我正在使用cpp-netlib在异步模式下使用C ++在Ubuntu 14.04上开发Web服务。该服务需要响应消息正文中通过HTTP POST发送的数据。当接收大于1K的输入时，我观察到性能不佳，我想解决这个问题。
如果数据相对较小，小于1K，服务器几乎立即接收数据。如果数据大于1K，则在将第一个实际数据块呈现给回调之前首次调用异步读回调函数后，大约会有一秒钟的延迟。在最初的延迟之后，后续的块几乎立即到达。
如何消除这种延迟？是使用curl发布测试数据的工件吗？如何轻松测试curl的性能以发布数据？
你可以在github上找到最小的source code来证明这个问题。这是我用来将数据发布到服务器的命令：
rcook$ curl -d @AsyncDaemon.h http://localhost:8787/foo
以下是其输出（注释）的示例：
rcook$ ./async_daemon
1431387368.321863: AsyncDaemon constructor
1431387368.322446: receive thread beginning
*** It's waiting for a connection here.
1431387371.536191: begin transaction 0 on thread 24050
1431387371.536237: transaction 0 constructor
1431387371.536273: received 1206 byte request for /foo from 127.0.0.1:49402
1431387371.536312: invoked asynchronous read
1431387371.536321: end transaction handler
1431387371.536335: begin asynchronous read callback on thread 24050
1431387371.536348: read 0 bytes
1431387371.536386: invoked asynchronous read
1431387371.536394: end asynchronous read callback
*** The asynchronous read callback is invoked quickly, but gets no data.
*** There is then a pause of just over one second before the asynchronous
*** read callback is invoked again.
1431387372.537203: begin asynchronous read callback on thread 24050
1431387372.537253: read 1024 bytes
1431387372.537307: invoked asynchronous read
1431387372.537317: end asynchronous read callback
*** There is no significant delay when reading the next chunk.
1431387372.537429: begin asynchronous read callback on thread 24050
1431387372.537469: read 182 bytes
1431387372.537478: finished reading the body
1431387372.537746: wrote response
1431387372.537763: transaction 0 destructor
1431387372.537772: end asynchronous read callback
*** The server is then killed with a keyboard interrupt.
^C1431387375.382186: terminating with signal 2
1431387375.382231: initiating shutdown
1431387375.382241: stopping server
1431387375.382363: server run finished
1431387375.382423: receive thread ending
1431387375.382522: AsyncDaemon destructor
正如您所看到的，在第一次调用异步读回调（并接收零字节数据，BTW）之后，它会请求另一块输入。此时在输入到达之前暂停一秒钟，在此示例中从1431387371.536394到1431387372.537203。那段时间发生了什么？我怎样才能消除这种延迟？
我已经在线完成了一些研究，并运行了几个实验（同步与异步模式与cpp-netlib（无效），卷曲与libcurl（无效）），但未能找到答案。< / p>
根据jxh的建议，我在示例事务中运行了一个tcp转储：
00:28:01.304446 IP6 localhost.52265 > localhost.8787: Flags [S], seq 3956487146, win 43690, options [mss 65476,sackOK,TS val 395479802 ecr 0,nop,wscale 7], length 0
00:28:01.304461 IP6 localhost.8787 > localhost.52265: Flags [R.], seq 0, ack 3956487147, win 0, length 0
00:28:01.305014 IP localhost.49421 > localhost.8787: Flags [S], seq 1668603425, win 43690, options [mss 65495,sackOK,TS val 395479803 ecr 0,nop,wscale 7], length 0
00:28:01.305039 IP localhost.8787 > localhost.49421: Flags [S.], seq 4010788604, ack 1668603426, win 43690, options [mss 65495,sackOK,TS val 395479803 ecr 395479803,nop,wscale 7], length 0
00:28:01.305079 IP localhost.49421 > localhost.8787: Flags [.], ack 1, win 342, options [nop,nop,TS val 395479803 ecr 395479803], length 0
