诊断 Ubuntu 中的数据包丢失/高延迟

诊断 Ubuntu 中的数据包丢失/高延迟

我们有一个运行 Nginx (1.5.2) 的 Linux 机器 (Ubuntu 12.04),它充当某些 Tornado 和 Apache 主机的反向代理/负载平衡器。上游服务器在物理和逻辑上都很接近 (同一 DC,有时是同一机架),并且它们之间的延迟为亚毫秒级:

 PING appserver (10.xx.xx.112) 56(84) bytes of data.
 64 bytes from appserver (10.xx.xx.112): icmp_req=1 ttl=64 time=0.180 ms
 64 bytes from appserver (10.xx.xx.112): icmp_req=2 ttl=64 time=0.165 ms
 64 bytes from appserver (10.xx.xx.112): icmp_req=3 ttl=64 time=0.153 ms

我们收到的持续负载约为每秒 500 个请求,目前互联网上经常出现数据包丢失/延迟峰值,甚至基本的 ping 也是如此:

sam@AM-KEEN ~> ping -c 1000 loadbalancer
PING 50.xx.xx.16 (50.xx.xx.16): 56 data bytes
64 bytes from loadbalancer: icmp_seq=0 ttl=56 time=11.624 ms
64 bytes from loadbalancer: icmp_seq=1 ttl=56 time=10.494 ms

... many packets later ...

Request timeout for icmp_seq 2
64 bytes from loadbalancer: icmp_seq=2 ttl=56 time=1536.516 ms
64 bytes from loadbalancer: icmp_seq=3 ttl=56 time=536.907 ms
64 bytes from loadbalancer: icmp_seq=4 ttl=56 time=9.389 ms

... many packets later ...

Request timeout for icmp_seq 919
64 bytes from loadbalancer: icmp_seq=918 ttl=56 time=2932.571 ms
64 bytes from loadbalancer: icmp_seq=919 ttl=56 time=1932.174 ms
64 bytes from loadbalancer: icmp_seq=920 ttl=56 time=932.018 ms
64 bytes from loadbalancer: icmp_seq=921 ttl=56 time=6.157 ms

--- 50.xx.xx.16 ping statistics ---
1000 packets transmitted, 997 packets received, 0.3% packet loss
round-trip min/avg/max/stddev = 5.119/52.712/2932.571/224.629 ms

模式总是相同的:一切运行良好一段时间(<20 毫秒),然后 ping 完全中断,然后是三到四次高延迟 ping(>1000 毫秒),然后它再次稳定下来。

流量通过绑定的公共接口(我们称之为bond0)进入,配置如下:

 bond0     Link encap:Ethernet  HWaddr 00:xx:xx:xx:xx:5d
           inet addr:50.xx.xx.16  Bcast:50.xx.xx.31  Mask:255.255.255.224
           inet6 addr: <ipv6 address> Scope:Global
           inet6 addr: <ipv6 address> Scope:Link
           UP BROADCAST RUNNING MASTER MULTICAST  MTU:1500  Metric:1
           RX packets:527181270 errors:1 dropped:4 overruns:0 frame:1
           TX packets:413335045 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:0 
           RX bytes:240016223540 (240.0 GB)  TX bytes:104301759647 (104.3 GB)

然后通过 HTTP 向私有网络上的上游服务器(我们可以称之为bond1)提交请求,其配置如下:

  bond1     Link encap:Ethernet  HWaddr 00:xx:xx:xx:xx:5c  
            inet addr:10.xx.xx.70  Bcast:10.xx.xx.127  Mask:255.255.255.192
            inet6 addr: <ipv6 address> Scope:Link
            UP BROADCAST RUNNING MASTER MULTICAST  MTU:1500  Metric:1
            RX packets:430293342 errors:1 dropped:2 overruns:0 frame:1
            TX packets:466983986 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:0 
            RX bytes:77714410892 (77.7 GB)  TX bytes:227349392334 (227.3 GB)

uname -a 的输出:

Linux <hostname> 3.5.0-42-generic #65~precise1-Ubuntu SMP Wed Oct 2 20:57:18 UTC 2013 x86_64 GNU/Linux

我们尝试进行自定义sysctl.conf以修复该问题,但没有成功。输出/etc/sysctl.conf(省略了不相关的配置):

# net: core
net.core.netdev_max_backlog = 10000

# net: ipv4 stack
net.ipv4.tcp_ecn = 2
net.ipv4.tcp_sack = 1
net.ipv4.tcp_fack = 1
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_tw_recycle = 0
net.ipv4.tcp_timestamps = 1
net.ipv4.tcp_window_scaling = 1
net.ipv4.tcp_no_metrics_save = 1
net.ipv4.tcp_max_syn_backlog = 10000
net.ipv4.tcp_congestion_control = cubic
net.ipv4.ip_local_port_range = 8000 65535

net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_synack_retries = 2

net.ipv4.tcp_thin_dupack = 1
net.ipv4.tcp_thin_linear_timeouts = 1

net.netfilter.nf_conntrack_max = 99999999
net.netfilter.nf_conntrack_tcp_timeout_established = 300

输出dmesg -d,抑制非 ICMP UFW 消息:

[508315.349295 <   19.852453>] [UFW BLOCK] IN=bond1 OUT= MAC=<mac addresses> SRC=118.xx.xx.143 DST=50.xx.xx.16 LEN=68 TOS=0x00 PREC=0x00 TTL=51 ID=43221 PROTO=ICMP TYPE=3 CODE=1 [SRC=50.xx.xx.16 DST=118.xx.xx.143 LEN=40 TOS=0x00 PREC=0x00 TTL=249 ID=10220 DF PROTO=TCP SPT=80 DPT=53817 WINDOW=8190 RES=0x00 ACK FIN URGP=0 ] 

[517787.732242 <    0.443127>] Peer 190.xx.xx.131:59705/80 unexpectedly shrunk window 1155488866:1155489425 (repaired)

在 Debian 系列 Linux 系统上,我怎样才能诊断这个问题的原因?

答案1

如果网络中任何部分已饱和,或者网络中的任何链接出现错误,则可能会出现数据包丢失。除非问题恰好出在交换机到服务器的电缆连接处,否则这不会显示在接口错误计数中。如果问题出在网络中的其他地方,则显示为数据包丢失。

如果您有 TCP 流量,您可能会发现此问题,因为内核中有计数器可以跟踪 TCP 采取恢复步骤来处理流中丢失的数据包。请查看-s上的 (stats) 选项netstat。显示的值是计数器,因此您需要观察它们一段时间才能了解什么是正常的,什么是异常的,但数据就在那里。retransmitdata loss计数器特别有用。

[sadadmin@busted ~]$ netstat -s | egrep -i 'loss|retran'
2058 segments retransmited
526 times recovered from packet loss due to SACK data
193 TCP data loss events
TCPLostRetransmit: 7
2 timeouts after reno fast retransmit
1 timeouts in loss state
731 fast retransmits
18 forward retransmits
97 retransmits in slow start
4 sack retransmits failed

有些工具会对这些值进行采样并为您绘制趋势图,这样您就可以轻松发现何时出现了问题。我使用穆宁

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