使用 curl 命令分析请求的耗时情况

发表于 2022-06-23 分类于 OS ， Linux 阅读次数： Valine：

最近工作中遇到一个问题，某个请求的响应特别慢，因此我就希望有一种方法能够分析到底请求的哪一步耗时比较长，好进一步找到问题的原因。

在网络上搜索了一下，发现了一个非常好用的方法，curl 命令就能帮助分析请求的各个部分耗时情况。

curl 参数

curl 命令提供了 -w 参数，这个参数在 manpage 是这样解释的：

-w, --write-out <format>
              Make curl display information on stdout after a completed transfer. The format is a string that may contain plain text mixed with any number of variables. The  format
              can  be  specified  as  a literal "string", or you can have curl read the format from a file with "@filename" and to tell curl to read the format from stdin you write
              "@-".

              The variables present in the output format will be substituted by the value or text that curl thinks fit, as described below. All variables are specified  as  %{vari‐
              able_name} and to output a normal % you just write them as %%. You can output a newline by using \n, a carriage return with \r and a tab space with \t.

它能够按照指定的格式打印某些信息，里面可以使用某些特定的变量，而且支持 \n、\t 和 \r 转义字符。提供的变量很多，比如 status_code、local_port、size_download 等等，这篇文章我们只关注和请求时间有关的变量（以 time_ 开头的变量）。

试用

$ curl -o /dev/null -s -w "time_connect: %{time_connect}\ntime_starttransfer: %{time_starttransfer}\ntime_nslookup:%{time_namelookup}\ntime_total: %{time_total}\n" "https://www.baidu.com"
time_connect: 0.009
time_starttransfer: 0.065
time_nslookup:0.007
time_total: 0.065

步骤

先往文本文件 curl.txt 写入下面的内容：

   time_namelookup:  %{time_namelookup}\n
      time_connect:  %{time_connect}\n
   time_appconnect:  %{time_appconnect}\n
     time_redirect:  %{time_redirect}\n
  time_pretransfer:  %{time_pretransfer}\n
time_starttransfer:  %{time_starttransfer}\n
                   ----------\n
        time_total:  %{time_total}\n

每个变量的解释如下：

time_namelookup：DNS 域名解析的时候，就是把 https://baidu.com 转换成 ip 地址的过程
time_connect：TCP 连接建立的时间，就是三次握手的时间
time_appconnect：SSL/SSH 等上层协议建立连接的时间，比如 connect/handshake 的时间
time_redirect：从开始到最后一个请求事务的时间
time_pretransfer：从请求开始到响应开始传输的时间
time_starttransfer：从请求开始到第一个字节将要传输的时间
time_total：这次请求花费的全部时间

示例

例子：

$ curl -w "@curl.txt" -o /dev/null -s -L  'http://www.baidu.com'
time_namelookup:  0.004
       time_connect:  0.015
    time_appconnect:  0.000
      time_redirect:  0.000
   time_pretransfer:  0.015
 time_starttransfer:  0.027
                    ----------
         time_total:  0.027

可以看到这次请求各个步骤的时间都打印出来了，每个数字的单位都是秒（seconds），这样可以分析哪一步比较耗时，方便定位问题。这个命令各个参数的意义：

-w：从文件中读取要打印信息的格式
-o /dev/null：把响应的内容丢弃，因为我们这里并不关心它，只关心请求的耗时情况
-s：不要打印进度条
从这个输出，我们可以算出各个步骤的时间：

DNS 查询：4ms
TCP 连接时间：pretransfter(15) - namelookup(4) = 11ms
服务器处理时间：starttransfter(27) - pretransfer(15) = 12ms
内容传输时间：total(27) - starttransfer(27) = 0ms

-w 参数详解

以下是 - w 参数对应的一些变量以及对应的解释：

url_effective 最终获取的 url 地址，尤其是当你指定给 curl 的地址存在 301 跳转，且通过 - L 继续追踪的情形。
http_code http 状态码，如 200 成功，301 转向，404 未找到，500 服务器错误等。(The numerical response code that was found in the last retrieved HTTP (S) or FTP (s) transfer. In 7.18.2 the alias response_code was added to show the same info.)
http_connect The numerical code that was found in the last response (from a proxy) to a curl CONNECT request. (Added in 7.12.4)
time_total 总时间，按秒计。精确到小数点后三位。（The total time, in seconds, that the full operation lasted. The time will be displayed with millisecond resolution.）
time_namelookup DNS 解析时间，从请求开始到 DNS 解析完毕所用时间。(The time, in seconds, it took from the start until the name resolving was completed.)
time_connect 连接时间，从开始到建立 TCP 连接完成所用时间，包括前边 DNS 解析时间，如果需要单纯的得到连接时间，用这个 time_connect 时间减去前边 time_namelookup 时间。以下同理，不再赘述。(The time, in seconds, it took from the start until the TCP connect to the remote host (or proxy) was completed.)
time_appconnect 连接建立完成时间，如 SSL/SSH 等建立连接或者完成三次握手时间。(The time, in seconds, it took from the start until the SSL/SSH/etc connect/handshake to the remote host was completed. (Added in 7.19.0))
time_pretransfer 从开始到准备传输的时间。(The time, in seconds, it took from the start until the file transfer was just about to begin. This includes all pre-transfer commands and negotiations that are specific to the particular protocol (s) involved.)
time_redirect 重定向时间，包括到最后一次传输前的几次重定向的 DNS 解析，连接，预传输，传输时间。(The time, in seconds, it took for all redirection steps include name lookup, connect, pretransfer and transfer before the final transaction was started. time_redirect shows the complete execution time for multiple redirections. (Added in 7.12.3))
time_starttransfer 开始传输时间。在发出请求之后，Web 服务器返回数据的第一个字节所用的时间 (The time, in seconds, it took from the start until the first byte was just about to be transferred. This includes time_pretransfer and also the time the server needed to calculate the result.)
size_download 下载大小。(The total amount of bytes that were downloaded.)
size_upload 上传大小。(The total amount of bytes that were uploaded.)
size_header 下载的 header 的大小 (The total amount of bytes of the downloaded headers.)
size_request 请求的大小。(The total amount of bytes that were sent in the HTTP request.)
speed_download 下载速度，单位 - 字节每秒。(The average download speed that curl measured for the complete download. Bytes per second.)
speed_upload 上传速度，单位 - 字节每秒。(The average upload speed that curl measured for the complete upload. Bytes per second.)
content_type 就是 content-Type，不用多说了，这是一个访问我博客首页返回的结果示例 (text/html; charset=UTF-8)；(The Content-Type of the requested document, if there was any.)
num_connects 最近的的一次传输中创建的连接数目。Number of new connects made in the recent transfer. (Added in 7.12.3)
num_redirects 在请求中跳转的次数。Number of redirects that were followed in the request. (Added in 7.12.3)
redirect_url When a HTTP request was made without -L to follow redirects, this variable will show the actual URL a redirect would take you to. (Added in 7.18.2)
ftp_entry_path 当连接到远程的 ftp 服务器时的初始路径。The initial path libcurl ended up in when logging on to the remote FTP server. (Added in 7.15.4)
ssl_verify_result ssl 认证结果，返回 0 表示认证成功。(The result of the SSL peer certificate verification that was requested. 0 means the verification was successful. (Added in 7.19.0))