一个linux 4.9内核的bbr带宽估计偏低问题
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linux 4.9内核，bbr的带宽估计问题。
一个正常的bbr流量图：
对应的ttl图形:
一个异常的bbr流量图：
可以看出，异常的bbr流量图，出现了一个很低的带宽，且稳定在这个带宽10s左右，而正常情况下，这个文件下载不应该超过10s，由于流量消耗大于流量的下载，导致了用户播放卡顿。
通过分析，我们确认了bbr在应对delay_ack时，出现带宽估计偏低的情况，比如正常的min_rtt是1.3ms，但是delay_ack的时候，是40ms左右，而由于probe_rtt需要10秒之后进行，那么对应的
bw乘以最小的rtt则处于偏低的状态，在稳定10s之后，bbr会主动探测rtt，
bbr探测rtt的时候，行为是什么样的呢？对于4.9内核版本来说，是最多发送4个mss包：
cwnd = max(cwnd_gain * bottleneck_bandwidth * min_rtt, 4)
/* Minimum time (in ms) spent at bbr_cwnd_min_target in BBR_PROBE_RTT mode: */
static const u32 bbr_probe_rtt_mode_ms = 200;
从10.3s左右的时间的流量图来看，可以看到一个很明显的向下缺口，就是探测最小rtt的行为，每个rtt之后4个包，流量再次下降。
这个持续的时间为0.2s，也就是代码里面如下的描述：
/* The goal of PROBE_RTT mode is to have BBR flows cooperatively and * periodically drain the bottleneck queue, to converge to measure the true * min_rtt (unloaded propagation delay). This allows the flows to keep queues * small (reducing queuing delay and packet loss) and achieve fairness among * BBR flows. * * The min_rtt filter window is 10 seconds. When the min_rtt estimate expires, * we enter PROBE_RTT mode and cap the cwnd at bbr_cwnd_min_target=4 packets. * After at least bbr_probe_rtt_mode_ms=200ms and at least one packet-timed * round trip elapsed with that flight size <= 4, we leave PROBE_RTT mode and * re-enter the previous mode. BBR uses 200ms to approximately bound the * performance penalty of PROBE_RTT\'s cwnd capping to roughly 2% (200ms/10s). * * Note that flows need only pay 2% if they are busy sending over the last 10 * seconds. Interactive applications (e.g., Web, RPCs, video chunks) often have * natural silences or low-rate periods within 10 seconds where the rate is low * enough for long enough to drain its queue in the bottleneck. We pick up * these min RTT measurements opportunistically with our min_rtt filter. :-) */ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
虽然经过10s，根据探测rtt之后的min_rtt应该修正为40ms左右，会导致流量上升，但由于我们的分片文件已经播放卡顿超时，客户端会断链。
结论：如果下载的流媒体文件过小，不建议开启bbr。特别是收包的客户端进入delay_ack非常早，并且不怎么出现乱序和丢包的情况。此现象在4.14内核依然存在。
回到问题本身，问什么会出现带宽估计过低的行为，我们通过什么样的手段去复现这种行为，由于现场的机顶盒我们没法拿到，那么只能通过自己来模拟：
首先，构建一个获取文件的请求，http，然后被动接收发包就行，结果复现不了，也就是客户端这边的delay_ack并不会在bbr的startup阶段出现。
所以，我们反过来想，我们能不能让服务器端收到的ack进行聚集，在startup阶段就收到聚集ack呢，顺着这种思路，我们仿照gro，将收到的ack进行聚集，并在40ms左右将ack报文送上协议栈
，这样从服务器的角度，让他认为客户端的ack就是这种频率发回来。
然后，我们在服务器bbr相关代码地方加printk：
首先增加四个地方，就是bbr的状态切换的地方：
static void bbr_reset_startup_mode(struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); bbr->mode = BBR_STARTUP; if(((struct inet_sock*)sk)->inet_dport == 3000) { printk("mode 2 mode=%d,min_rtt_us=%d,full_bw=%d,cycle_idx=%d,pacing_gain=%d,cwnd_gain=%d,rtt_cnt=%d\\r\\n",bbr->mode,bbr->min_rtt_us,bbr->full_bw,bbr->cycle_idx,bbr->pacing_gain,bbr->cwnd_gain,bbr->rtt_cnt); } 。。。。 } static void bbr_reset_probe_bw_mode(struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); bbr->mode = BBR_PROBE_BW; if(((struct inet_sock*)sk)->inet_dport == 3000) { printk("mode 2 mode=%d,min_rtt_us=%d,full_bw=%d,cycle_idx=%d,pacing_gain=%d,cwnd_gain=%d,rtt_cnt=%d\\r\\n",bbr->mode,bbr->min_rtt_us,bbr->full_bw,bbr->cycle_idx,bbr->pacing_gain,bbr->cwnd_gain,bbr->rtt_cnt); } 。。。。 } static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs) { struct bbr *bbr = inet_csk_ca(sk); if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) { bbr->mode = BBR_DRAIN; /* drain queue we created */ if(((struct inet_sock*)sk)->inet_dport == 3000) { printk("mode 2 mode=%d,min_rtt_us=%d,full_bw=%d,cycle_idx=%d,pacing_gain=%d,cwnd_gain=%d,rtt_cnt=%d\\r\\n",bbr->mode,bbr->min_rtt_us,bbr->full_bw,bbr->cycle_idx,bbr->pacing_gain,bbr->cwnd_gain,bbr->rtt_cnt); } 。。。 } static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); bool filter_expired; static int count_expire=0; 。。。。。。if (rs->rtt_us >= 0 && (rs->rtt_us <= bbr->min_rtt_us || filter_expired)) { bbr->min_rtt_us = rs->rtt_us; bbr->min_rtt_stamp = tcp_time_stamp; } if (bbr_probe_rtt_mode_ms > 0 && filter_expired && !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) { bbr->mode = BBR_PROBE_RTT; /* dip, drain queue */ if(((struct inet_sock*)sk)->inet_dport == 3000) { printk("mode 2 mode=%d,min_rtt_us=%d,full_bw=%d,cycle_idx=%d,pacing_gain=%d,cwnd_gain=%d,rtt_cnt=%d\\r\\n",bbr->mode,bbr->min_rtt_us,bbr->full_bw,bbr->cycle_idx,bbr->pacing_gain,bbr->cwnd_gain,bbr->rtt_cnt); } bbr->pacing_gain = BBR_UNIT;
.....
}
static void bbr_main(struct sock *sk, const struct rate_sample *rs)
{
struct bbr *bbr = inet_csk_ca(sk);
u32 bw;
bbr_update_model(sk, rs);
bw = bbr_bw(sk);
bbr_set_pacing_rate(sk, bw, bbr->pacing_gain);
bbr_set_tso_segs_goal(sk);
bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain);
if(((struct inet_sock*)sk)->inet_dport == 3000)
{
printk("main mode=%d,min_rtt_us=%d,cur_bw=%d,cycle_idx=%d,pacing_gain=%d,cwnd_gain=%d,rtt_cnt=%d,snd_cwnd=%d, snd_nxt=%u,snd_una=%u\\r\\n",bbr->mode,bbr->min_rtt_us,bbr_bw(sk),bbr->cycle_idx,bbr->pacing_gain,bbr->cwnd_gain,bbr->rtt_cnt,tcp_sk(sk)->snd_cwnd, tcp_sk(sk)->snd_nxt, tcp_sk(sk)->snd_una );
}
}
除了四个状态转换的地方，由于我们收到ack，都会进入bbr_main,所以我们也在这个地方加了打印。
打印结果如下：
[11241.360364] mode 2 mode=3,min_rtt_us=553,full_bw=0,cycle_idx=0,pacing_gain=0,cwnd_gain=0,rtt_cnt=0 [11241.360373] main mode=3,min_rtt_us=553,cur_bw=0,cycle_idx=0,pacing_gain=256,cwnd_gain=256,rtt_cnt=0,snd_cwnd=4 [11241.360377] mode 2 mode=0,min_rtt_us=553,full_bw=0,cycle_idx=0,pacing_gain=256,cwnd_gain=256,rtt_cnt=0 [11241.367057] main mode=0,min_rtt_us=553,cur_bw=91528,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=1,snd_cwnd=20 [11241.369053] main mode=0,min_rtt_us=553,cur_bw=109875,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=2,snd_cwnd=33 [11241.375055] main mode=0,min_rtt_us=553,cur_bw=267597,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=3,snd_cwnd=56 [11241.383053] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=4,snd_cwnd=79 [11241.392054] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=5,snd_cwnd=102 [11241.401053] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=6,snd_cwnd=125 [11241.409054] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=7,snd_cwnd=148 [11241.417055] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=8,snd_cwnd=171 [11241.426086] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=9,snd_cwnd=171 [11241.434054] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=10,snd_cwnd=171 [11241.442055] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=11,snd_cwnd=171 。。。。。 