我们线上服务器net.core.somaxconn都是默认的128,这个参数会影响到所有AF_INET类型socket的listen队列
Man 2 listen可以知道:
int listen(int s, int backlog);
The backlog parameter defines the maximum length the queue of pending connections may grow to. If a connection request
arrives with the queue full the client may receive an error with an indication of ECONNREFUSED or, if the underlying pro-
tocol supports retransmission, the request may be ignored so that retries succeed.
BUGS
If the socket is of type AF_INET, and the backlog argument is greater than the constant SOMAXCONN (128 in Linux 2.0 &
2.2), it is silently truncated to SOMAXCONN.
也就是说,web应用中listen函数的backlog会给我们内核参数的net.core.somaxconn 限制到128,在高突发的请求中可能会导致链接超时或者触发重传
比如nginx 定义NGX_LISTEN_BACKLOG默认到511, 却由于我们参数未曾优化会限制到128,只有128个connections can be queued in kernel listen
queue(by Igor Sysoev).
#define NGX_LISTEN_BACKLOG 511/ls.backlog = NGX_LISTEN_BACKLOG;/ if (listen(s, ls.backlog) == -1) {
相信其他应用比如squid也会有类似问题,突发的大并发connect请求会由于内核listen队列的限制导致链接超时或者重传,从而影响用户体验
以下是实验测试情况,使用2台机器分别以1000个并发,benchmark方式,请求服务器 ( 相当于2000个并发请求同时请求服务器 )
情景1,默认配置, net.core.somaxconn=128,服务器为nginx
测试客户端A:
Transactions: 2072870 hits
Availability: 99.99 %
Elapsed time: 179.59 secs
Data transferred: 6096.59 MB
Response time: 0.08 secs
Transaction rate: 11542.24 trans/sec
Throughput: 33.95 MB/sec
Concurrency: 927.34
Successful transactions: 2072871
Failed transactions: 300
Longest transaction: 45.30
Shortest transaction: 0.00
错误率大概是1.5%
测试客户端B:
Transactions: 1859454 hits
Availability: 99.99 %
Elapsed time: 179.11 secs
Data transferred: 5468.90 MB
Response time: 0.09 secs
Transaction rate: 10381.63 trans/sec
Throughput: 30.53 MB/sec
Concurrency: 904.45
Successful transactions: 1859454
Failed transactions: 276
Longest transaction: 49.60
Shortest transaction: 0.00
错误率大概也是1.5%
错误提示大都为:
socket: connection timed out
warning: socket: -1803417280 select timed out: Connection timed out
情景2,调整配置, net.core.somaxconn=8192, nginx显式配置 listen 80 default backlog=8192;
测试客户端A:
** SIEGE 2.69
** Preparing 1000 concurrent users for battle.
The server is now under siege…
Lifting the server siege… done.
Transactions: 1789818 hits
Availability: 100.00 %
Elapsed time: 180.00 secs
Data transferred: 5264.09 MB
Response time: 0.10 secs
Transaction rate: 9943.43 trans/sec
Throughput: 29.24 MB/sec
Concurrency: 997.06
Successful transactions: 1789818
Failed transactions: 0
Longest transaction: 0.87
Shortest transaction: 0.00
错误率是0
测试客户端B:
** SIEGE 2.69
** Preparing 1000 concurrent users for battle.
The server is now under siege…
Lifting the server siege… done.
Transactions: 1768585 hits
Availability: 100.00 %
Elapsed time: 179.31 secs
Data transferred: 5201.65 MB
Response time: 0.10 secs
Transaction rate: 9863.28 trans/sec
Throughput: 29.01 MB/sec
Concurrency: 998.30
Successful transactions: 1768588
Failed transactions: 0
Longest transaction: 3.10
Shortest transaction: 0.03
错误率是0