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Date: Thu, 4 Jun 2015 16:46:10 -0700
From: Yuchung Cheng <ycheng@...gle.com>
To: Kenneth Klette Jonassen <kennetkl@....uio.no>
Cc: netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Stephen Hemminger <stephen@...workplumber.org>,
Neal Cardwell <ncardwell@...gle.com>,
David Hayes <davihay@....uio.no>,
Andreas Petlund <apetlund@...ula.no>,
Dave Taht <dave.taht@...ferbloat.net>,
Nicolas Kuhn <nicolas.kuhn@...ecom-bretagne.eu>
Subject: Re: [PATCH net-next 2/2] tcp: add CDG congestion control
On Thu, Jun 4, 2015 at 2:01 PM, Kenneth Klette Jonassen
<kennetkl@....uio.no> wrote:
> CAIA Delay-Gradient (CDG) is a TCP congestion control that modifies
> the TCP sender in order to [1]:
>
> o Use the delay gradient as a congestion signal.
> o Back off with an average probability that is independent of the RTT.
> o Coexist with flows that use loss-based congestion control, i.e.,
> flows that are unresponsive to the delay signal.
> o Tolerate packet loss unrelated to congestion. (Disabled by default.)
>
> Its FreeBSD implementation was presented for the ICCRG in July 2012;
> slides are available at http://www.ietf.org/proceedings/84/iccrg.html
>
> Running the experiment scenarios in [1] suggests that our implementation
> achieves more goodput compared with FreeBSD 10.0 senders, although it also
> causes more queueing delay for a given backoff factor.
>
> The loss tolerance heuristic is disabled by default due to safety concerns
> for its use in the Internet [2, p. 45-46].
>
> We use a variant of the Hybrid Slow start algorithm in tcp_cubic to reduce
> the probability of slow start overshoot.
nice patch. I would like to review it more thoroughly but I have some
quick comments.
given that cdg didn't include hystart. it'd be nice to have a module knob to
enable and disable hystart for experiments.
>
> [1] D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
> delay gradients." In Networking 2011, pages 328-341. Springer, 2011.
> [2] K.K. Jonassen. "Implementing CAIA Delay-Gradient in Linux."
> MSc thesis. Department of Informatics, University of Oslo, 2015.
>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Stephen Hemminger <stephen@...workplumber.org>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> Cc: David Hayes <davihay@....uio.no>
> Cc: Andreas Petlund <apetlund@...ula.no>
> Cc: Dave Taht <dave.taht@...ferbloat.net>
> Cc: Nicolas Kuhn <nicolas.kuhn@...ecom-bretagne.eu>
> Signed-off-by: Kenneth Klette Jonassen <kennetkl@....uio.no>
>
> ---
> V0: RFC
> V1: Feedback from Dumazet, Cheng, and Hemminger [3], Shadow window, HyStart.
>
> [1] http://caia.swin.edu.au/cv/dahayes/content/networking2011-cdg-preprint.pdf
> [2] http://folk.uio.no/kennetkl/jonassen_thesis.pdf
> [3] http://thread.gmane.org/gmane.linux.network/363729
> ---
> net/ipv4/Kconfig | 20 +++
> net/ipv4/Makefile | 1 +
> net/ipv4/tcp_cdg.c | 427 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 448 insertions(+)
> create mode 100644 net/ipv4/tcp_cdg.c
>
> diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
> index d83071d..6fb3c90 100644
> --- a/net/ipv4/Kconfig
> +++ b/net/ipv4/Kconfig
> @@ -615,6 +615,22 @@ config TCP_CONG_DCTCP
> For further details see:
> http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
>
> +config TCP_CONG_CDG
> + tristate "CAIA Delay-Gradient (CDG)"
> + default n
> + ---help---
> + CAIA Delay-Gradient (CDG) is a TCP congestion control that modifies
> + the TCP sender in order to:
> +
> + o Use the delay gradient as a congestion signal.
> + o Back off with an average probability that is independent of the RTT.
> + o Coexist with flows that use loss-based congestion control.
> + o Tolerate packet loss unrelated to congestion.