00:28:01.305185 IP localhost.49421 > localhost.8787: Flags [P.], seq 1:176, ack 1, win 342, options [nop,nop,TS val 395479803 ecr 395479803], length 175
00:28:01.305210 IP localhost.8787 > localhost.49421: Flags [.], ack 176, win 350, options [nop,nop,TS val 395479803 ecr 395479803], length 0
00:28:02.306555 IP localhost.49421 > localhost.8787: Flags [P.], seq 176:1382, ack 1, win 342, options [nop,nop,TS val 395480053 ecr 395479803], length 1206
00:28:02.306620 IP localhost.8787 > localhost.49421: Flags [.], ack 1382, win 1373, options [nop,nop,TS val 395480053 ecr 395480053], length 0
00:28:02.307223 IP localhost.8787 > localhost.49421: Flags [P.], seq 1:52, ack 1382, win 1373, options [nop,nop,TS val 395480053 ecr 395480053], length 51
00:28:02.307270 IP localhost.49421 > localhost.8787: Flags [.], ack 52, win 342, options [nop,nop,TS val 395480053 ecr 395480053], length 0
00:28:02.307494 IP localhost.8787 > localhost.49421: Flags [P.], seq 52:66, ack 1382, win 1373, options [nop,nop,TS val 395480053 ecr 395480053], length 14
00:28:02.307522 IP localhost.49421 > localhost.8787: Flags [.], ack 66, win 342, options [nop,nop,TS val 395480053 ecr 395480053], length 0
00:28:02.307765 IP localhost.8787 > localhost.49421: Flags [F.], seq 66, ack 1382, win 1373, options [nop,nop,TS val 395480053 ecr 395480053], length 0
00:28:02.307867 IP localhost.49421 > localhost.8787: Flags [F.], seq 1382, ack 67, win 342, options [nop,nop,TS val 395480053 ecr 395480053], length 0
00:28:02.307917 IP localhost.8787 > localhost.49421: Flags [.], ack 1383, win 1373, options [nop,nop,TS val 395480053 ecr 395480053], length 0
我对tcpdump不是很熟悉，但看起来像175字节流到服务器（HTTP头？），然后经过一秒多一点的延迟，1206字节流到服务器，然后是51具有最小延迟的字节块，然后是服务器响应。
这告诉我延迟是在客户端引入的，可能是卷曲。有谁知道为什么？
答案 0 :(得分：19)
由于@jxh建议的调试和诊断技术，问题解决了。
将--trace - --trace-time添加到curl命令后发现curl正在花费那个神秘的第二个等待服务器在发送剩余请求之前返回100 Continue响应：
01:31:44.043611 == Info: Connected to localhost (127.0.0.1) port 8787 (#0)
01:31:44.043726 => Send header, 175 bytes (0xaf)
0000: 50 4f 53 54 20 2f 66 6f 6f 20 48 54 54 50 2f 31 POST /foo HTTP/1
0010: 2e 31 0d 0a 55 73 65 72 2d 41 67 65 6e 74 3a 20 .1..User-Agent:
0020: 63 75 72 6c 2f 37 2e 33 35 2e 30 0d 0a 48 6f 73 curl/7.35.0..Hos
0030: 74 3a 20 6c 6f 63 61 6c 68 6f 73 74 3a 38 37 38 t: localhost:878
0040: 37 0d 0a 41 63 63 65 70 74 3a 20 2a 2f 2a 0d 0a 7..Accept: */*..
0050: 43 6f 6e 74 65 6e 74 2d 4c 65 6e 67 74 68 3a 20 Content-Length:
0060: 31 32 30 36 0d 0a 43 6f 6e 74 65 6e 74 2d 54 79 1206..Content-Ty
0070: 70 65 3a 20 61 70 70 6c 69 63 61 74 69 6f 6e 2f pe: application/
0080: 78 2d 77 77 77 2d 66 6f 72 6d 2d 75 72 6c 65 6e x-www-form-urlen
0090: 63 6f 64 65 64 0d 0a 45 78 70 65 63 74 3a 20 31 coded..Expect: 1
00a0: 30 30 2d 63 6f 6e 74 69 6e 75 65 0d 0a 0d 0a 00-continue....
01:31:45.045626 == Info: Done waiting for 100-continue
01:31:45.045831 => Send data, 1206 bytes (0x4b6)
cpp-netlib中的known deficiency（至少从版本0.11.0开始）它不支持发送curl期待的100个Continue响应。
然后解决方案变得令人信服，不要等待100继续响应。当我discovered here时，将-H 'Expect:'添加到curl命令行就可以了。有了这个，整个交易大约需要一毫秒。
由于我回答了自己的问题，几个星期后我没有接受我的回答，让其他人有机会做出更好的贡献。