中间只增加rtt_cnt的部分省略： [11243.420044] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=246,snd_cwnd=187 [11243.429045] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=247,snd_cwnd=187 [11243.437044] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=248,snd_cwnd=187 [11243.446046] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=249,snd_cwnd=187 [11243.454046] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=250,snd_cwnd=187 [11243.502075] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=251,snd_cwnd=195------------rtt从8ms变成48ms [11243.544064] main mode=0,min_rtt_us=553,cur_bw=364377,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=252,snd_cwnd=195-----------42ms [11243.586056] main mode=0,min_rtt_us=553,cur_bw=39570,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=253,snd_cwnd=195------------42ms [11243.628057] main mode=0,min_rtt_us=553,cur_bw=50749,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=254,snd_cwnd=195------------42ms [11243.670058] main mode=0,min_rtt_us=553,cur_bw=50749,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=255,snd_cwnd=195------------42ms [11243.753064] main mode=0,min_rtt_us=553,cur_bw=50749,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=256,snd_cwnd=195------------83ms [11243.795063] mode 2 mode=1,min_rtt_us=553,full_bw=109875,cycle_idx=0,pacing_gain=739,cwnd_gain=739,rtt_cnt=257----------------从startup模式切换到drain模式 [11243.795068] mode 2 mode=2,min_rtt_us=553,full_bw=109875,cycle_idx=0,pacing_gain=88,cwnd_gain=739,rtt_cnt=257----------------立刻从drain模式切换到探测bw模式 [11243.795072] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=4,pacing_gain=256,cwnd_gain=512,rtt_cnt=257,snd_cwnd=14 [11243.837061] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=5,pacing_gain=256,cwnd_gain=512,rtt_cnt=258,snd_cwnd=14 [11243.879063] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=6,pacing_gain=256,cwnd_gain=512,rtt_cnt=259,snd_cwnd=14 [11243.921061] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=7,pacing_gain=256,cwnd_gain=512,rtt_cnt=260,snd_cwnd=14 [11243.963063] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=0,pacing_gain=320,cwnd_gain=512,rtt_cnt=261,snd_cwnd=14 [11244.005063] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=1,pacing_gain=192,cwnd_gain=512,rtt_cnt=262,snd_cwnd=10 [11244.047065] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=2,pacing_gain=256,cwnd_gain=512,rtt_cnt=263,snd_cwnd=14 [11244.089065] main mode=2,min_rtt_us=553,cur_bw=50749,cycle_idx=3,pacing_gain=256,cwnd_gain=512,rtt_cnt=264,snd_cwnd=14 [11244.131066] main mode=2,min_rtt_us=553,cur_bw=50352,cycle_idx=4,pacing_gain=256,cwnd_gain=512,rtt_cnt=265,snd_cwnd=14 [11244.173065] main mode=2,min_rtt_us=553,cur_bw=50352,cycle_idx=5,pacing_gain=256,cwnd_gain=512,rtt_cnt=266,snd_cwnd=14 [11244.215069] main