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Date: Fri, 21 Aug 2015 17:38:02 -0700
From: Eric Dumazet <eric.dumazet@...il.com>
To: David Miller <davem@...emloft.net>
Cc: netdev <netdev@...r.kernel.org>,
Neal Cardwell <ncardwell@...gle.com>,
Yuchung Cheng <ycheng@...gle.com>
Subject: [PATCH net-next] tcp: refine pacing rate determination
From: Eric Dumazet <edumazet@...gle.com>
When TCP pacing was added back in linux-3.12, we chose
to apply a fixed ratio of 200 % against current rate,
to allow probing for optimal throughput even during
slow start phase, where cwnd can be doubled every other gRTT.
At Google, we found it was better applying a different ratio
while in Congestion Avoidance phase.
This ratio was set to 120 %.
We've used the normal tcp_in_slow_start() helper for a while,
then tuned the condition to select the conservative ratio
as soon as cwnd >= ssthresh/2 :
- After cwnd reduction, it is safer to ramp up more slowly,
as we approach optimal cwnd.
- Initial ramp up (ssthresh == INFINITY) still allows doubling
cwnd every other RTT.
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
---
Documentation/networking/ip-sysctl.txt | 15 +++++++++++++++
include/net/tcp.h | 2 ++
net/ipv4/sysctl_net_ipv4.c | 19 +++++++++++++++++++
net/ipv4/tcp_input.c | 18 +++++++++++++++++-
4 files changed, 53 insertions(+), 1 deletion(-)
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 46e88ed7f41d..ac77a13d2ea2 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -586,6 +586,21 @@ tcp_min_tso_segs - INTEGER
if available window is too small.
Default: 2
+tcp_pacing_ss_ratio - INTEGER
+ sk->sk_pacing_rate is set by TCP stack using a ratio applied
+ to current rate. (current_rate = cwnd * mss / srtt)
+ If TCP is in slow start, tcp_pacing_ss_ratio is applied
+ to let TCP probe for bigger speeds, assuming cwnd can be
+ doubled every other RTT.
+ Default: 200
+
+tcp_pacing_ca_ratio - INTEGER
+ sk->sk_pacing_rate is set by TCP stack using a ratio applied
+ to current rate. (current_rate = cwnd * mss / srtt)
+ If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
+ is applied to conservatively probe for bigger throughput.
+ Default: 120
+
tcp_tso_win_divisor - INTEGER
This allows control over what percentage of the congestion window
can be consumed by a single TSO frame.
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 364426a2be5a..3e2b3ba43ae5 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -281,6 +281,8 @@ extern unsigned int sysctl_tcp_notsent_lowat;
extern int sysctl_tcp_min_tso_segs;
extern int sysctl_tcp_autocorking;
extern int sysctl_tcp_invalid_ratelimit;
+extern int sysctl_tcp_pacing_ss_ratio;
+extern int sysctl_tcp_pacing_ca_ratio;
extern atomic_long_t tcp_memory_allocated;
extern struct percpu_counter tcp_sockets_allocated;
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 0330ab2e2b63..879bdc5c95b1 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -29,6 +29,7 @@
static int zero;
static int one = 1;
static int four = 4;
+static int thousand = 1000;
static int gso_max_segs = GSO_MAX_SEGS;
static int tcp_retr1_max = 255;
static int ip_local_port_range_min[] = { 1, 1 };
@@ -712,6 +713,24 @@ static struct ctl_table ipv4_table[] = {
.extra2 = &gso_max_segs,
},
{
+ .procname = "tcp_pacing_ss_ratio",
+ .data = &sysctl_tcp_pacing_ss_ratio,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &thousand,
+ },
+ {
+ .procname = "tcp_pacing_ca_ratio",
+ .data = &sysctl_tcp_pacing_ca_ratio,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &thousand,
+ },
+ {
.procname = "tcp_autocorking",
.data = &sysctl_tcp_autocorking,
.maxlen = sizeof(int),
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 4e4d6bcd0ca9..7e1623775744 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -753,13 +753,29 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
* TCP pacing, to smooth the burst on large writes when packets
* in flight is significantly lower than cwnd (or rwin)
*/
+int sysctl_tcp_pacing_ss_ratio __read_mostly = 200;
+int sysctl_tcp_pacing_ca_ratio __read_mostly = 120;
+
static void tcp_update_pacing_rate(struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);
u64 rate;
/* set sk_pacing_rate to 200 % of current rate (mss * cwnd / srtt) */
- rate = (u64)tp->mss_cache * 2 * (USEC_PER_SEC << 3);
+ rate = (u64)tp->mss_cache * ((USEC_PER_SEC / 100) << 3);
+
+ /* current rate is (cwnd * mss) / srtt
+ * In Slow Start [1], set sk_pacing_rate to 200 % the current rate.
+ * In Congestion Avoidance phase, set it to 120 % the current rate.
+ *
+ * [1] : Normal Slow Start condition is (tp->snd_cwnd < tp->snd_ssthresh)
+ * If snd_cwnd >= (tp->snd_ssthresh / 2), we are approaching
+ * end of slow start and should slow down.
+ */
+ if (tp->snd_cwnd < tp->snd_ssthresh / 2)
+ rate *= sysctl_tcp_pacing_ss_ratio;
+ else
+ rate *= sysctl_tcp_pacing_ca_ratio;
rate *= max(tp->snd_cwnd, tp->packets_out);
--
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