描述

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以下是提交者Stephen Hemminger對這個patch的描述：

enable high resolution ack time if needed

This is a refined version of an earlier patch by Lucas Nussbaum.

Cubic needs RTT values in miliseconds. If HZ < 1000 then the values will be too coarse.

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代碼

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--- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -459,6 +459,10 @@ static int __init cubictcp_register(void)/* divide by bic_scale and by constant Srtt (100ms) */do_div(cube_factor, bic_scale * 10);+ /* hystart needs ms clock resolution */ + if (hystart && HZ < 1000) + cubictcp.flags |= TCP_CONG_RTT_STAMP; +return tcp_register_congestion_control(&cubictcp);}

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分析

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@include/net/tcp.h： #define TCP_CONG_RTT_STAMP 0x02struct tcp_congestion_ops {...unsigned long flags;... };/*** @tstamp: Time we arrived*/ struct sk_buff {...ktime_t tstamp;... };static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets, u32 prior_snd_una) {ktime_t last_ackt = net_invalid_timestamp();...ca_seq_rtt = now - scb->when;last_ackt = skb->tstamp;...if (ca_ops->pkts_acked) {s32 rtt_us = -1;/* Is the ACK triggering packet unambiguous? */if (! (flag & FLAG_RETRANS_DATA_ACKED)) {/* High resolution needed and available? */if (ca_ops->flags & TCP_CONG_RTT_STAMP &&! ktime_equal(last_ackt, net_invalid_timestamp()))rtt_us = ktime_us_delta(ktime_get_real(), last_ackt); /* 精確到微秒*/else if (ca_seq_rtt >= 0)rtt_us = jiffies_to_usecs(ca_seq_rtt); /*精確度只有jiffies，一般是毫秒 */}ca_ops->pkts_acked(sk, pkts_acked, rtt_us);}... }/*** ktime_equal - Compares two ktime_t variables to see if they are equal* @cmp1: comparable1* @cmp2: comparable2* Compare two ktime_t variables, returns 1 if equal*/ static inline int ktime_equal(const ktime_t cmp1, const ktime_t cmp2) {return cmp1.tv64 == cmp2.tv64; }static inline ktime_t net_invalid_timestamp(void) {return ktime_set(0, 0); }/* Set a ktime_t variable to a value in sec/nsec representation: */ static inline ktime_t ktime_set (const long secs, const unsigned long nsecs) {return (ktime_t) { .tv = { .sec = secs, .nsec = nsecs} }; }@tcp_transmit_skb /* If congestion control is doing timestamping, we must take such a timestamp* before we potentially clone/copy.*/ if (icsk->icsk_ca_ops->flags & TCP_CONG_RTT_STAMP)__net_timestamp(skb); /*設(shè)置skb->tstamp，記錄發(fā)送時間*/static inline void __net_timestamp(struct sk_buff *skb) {skb->tstamp = ktime_get_real(); /* 獲取當(dāng)前時間的ktime_t表示 */ }

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評價

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我們知道當(dāng)HZ < 1000時，計算得到的RTT的精確度就在1ms以上。這個時候可以通過設(shè)置

TCP_CONG_RTT_STAMP標(biāo)志來獲得更高精度的RTT，測量得到的RTT可以精確到微秒。

但是由于x86機器的HZ一般為1000，所以這個補丁的影響有限。

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Author

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zhangskd @ csdn blog

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轉(zhuǎn)載于:https://www.cnblogs.com/aiwz/archive/2013/03/02/6333350.html

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