mode=2,min_rtt_us=553,cur_bw=50352,cycle_idx=6,pacing_gain=256,cwnd_gain=512,rtt_cnt=267,snd_cwnd=14 [11244.257067] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=7,pacing_gain=256,cwnd_gain=512,rtt_cnt=268,snd_cwnd=8 [11244.299067] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=0,pacing_gain=320,cwnd_gain=512,rtt_cnt=269,snd_cwnd=8 [11244.341077] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=1,pacing_gain=192,cwnd_gain=512,rtt_cnt=270,snd_cwnd=8 [11244.383073] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=2,pacing_gain=256,cwnd_gain=512,rtt_cnt=271,snd_cwnd=8 [11244.425070] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=3,pacing_gain=256,cwnd_gain=512,rtt_cnt=272,snd_cwnd=8 [11244.467071] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=4,pacing_gain=256,cwnd_gain=512,rtt_cnt=273,snd_cwnd=8 [11244.509073] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=5,pacing_gain=256,cwnd_gain=512,rtt_cnt=274,snd_cwnd=8 [11244.551076] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=6,pacing_gain=256,cwnd_gain=512,rtt_cnt=275,snd_cwnd=8 [11244.593078] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=7,pacing_gain=256,cwnd_gain=512,rtt_cnt=276,snd_cwnd=8 [11244.635078] main mode=2,min_rtt_us=553,cur_bw=5594,cycle_idx=0,pacing_gain=320,cwnd_gain=512,rtt_cnt=277,snd_cwnd=8 [11244.677079] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=1,pacing_gain=192,cwnd_gain=512,rtt_cnt=278,snd_cwnd=8 [11244.719083] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=2,pacing_gain=256,cwnd_gain=512,rtt_cnt=279,snd_cwnd=8 [11244.761082] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=3,pacing_gain=256,cwnd_gain=512,rtt_cnt=280,snd_cwnd=8
黄色颜色部分就是我们突然使用delay_ack的地方，这个可以通过前后两个ack的时间相差来确定。
可以看到，bbr反应相当剧烈，在一个RTO左右的时间，将snd_cwnd迅速降低，
ps：初始状态并不是0，这个并不是我们代码打印有问题，而是内核中真的就这么执行的，个人觉得是个bug，具体分析可以参见《https://www.cnblogs.com/10087622blog/p/10412440.html》。
我们复现的带宽图：
对应的rtt图：
怎么解释在10s附近，带宽增长很多呢？继续根据我们prink的数据来分析：
[11251.018193] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=0,pacing_gain=320,cwnd_gain=512,rtt_cnt=429,snd_cwnd=8 [11251.060199] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=1,pacing_gain=192,cwnd_gain=512,rtt_cnt=430,snd_cwnd=8 [11251.102199] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=2,pacing_gain=256,cwnd_gain=512,rtt_cnt=431,snd_cwnd=8 [11251.144200] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=3,pacing_gain=256,cwnd_gain=512,rtt_cnt=432,snd_cwnd=8 [11251.186198] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=4,pacing_gain=256,cwnd_gain=512,rtt_cnt=433,snd_cwnd=8 [11251.228199] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=5,pacing_gain=256,cwnd_gain=512,rtt_cnt=434,snd_cwnd=8 [11251.270202] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=6,pacing_gain=256,cwnd_gain=512,rtt_cnt=435,snd_cwnd=8 [11251.312199] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=7,pacing_gain=256,cwnd_gain=512,rtt_cnt=436,snd_cwnd=8 [11251.354205] main mode=2,min_rtt_us=553,cur_bw=3197,cycle_idx=0,pacing_gain=320,cwnd_gain=512,rtt_cnt=437,snd_cwnd=8 [11251.396210] mode 2 