【发布时间】：2023-03-28 02:20:01
【问题描述】：
我在我的 Mint/Debian 盒子上安装了非常简单的 CouchDB。我的 Java webapp 在查询 CouchDB 时遇到了相当长的延迟，所以我开始寻找原因。
编辑：查询模式是大量的小查询和小的 JSON 对象（例如 300 字节向上/1Kbyte 向下）。
Wireshark 转储非常好，主要显示 3-5 毫秒的请求-响应周转时间。 JVM 帧采样向我展示了套接字代码（对 Couch 的客户端查询）有些忙，但没有什么了不起的。然后我尝试使用 ApacheBench 和 oops 进行分析：我目前看到 keep-alive 在非持久设置上引入了稳定的额外 39 毫秒延迟。
有人知道怎么解释吗？也许持久连接会增加 TCP 层的拥塞窗口，然后由于 TCP_WAIT 和较小的请求/响应大小或类似的东西而空闲？ 是否应该为环回 tcp 连接打开此选项 (TCP_WAIT)？
w@mint ~ $ uname -a
Linux mint 2.6.39-2-486 #1 Tue Jul 5 02:52:23 UTC 2011 i686 GNU/Linux
w@mint ~ $ curl http://127.0.0.1:5984/
{"couchdb":"Welcome","version":"1.1.1"}
在保持活动状态下运行，每个请求平均 40 毫秒
w@mint ~ $ ab -n 1024 -c 1 -k http://127.0.0.1:5984/
>>>snip
Server Software: CouchDB/1.1.1
Server Hostname: 127.0.0.1
Server Port: 5984
Document Path: /
Document Length: 40 bytes
Concurrency Level: 1
Time taken for tests: 41.001 seconds
Complete requests: 1024
Failed requests: 0
Write errors: 0
Keep-Alive requests: 1024
Total transferred: 261120 bytes
HTML transferred: 40960 bytes
Requests per second: 24.98 [#/sec] (mean)
Time per request: 40.040 [ms] (mean)
Time per request: 40.040 [ms] (mean, across all concurrent requests)
Transfer rate: 6.22 [Kbytes/sec] received
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 0.0 0 0
Processing: 1 40 1.4 40 48
Waiting: 0 1 0.7 1 8
Total: 1 40 1.3 40 48
Percentage of the requests served within a certain time (ms)
50% 40
>>>snip
95% 40
98% 41
99% 44
100% 48 (longest request)
没有 keepalive，瞧 - 每个请求 1 毫秒，大部分时间。
w@mint ~ $ ab -n 1024 -c 1 http://127.0.0.1:5984/
>>>snip
Time taken for tests: 1.080 seconds
Complete requests: 1024
Failed requests: 0
Write errors: 0
Total transferred: 236544 bytes
HTML transferred: 40960 bytes
Requests per second: 948.15 [#/sec] (mean)
Time per request: 1.055 [ms] (mean)
Time per request: 1.055 [ms] (mean, across all concurrent requests)
Transfer rate: 213.89 [Kbytes/sec] received
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 0.0 0 0
Processing: 1 1 1.0 1 11
Waiting: 1 1 0.9 1 11
Total: 1 1 1.0 1 11
Percentage of the requests served within a certain time (ms)
50% 1
>>>snip
80% 1
90% 2
95% 3
98% 5
99% 6
100% 11 (longest request)
好的，现在启用 keep-alive，但还要求通过 http 标头关闭连接。每个请求也需要 1 毫秒左右。
w@mint ~ $ ab -n 1024 -c 1 -k -H 'Connection: close' http://127.0.0.1:5984/
>>>snip
Time taken for tests: 1.131 seconds
Complete requests: 1024
Failed requests: 0
Write errors: 0
Keep-Alive requests: 0
Total transferred: 236544 bytes
HTML transferred: 40960 bytes
Requests per second: 905.03 [#/sec] (mean)
Time per request: 1.105 [ms] (mean)
Time per request: 1.105 [ms] (mean, across all concurrent requests)
Transfer rate: 204.16 [Kbytes/sec] received
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 0 0.0 0 0
Processing: 1 1 1.2 1 14
Waiting: 0 1 1.1 1 13
Total: 1 1 1.2 1 14
Percentage of the requests served within a certain time (ms)
50% 1
>>>snip
80% 1
90% 2
95% 3
98% 6
99% 7
100% 14 (longest request)
【问题讨论】：
标签： http tcp profiling couchdb congestion-control