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Linux用strace来跟踪进程执行时的系统调用和所接收的信号

strace常用来跟踪进程执行时的系统调用和所接收的信号。 在Linux世界,进程不能直接访问硬件设备,当进程需要访问硬件设备(比如读取磁盘文件,接收网络数据等等)时,必须由用户态模式切换至内核态模式,通过系统调用访问硬件设备。strace可以跟踪到一个进程产生的系统调用,包括参数,返回值,执行消耗的时间。

strace使用参数

  • -p 跟踪指定的进程
  • -f 跟踪由fork子进程系统调用
  • -F 尝试跟踪vfork子进程系统调吸入,与-f同时出现时, vfork不被跟踪
  • -o filename 默认strace将结果输出到stdout。通过-o可以将输出写入到filename文件中
  • -ff 常与-o选项一起使用,不同进程(子进程)产生的系统调用输出到filename.PID文件
  • -r 打印每一个系统调用的相对时间
  • -t 在输出中的每一行前加上时间信息。 -tt 时间确定到微秒级。还可以使用-ttt打印相对时间
  • -v 输出所有系统调用。默认情况下,一些频繁调用的系统调用不会输出
  • -s 指定每一行输出字符串的长度,默认是32。文件名一直全部输出
  • -c 统计每种系统调用所执行的时间,调用次数,出错次数。
  • -e expr 输出过滤器,通过表达式,可以过滤出掉你不想要输出

应用场景

#1.跟踪你的web服务器系统调用
系统调用优化,也是web性能优化的一个较为重要的方向,尤其是在I/O密集型web应用的情况。我们这里的测试环境是CentOS5.4+Nginx+FastCGI。

<?PHP
//file:hello.php
<a href="http://www.php.net/define">define</a>('DOCUMENT_ROOT', <a href="http://www.php.net/dirname">dirname</a>(__FILE__));
include("hello.inc");
include("./hello.inc");
include(DOCUMENT_ROOT . "/hello.inc");
?&gt;
#strace -f -F -o strace_nginx strace /wwwchroot/nginx/sbin/nginx -c /wwwchroot/nginx/nginx.conf
... (有部分不重要的数据影响排版,在这里使用...代替)
//--接受来自客户端的http请求
4165  recv(16, "GET /hello.php HTTP/1.1rnHost: f"..., 32768, 0) = 391
4165  epoll_ctl(9, EPOLL_CTL_MOD, 16, {EPOLLIN|EPOLLOUT|EPOLLET, {u32=3081162952, u64=698098541354471624}}) = 0
//--进行DNS查找
4165  getsockname(16, {sa_family=AF_INET, sin_port=htons(80), sin_addr=inet_addr("222.73.211.214")}, [16]) = 0
//--新建一个socket,连接Fast-CGI,端口号为9000
4165  socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 17
4165  ioctl(17, FIONBIO, [1])           = 0
4165  epoll_ctl(9, EPOLL_CTL_ADD, 17, {EPOLLIN|EPOLLOUT|EPOLLET, {u32=3081163048, u64=697886249710965032}}) = 0
4165  connect(17, {sa_family=AF_INET, sin_port=htons(9000), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 )
4165  epoll_wait(9, {{EPOLLOUT, {u32=3081163048, u64=697886249710965032}}, {...}, 5
12, 300000) = 2
4165  gettimeofday({1295420285, 130967}, NULL) = 0
4165  recv(16, 0xbfdd7d8b, 1, MSG_PEEK) = -1 EAGAIN (Resource temporarily unavailable)
4165  getsockopt(17, SOL_SOCKET, SO_ERROR, [0], [4]) = 0
//--将用户http请求交给Fast-CGI
4165  writev(17, [{"111101141330217GATEWA"..., 832}], 1) = 832
4165  epoll_wait(9, {{EPOLLIN|EPOLLOUT, {u32=3081163048, u64=697886249710965032}}}, 512, 300000) = 1
4165  gettimeofday({1295420285, 131559}, NULL) = 0
//--接收Fast-CGI响应
4165  recv(17, "161V2X-Powered-By: PHP/5.2.10"..., 65536, 0) = 112
4165  readv(17, [{""..., 65424}], 1) = 0
4165  mmap2(NULL, 274432, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7514000
4165  close(17)                         = 0
4165  munmap(0xb7514000, 274432)        = 0
//-- 响应客户端http请求,即http响应
4165  writev(16, [{"HTTP/1.1 200 OKrnServer: nginx/0"..., 228}, {"22rn", 4}, ..., 5) = 273
4165  write(5, "116.66.34.82 - - [19/Jan/2011:14"..., 191) = 191
4165  setsockopt(16, SOL_TCP, TCP_NODELAY, [1], 4) = 0
4165  recv(16, 0x9b024e8, 32768, 0)     = -1 EAGAIN (Resource temporarily unavailable)
...