> +
> + For further details see:
> + D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
> + delay gradients." In Networking 2011. Preprint: http://goo.gl/No3vdg
> +
> choice
> prompt "Default TCP congestion control"
> default DEFAULT_CUBIC
> @@ -646,6 +662,9 @@ choice
> config DEFAULT_DCTCP
> bool "DCTCP" if TCP_CONG_DCTCP=y
>
> + config DEFAULT_CDG
> + bool "CDG" if TCP_CONG_CDG=y
> +
> config DEFAULT_RENO
> bool "Reno"
> endchoice
> @@ -668,6 +687,7 @@ config DEFAULT_TCP_CONG
> default "veno" if DEFAULT_VENO
> default "reno" if DEFAULT_RENO
> default "dctcp" if DEFAULT_DCTCP
> + default "cdg" if DEFAULT_CDG
> default "cubic"
>
> config TCP_MD5SIG
> diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
> index b36236d..efc43f3 100644
> --- a/net/ipv4/Makefile
> +++ b/net/ipv4/Makefile
> @@ -42,6 +42,7 @@ obj-$(CONFIG_INET_TCP_DIAG) += tcp_diag.o
> obj-$(CONFIG_INET_UDP_DIAG) += udp_diag.o
> obj-$(CONFIG_NET_TCPPROBE) += tcp_probe.o
> obj-$(CONFIG_TCP_CONG_BIC) += tcp_bic.o
> +obj-$(CONFIG_TCP_CONG_CDG) += tcp_cdg.o
> obj-$(CONFIG_TCP_CONG_CUBIC) += tcp_cubic.o
> obj-$(CONFIG_TCP_CONG_DCTCP) += tcp_dctcp.o
> obj-$(CONFIG_TCP_CONG_WESTWOOD) += tcp_westwood.o
> diff --git a/net/ipv4/tcp_cdg.c b/net/ipv4/tcp_cdg.c
> new file mode 100644
> index 0000000..ae05b09
> --- /dev/null
> +++ b/net/ipv4/tcp_cdg.c
> @@ -0,0 +1,427 @@
> +/*
> + * CAIA Delay-Gradient (CDG) congestion control
> + *
> + * This implementation is based on the paper:
> + * D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
> + * delay gradients." In IFIP Networking, pages 328-341. Springer, 2011.
> + *
> + * Background traffic (Less-than-Best-Effort) should disable coexistence
> + * heuristics using parameters use_shadow=0 detect_ineff=0.
> + *
> + * Parameters window, backoff_beta, and backoff_factor are crucial for
> + * throughput and delay. Future work is needed to determine better defaults,
> + * and to provide guidelines for use in different environments.
> + *
> + * window is only configurable when loading CDG as a module.
> + *
> + * Notable differences from paper/FreeBSD:
> + * o Using Hybrid Slow start and Proportional Rate Reduction.
> + * o Add toggle for shadow window mechanism. Suggested by David Hayes.
> + * o Add toggle for non-congestion loss tolerance.
> + * o Scaling parameter G is changed to a backoff factor;
> + * conversion is given by: backoff_factor = 1000/(G * window).
> + * o Limit shadow window to 2 * cwnd, or to cwnd when application limited.
> + * o Bypass loss tolerance heuristic on ECN signal.
> + * o More accurate e^-x.
> + */
> +#include <linux/kernel.h>
> +#include <linux/random.h>
> +#include <linux/module.h>
> +#include <net/tcp.h>
> +
> +static int window __read_mostly = 8;
> +static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
> +static unsigned int backoff_factor __read_mostly = 42;
> +static unsigned int detect_ineff __read_mostly = 5;
> +static bool use_shadow __read_mostly = true;
> +static bool use_tolerance __read_mostly;
> +
> +module_param(window, int, 0444);
> +MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
> +module_param(backoff_beta, uint, 0644);
> +MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
> +module_param(backoff_factor, uint, 0644);
> +MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
> +module_param(detect_ineff, uint, 0644);
> +MODULE_PARM_DESC(detect_ineff, "ineffectual backoff detection threshold");
> +module_param(use_shadow, bool, 0644);
> +MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
> +module_param(use_tolerance, bool, 0644);
> +MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
> +
> +struct minmax {
> + union {
> + struct {
> + s32 min;
> + s32 max;
> + };
> + u64 v64;
> + };
> +};
> +
> +enum cdg_state {
> + CDG_UNKNOWN = 0,
> + CDG_FULL = 0,
> + CDG_NONFULL = 1,
> + CDG_BACKOFF = 2,
> +};
> +
> +struct cdg {
> + struct minmax rtt;
> + struct minmax rtt_prev;
> + struct minmax *gradients;
> + struct minmax gsum;
> + bool gfilled;
> + u8 tail;
> + u8 state;
> + u8 delack;
> + u32 rtt_seq;
> + u32 loss_cwnd;
> + u32 shadow_wnd;
> + u16 backoff_cnt;
> + u16 sample_cnt;
> + s32 delay_min;
> + u32 last_ack;
> + u32 round_start;
> +};
> +
> +/**
> + * nexp_u32 - negative base-e exponential
> + * @ux: x in units of micro
> + *
> + * Returns exp(ux * -1e-6) * U32_MAX.