mode=3,min_rtt_us=41442,full_bw=109875,cycle_idx=1,pacing_gain=192,cwnd_gain=512,rtt_cnt=438------mode从探测bw到探测rtt [11251.396215] main mode=3,min_rtt_us=41442,cur_bw=3197,cycle_idx=1,pacing_gain=256,cwnd_gain=256,rtt_cnt=438,snd_cwnd=4----取4个mss来发包 [11251.438203] main mode=3,min_rtt_us=41442,cur_bw=3197,cycle_idx=1,pacing_gain=256,cwnd_gain=256,rtt_cnt=439,snd_cwnd=4----稳定的min_rtt变成41442us [11251.480205] main mode=3,min_rtt_us=41442,cur_bw=3197,cycle_idx=1,pacing_gain=256,cwnd_gain=256,rtt_cnt=440,snd_cwnd=4 [11251.522214] main mode=3,min_rtt_us=41442,cur_bw=3197,cycle_idx=1,pacing_gain=256,cwnd_gain=256,rtt_cnt=441,snd_cwnd=4 [11251.564202] main mode=3,min_rtt_us=41442,cur_bw=3197,cycle_idx=1,pacing_gain=256,cwnd_gain=256,rtt_cnt=442,snd_cwnd=4----探测持续200ms，符合代码 [11251.606206] mode 2 mode=2,min_rtt_us=41442,full_bw=109875,cycle_idx=1,pacing_gain=256,cwnd_gain=256,rtt_cnt=443-------mode从探测rtt回到探测bw [11251.606215] main mode=2,min_rtt_us=41442,cur_bw=3197,cycle_idx=2,pacing_gain=256,cwnd_gain=512,rtt_cnt=443,snd_cwnd=12---开始指数增长我们的cwnd [11251.648208] main mode=2,min_rtt_us=40906,cur_bw=4795,cycle_idx=3,pacing_gain=256,cwnd_gain=512,rtt_cnt=444,snd_cwnd=24 [11251.690214] main mode=2,min_rtt_us=39861,cur_bw=9589,cycle_idx=4,pacing_gain=256,cwnd_gain=512,rtt_cnt=445,snd_cwnd=48 [11251.732220] main mode=2,min_rtt_us=36713,cur_bw=19184,cycle_idx=5,pacing_gain=256,cwnd_gain=512,rtt_cnt=446,snd_cwnd=90 [11251.774230] main mode=2,min_rtt_us=31462,cur_bw=35975,cycle_idx=6,pacing_gain=256,cwnd_gain=512,rtt_cnt=447,snd_cwnd=142 [11251.816239] main mode=2,min_rtt_us=25016,cur_bw=50376,cycle_idx=7,pacing_gain=256,cwnd_gain=512,rtt_cnt=448,snd_cwnd=160 [11251.858244] main mode=2,min_rtt_us=25016,cur_bw=50376,cycle_idx=0,pacing_gain=320,cwnd_gain=512,rtt_cnt=449,snd_cwnd=160 [11251.900231] main mode=2,min_rtt_us=23843,cur_bw=50376,cycle_idx=1,pacing_gain=192,cwnd_gain=512,rtt_cnt=450,snd_cwnd=150 [11251.942238] main mode=2,min_rtt_us=23843,cur_bw=50376,cycle_idx=2,pacing_gain=256,cwnd_gain=512,rtt_cnt=451,snd_cwnd=154 [11251.984226] main mode=2,min_rtt_us=23841,cur_bw=50376,cycle_idx=3,pacing_gain=256,cwnd_gain=512,rtt_cnt=452,snd_cwnd=154 [11252.026236] main mode=2,min_rtt_us=23841,cur_bw=50376,cycle_idx=4,pacing_gain=256,cwnd_gain=512,rtt_cnt=453,snd_cwnd=154 [11252.068228] main mode=2,min_rtt_us=23828,cur_bw=50382,cycle_idx=5,pacing_gain=256,cwnd_gain=512,rtt_cnt=454,snd_cwnd=154 [11252.110231] main mode=2,min_rtt_us=23828,cur_bw=50382,cycle_idx=6,pacing_gain=256,cwnd_gain=512,rtt_cnt=455,snd_cwnd=154 [11252.152229] main mode=2,min_rtt_us=23828,cur_bw=50382,cycle_idx=7,pacing_gain=256,cwnd_gain=512,rtt_cnt=456,snd_cwnd=154 [11252.194235] main mode=2,min_rtt_us=23828,cur_bw=50382,cycle_idx=0,pacing_gain=320,cwnd_gain=512,rtt_cnt=457,snd_cwnd=154 [11252.236229] main mode=2,min_rtt_us=23828,cur_bw=50382,cycle_idx=1,pacing_gain=192,cwnd_gain=512,rtt_cnt=458,snd_cwnd=150 [11252.278264] main mode=2,min_rtt_us=23828,cur_bw=50382,cycle_idx=2,pacing_gain=256,cwnd_gain=512,rtt_cnt=459,snd_cwnd=154 [11252.320232] main mode=2,min_rtt_us=23828,cur_bw=50407,cycle_idx=sh 将Ubuntu Linux内核更新到4.9包含BBR算法，它可以显着提高TCP连接性能。 （就像服务一样