通过这些,我们只能够大概地了解,Nginx这里启用了epoll。同时,还可以了解到Nginx和Fast-CGI底层是如何运作的。奇怪,hello.php文件中有三个inclue,即加载了三次文件,这里没有看到相应的i/o逻辑操作,是为什么呢?这是因为,Nginx并没解析处理PHP脚本,而是交给Fast-CGI去做这部事情了。

#strace -f -F -o php-cgi-strace /wwwchroot/php/bin/php-cgi --fpm-config /wwwchroot/php/etc/php-fpm.conf
//--接收来自Nginx发出的请求
4510  <... accept resumed> {sa_family=AF_INET, sin_port=htons(35983), sin_addr=inet_addr("127.0.0.1")}, [16]) = 3
4510  clock_gettime(CLOCK_MONOTONIC, {22638545, 869965681}) = 0
4510  poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
4510  read(3, "11110", 8)   = 8
4510  read(3, "1", 8)    = 8
4510  read(3, "1410353", 8)   = 8
4510  read(3, "217GATEWAY_INTERFACECGI/1.1175SERV"..., 824) = 824
4510  read(3, "141", 8)    = 8
4510  time(NULL)                        = 1295425149
//--加载请求资源文件hello.php
4510  lstat64("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
4510  lstat64("/var/www", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
4510  lstat64("/var/www/ep", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
4510  lstat64("/var/www/ep/hello.php", {st_mode=S_IFREG|0644, st_size=119, ...}) = 0
4510  clock_gettime(CLOCK_MONOTONIC, {22638545, 870893872}) = 0
4510  setitimer(ITIMER_PROF, {it_interval={0, 0}, it_value={60, 0}}, NULL) = 0
4510  rt_sigaction(SIGPROF, {0x835c120, [PROF], SA_RESTART}, {SIG_DFL, [], 0}, 8) = 0
4510  rt_sigprocmask(SIG_UNBLOCK, [PROF], NULL, 8) = 0
4510  time(NULL)                        = 1295425149
4510  open("/var/www/ep/hello.php", O_RDONLY) = 4
4510  fstat64(4, {st_mode=S_IFREG|0644, st_size=119, ...}) = 0
4510  time(NULL)                        = 1295425149
4510  chdir("/var/www/ep")              = 0
4510  fstat64(4, {st_mode=S_IFREG|0644, st_size=119, ...}) = 0
4510  mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7fe7000
4510  read(4, "n", 8192) = 29
4510  read(4, "", 8192)                 = 0
4510  read(4, "", 8192)                 = 0
4510  close(4)                          = 0
//--  加载hello.inc, 对应php代码include './hello.inc'
4510  getcwd("/var/www/ep"..., 4096)    = 12
4510  time(NULL)                        = 1295425149
4510  open("/var/www/ep/hello.inc", O_RDONLY) = 4
4510  fstat64(4, {st_mode=S_IFREG|0644, st_size=29, ...}) = 0
4510  read(4, "n", 8192) = 29
4510  read(4, "", 8192)                 = 0
4510  read(4, "", 8192)                 = 0
4510  close(4)                          = 0
4510  time(NULL)                        = 1295425149
//-- 加载hello.inc, 对应php代码include DOCUMENT_ROOT . '/hello.inc'
4510  lstat64("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
4510  lstat64("/var/www", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
4510  lstat64("/var/www/ep", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
4510  lstat64("/var/www/ep/hello.inc", {st_mode=S_IFREG|0644, st_size=29, ...}) = 0
4510  open("/var/www/ep/hello.inc", O_RDONLY) = 4
4510  fstat64(4, {st_mode=S_IFREG|0644, st_size=29, ...}) = 0
4510  read(4, "n", 8192) = 29
4510  read(4, "", 8192)                 = 0
4510  read(4, "", 8192)                 = 0
4510  close(4)                          = 0
//-- 将响结果输出给Nginx,并且关闭连接
4510  write(3, "161V2X-Powered-By: PHP/5.2.10"..., 96) = 96
4510  setitimer(ITIMER_PROF, {it_interval={0, 0}, it_value={0, 0}}, NULL) = 0
4510  write(3, "13110ere", 16) = 16
4510  shutdown(3, 1 /* send */)         = 0
4510  recv(3, "151", 8, 0) = 8
4510  recv(3, "", 8, 0)                 = 0
4510  close(3)                          = 0

通过跟踪php-cgi,我们可以知道,相较与其它二种方法include ‘./hello.inc’的性能是最高的。这里看到strace输出都被截断了,如果你需要看到更多的输出,可以通过-s选项,让strace输出更多内容。

当你发现某个http请求造成CPU占用效骤然升高,你可以通过strace跟踪查找问题的根源。同时,你也可以通过strace -c统计监控你的优化是否生效

2. mysql执行语句列表

当发生个http请求的时候,很多时候希望得到这个http请求发生了多少次数据库SELECT操作,是否在同一个mysql connection连接里面完成。这里以访问本页为例子,通过strace来跟踪这些MySQL SELECT查询语句。

//-9514是mysqld的进程号,为了看到整条SQL语句,我们通过-s 1024希望输出更多内容
#strace -f -F -ff -o strace-mysqld -s 1024 -p 9514
#find . -name "strace-mysqld*" -type f -print |xargs grep -n "SELECT.*FROM wp_"
./strace-mysqld.19203:64:
read(19, "3SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes'", 72) = 72
./strace-mysqld.19203:165:
read(19, "3SELECT * FROM wp_users WHERE user_login = 'admin'", 50) = 50
./strace-mysqld.19203:184:
read(19, "3SELECT meta_key, meta_value FROM wp_usermeta WHERE user_id = 1", 63) = 63
./strace-mysqld.19203:295:
read(19, "3SELECT option_value FROM wp_options WHERE option_name = 'rewrite_rules' LIMIT 1", 80) = 80
./strace-mysqld.19203:311:
read(19, "3 SELECT   wp_posts.* FROM wp_posts  WHERE 1=1  AND wp_posts.ID = 501
     AND wp_posts.post_type = 'post'  ORDER BY wp_posts.post_date DESC ", 136) = 136
... (这里省去了一些)

其他

strace远不止这么强大,你可以善用之,我想你会离不开它的。同时,你还可以联合gdb和ltrace,你的工作会更加高效。

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