> + */
> +static u32 __pure nexp_u32(u32 ux)
> +{
> + static const u16 v[] = {
> + /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
> + 65535,
> + 65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
> + 61378, 57484, 50423, 38795, 22965, 8047, 987, 14,
> + };
> + u32 msb = ux >> 8;
> + u32 res;
> + int i;
> +
> + /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
> + if (msb > U16_MAX)
> + return 0;
> +
> + /* Scale first eight bits linearly: */
> + res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);
> +
> + /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
> + for (i = 1; msb; i++, msb >>= 1) {
> + u32 y = v[i & -(msb & 1)] + U32_C(1);
> +
> + res = ((u64)res * y) >> 16;
> + }
> +
> + return res;
> +}
> +
> +/* Based on the HyStart algorithm (by Ha et al.) that is implemented in
> + * tcp_cubic. Differences/experimental changes:
> + * o Using Hayes' delayed ACK filter.
> + * o Using a usec clock for the ACK train.
> + * o Reset ACK train when application limited.
> + * o Invoked at any cwnd (i.e. cwnd < 16).
> + * o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
> + */
> +static void tcp_cdg_hystart_update(struct sock *sk)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> + struct tcp_sock *tp = tcp_sk(sk);
> + u32 now_us = local_clock() / NSEC_PER_USEC;
now_us = mstamp_us_get();
> +
> + ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
> +
> + if (ca->last_ack == 0 || ca->delay_min == 0) {
> + ca->sample_cnt = 0;
> + ca->last_ack = now_us;
> + ca->round_start = now_us;
> + return;
> + }
> +
> + if (!tcp_is_cwnd_limited(sk)) {
> + ca->last_ack = now_us;
> + ca->round_start = now_us;
> + } else if (before(now_us, ca->last_ack + 3000)) {
> + u32 base_owd = max(125U, ca->delay_min / 2U);
> +
> + ca->last_ack = now_us;
> + if (after(now_us, ca->round_start + base_owd)) {
> + NET_INC_STATS_BH(sock_net(sk),
> + LINUX_MIB_TCPHYSTARTTRAINDETECT);
> + NET_ADD_STATS_BH(sock_net(sk),
> + LINUX_MIB_TCPHYSTARTTRAINCWND,
> + tp->snd_cwnd);
> + tp->snd_ssthresh = tp->snd_cwnd;
> + return;
> + }
> + }
> +
> + if (ca->sample_cnt < 8) {
> + ca->sample_cnt++;
> + } else {
> + s32 thresh = max(125U, ca->delay_min + ca->delay_min / 8U);
> +
> + if (ca->rtt.min > thresh) {
> + NET_INC_STATS_BH(sock_net(sk),
> + LINUX_MIB_TCPHYSTARTDELAYDETECT);
> + NET_ADD_STATS_BH(sock_net(sk),
> + LINUX_MIB_TCPHYSTARTDELAYCWND,
> + tp->snd_cwnd);
> + tp->snd_ssthresh = tp->snd_cwnd;
> + }
> + }
> +}
> +
> +static s32 tcp_cdg_grad(struct cdg *ca)
> +{
> + s32 gmin = ca->rtt.min - ca->rtt_prev.min;
> + s32 gmax = ca->rtt.max - ca->rtt_prev.max;
> + s32 grad;
> +
> + if (ca->gradients) {
> + ca->gsum.min += gmin - ca->gradients[ca->tail].min;
> + ca->gsum.max += gmax - ca->gradients[ca->tail].max;
> + ca->gradients[ca->tail].min = gmin;
> + ca->gradients[ca->tail].max = gmax;
> + ca->tail = (ca->tail + 1) & (window - 1);
> + gmin = ca->gsum.min;
> + gmax = ca->gsum.max;
> + }
> +
> + /* We keep sums to ignore gradients during cwnd reductions;
> + * the paper's smoothed gradients otherwise simplify to:
> + * (rtt_latest - rtt_oldest) / window.
