename 'offload_mr_fwd_mark' to 'offload_l3_fwd_mark'	2018-12-04 08:36:36 -08:00
math-emu
media	media: Rename vb2_m2m_request_queue -> v4l2_m2m_request_queue	2018-11-06 05:24:22 -05:00
memory
misc
net	tcp: reduce POLLOUT events caused by TCP_NOTSENT_LOWAT	2018-12-04 21:21:18 -08:00
pcmcia
ras
rdma	First merge window pull request	2018-10-26 07:38:19 -07:00
scsi
soc	soc/qman: add return value to interrupt coalesce changing APIs	2018-11-23 11:17:06 -08:00
sound	ASoC: Updates for v5.0/v4.20	2018-10-22 23:26:37 +02:00
target
trace	net: Add trace events for all receive exit points	2018-11-30 13:23:25 -08:00
uapi	devlink: Add 'fw_load_policy' generic parameter	2018-12-03 13:55:43 -08:00
video
xen	CONFIG_XEN_PV breaks xen_create_contiguous_region on ARM	2018-11-02 17:18:34 +01:00