> + *
> + * We also drop division by window to make backoff_factor
> + * independent of window size.
> + */
> + grad = gmin > 0 ? gmin : gmax;
> +
> + /* Extrapolate missing values in gradient window: */
> + if (!ca->gfilled) {
> + if (ca->tail == 0)
> + ca->gfilled = true;
> + else
> + grad = (grad * window) / (int)ca->tail;
> + }
> +
> + /* Backoff was effectual: */
> + if (gmin <= -125 || gmax <= -125)
> + ca->backoff_cnt = 0;
> +
> + if (use_tolerance) {
> + /* Reduce small variations to zero: */
> + gmin = DIV_ROUND_CLOSEST(gmin, 64);
> + gmax = DIV_ROUND_CLOSEST(gmax, 64);
> +
> + if (gmin > 0 && gmax <= 0)
> + ca->state = CDG_FULL;
> + else if ((gmin > 0 && gmax > 0) || gmax < 0)
> + ca->state = CDG_NONFULL;
> + }
> + return grad;
> +}
> +
> +static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> +
> + if (prandom_u32() <= nexp_u32(grad * backoff_factor))
> + return false;
> +
> + if (detect_ineff) {
> + ca->backoff_cnt++;
> + if (ca->backoff_cnt > detect_ineff)
> + return false;
> + }
> +
> + ca->shadow_wnd = max(ca->shadow_wnd, tcp_sk(sk)->snd_cwnd);
> + ca->state = CDG_BACKOFF;
> + tcp_enter_cwr(sk);
> + return true;
> +}
> +
> +/* Not called in CWR or Recovery state. */
> +static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> + struct tcp_sock *tp = tcp_sk(sk);
> + u32 prior_snd_cwnd;
> +
> + if (tp->snd_cwnd < tp->snd_ssthresh)
> + tcp_cdg_hystart_update(sk);
nit: reno and cubic slow starts w/ snd_cwnd == ssthresh. maybe keep
this consistent?
> +
> + if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
> + s32 grad = 0;
> +
> + if (ca->rtt_prev.v64)
> + grad = tcp_cdg_grad(ca);
> + ca->rtt_seq = tp->snd_nxt;
> + ca->rtt_prev = ca->rtt;
> + ca->rtt.v64 = 0;
> + ca->last_ack = 0;
> +
> + if (grad > 0 && tcp_cdg_backoff(sk, grad))
> + return;
> + }
> +
> + if (!tcp_is_cwnd_limited(sk)) {
> + ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
> + return;
> + }
> +
> + prior_snd_cwnd = tp->snd_cwnd;
> + tcp_reno_cong_avoid(sk, ack, acked);
> +
> + if (ca->shadow_wnd) {
> + ca->shadow_wnd += tp->snd_cwnd - prior_snd_cwnd;
> + if (ca->shadow_wnd > tp->snd_cwnd_clamp)
> + ca->shadow_wnd = tp->snd_cwnd_clamp;
Not sure why use snd_cwnd_clamp here. looks like it's applied in
tcp_reno_cong_avoid().
> + }
> +}
> +
> +static void tcp_cdg_acked(struct sock *sk, u32 num_acked, s32 rtt_us)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> +
> + if (rtt_us <= 0)
> + return;
> +
> + /* A heuristic for filtering delayed ACKs, adapted from:
> + * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
> + * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
> + *
> + * Assume num_acked == 0 indicates RTT measurement from SACK.
> + */
another heuristic to try is to look at tp->sacked_out. when it's
positive the receiver should not be delaying acks (hopefully).
> + if (num_acked == 1 && ca->delack) {
> + /* A delayed ACK is only used for the minimum if it is
> + * provenly lower than an existing non-zero minimum.
> + */
> + ca->rtt.min = min(ca->rtt.min, rtt_us);
> + ca->delack--;
> + return;
> + } else if (num_acked > 1 && ca->delack < 5) {
> + ca->delack++;
> + }
> +
> + ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us);
> + ca->rtt.max = max(ca->rtt.max, rtt_us);
> +}
> +
> +static u32 tcp_cdg_ssthresh(struct sock *sk)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> + struct tcp_sock *tp = tcp_sk(sk);
> +
> + if (ca->state == CDG_BACKOFF)
> + return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);
> +
> + /* If non-ECN: */
> + if (tp->prior_ssthresh) {
no need to check prior_ssthresh here b/c it's checked in
tcp_undo_cwnd_reduction() already.
> + ca->loss_cwnd = tp->snd_cwnd;
> +
> + if (use_tolerance && ca->state == CDG_NONFULL) {
> + tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
> + return tp->snd_cwnd;
> + }
> + }
> +
> + if (!use_shadow)
> + return max(2U, tp->snd_cwnd >> 1);
> +
> + ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
> + return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
> +}
> +
> +static u32 tcp_cdg_undo_cwnd(struct sock *sk)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> +
> + return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
> +}
> +
> +static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> + struct tcp_sock *tp = tcp_sk(sk);
> + struct minmax *gradients;
> +
> + switch (ev) {
> + case CA_EVENT_CWND_RESTART:
> + gradients = ca->gradients;
> + if (gradients)
> + memset(gradients, 0, window * sizeof(gradients[0]));
> + memset(ca, 0, sizeof(*ca));
> +
> + ca->gradients = gradients;
> + ca->rtt_seq = tp->snd_nxt;
> + break;
> + case CA_EVENT_COMPLETE_CWR:
> + ca->state = CDG_UNKNOWN;
> + ca->rtt_seq = tp->snd_nxt;
> + ca->rtt_prev = ca->rtt;
> + ca->rtt.v64 = 0;
> + break;
> + default:
> + break;
> + }
> +}
> +
> +static void tcp_cdg_init(struct sock *sk)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> + struct tcp_sock *tp = tcp_sk(sk);
> +
> + /* We silently fall back to window = 1 if allocation fails. */
> + if (window > 1)
> + ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
> + GFP_NOWAIT | __GFP_NOWARN);
> + ca->rtt_seq = tp->snd_nxt;
> +}
> +
> +static void tcp_cdg_release(struct sock *sk)
> +{
> + struct cdg *ca = inet_csk_ca(sk);
> +
> + kfree(ca->gradients);
> +}
> +
> +struct tcp_congestion_ops tcp_cdg __read_mostly = {
> + .cong_avoid = tcp_cdg_cong_avoid,
> + .cwnd_event = tcp_cdg_cwnd_event,
> + .pkts_acked = tcp_cdg_acked,
> + .undo_cwnd = tcp_cdg_undo_cwnd,
> + .ssthresh = tcp_cdg_ssthresh,
> + .release = tcp_cdg_release,
> + .init = tcp_cdg_init,
> + .owner = THIS_MODULE,
> + .name = "cdg",
> +};
> +
> +static int __init tcp_cdg_register(void)
> +{
> + if (backoff_beta > 1024 || window < 1 || window > 256)
> + return -ERANGE;
> + if (!is_power_of_2(window))
> + return -EINVAL;
> +
> + BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
> + tcp_register_congestion_control(&tcp_cdg);
> + return 0;
> +}
> +
> +static void __exit tcp_cdg_unregister(void)
> +{
> + tcp_unregister_congestion_control(&tcp_cdg);
> +}
> +
> +module_init(tcp_cdg_register);
> +module_exit(tcp_cdg_unregister);
> +MODULE_AUTHOR("Kenneth Klette Jonassen");
> +MODULE_LICENSE("GPL");
> +MODULE_DESCRIPTION("TCP CDG");
> --
> 2.1.0
>
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