POSIX Threads,通常简称为Pthreads,它是指开放系统接口标准―POSIX线程,1995年规范的POSIX.1c,即线程扩展标准(IEEE Std 1003.1c-1995),其中定义了关于创建和操作线程的相关API。
如今,在很多类Unix的POSIX兼容操作系统上,如FreeBSD,NetBSD,OpenBSD,GNU/Linux,Mac OS X和Solaris等,都已实现Pthreads并且可用。
但在早期的Linux系统上,它所提供的线程机制LinuxThreads仅只是部分实现了Pthreads,具体来说,LinuxThreads是通过系统调用clone来实现的,并不是Linux内核真正提供支持,直到Linux 2.6时NPTL的加入为止。也就是说,在Linux 2.6以前,操作系统最小的调度单位是进程而不是线程。
LinuxThreads的主要开发者为大牛Xavier Leroy。LinuxThreads有一些缺点,比如信号处理、进程调度和进程间同步原语等方面(参考1),因此出现了
另外两个互相竞争的项目:一个IBM的组的项目叫做NGPT(Next Generation POSIX Threads,下一代POSIX线程),另一个组是由Red Hat程序员组成的。2003年中NGPT被放弃,几乎与此同时NPTL公布了。
(参考2)
关于这部分发展历史不做多说,反正就是在当前的Linux系统中,Pthreads是由NPTL实现的,下面逐一讨论几个主题。
1,NPTL的代码位置以及维护者
NPTL的实现代码位于glibc,从man pthreads可以看到,从glibc-2.3.2开始,NPTL已经可用,而从glibc-2.4开始,旧的LinuxThreads不再可用。
NPTL由glibc开发组持续发展和维护,这其中包括另一位大牛Ulrich Drepper。
2,NPTL的线程模型
在讨论这个问题之前需要先了解kernel thread(核心线程)和user thread(用户线程),其
分类标准主要是线程的调度者在核内还是在核外。前者更利于并发使用多处理器的资源,而后者则更多考虑的是上下文切换开销。
(参考1,参考6)
线程模型分为三种:1:1 (Kernel-level threading)、N:1 (User-level threading)和M:N (Hybrid threading)。
对于Linux系统而言,采用的是1:1线程模型,这意味着对于有N个用户线程的程序,就有N个核心线程与此一一对应,这也被认为是最容易用代码实现并且在实践中最高效的模型。
3,同一个进程的多个线程之间共享了哪些数据,各自单独拥有哪些数据。
共享数据(参考4):
程序指令(Process instructions)
大部分数据(Most data)
打开文件描述符(open files (descriptors))
信号与信号处理(signals and signal handlers)
当前工作目录(current working directory)
用户id和组id(User and group id)
单独享有(参考4):
线程id(Thread ID)
部分寄存器和栈指针(set of registers, stack pointer)
栈局部变量和返回地址(stack for local variables, return addresses)
信号掩码(signal mask)
调度优先级(priority)
返回值(Return value: errno)
实例:
[root@localhost pthread]# rz rz waiting to receive. [root@localhost pthread]# rpm -q glibc glibc-2.12-1.7.el6.x86_64 glibc-2.12-1.7.el6.i686 [root@localhost pthread]# cat pthread1.c /** * gcc pthread1.c -o pthread1 -lpthread * modified by: http://lenky.info/ */ #include <stdio.h> #include <stdlib.h> #include <pthread.h> void *print_message_function( void *ptr ); main() { pthread_t thread1, thread2; char *message1 = "Thread 1" ; char *message2 = "Thread 2" ; int iret1, iret2; /* Create independent threads each of which will execute function */ iret1 = pthread_create( &thread1, NULL, print_message_function, ( void *) message1); iret2 = pthread_create( &thread2, NULL, print_message_function, ( void *) message2); /* Wait till threads are complete before main continues. Unless we */ /* wait we run the risk of executing an exit which will terminate */ /* the process and all threads before the threads have completed. */ pthread_join( thread1, NULL); pthread_join( thread2, NULL); printf ( "Thread 1 returns: %d\n" ,iret1); printf ( "Thread 2 returns: %d\n" ,iret2); exit (0); } void *print_message_function( void *ptr ) { char *message; message = ( char *) ptr; printf ( "%s \n" , message); } [root@localhost pthread]# gcc pthread1.c -o pthread1 -lpthread -g [root@localhost pthread]# gdb ./pthread1 -q Reading symbols from /home/work/pthread/pthread1...done. (gdb) b 40 Breakpoint 1 at 0x4006ca: file pthread1.c, line 40. (gdb) r Starting program: /home/work/pthread/pthread1 [Thread debugging using libthread_db enabled] [New Thread 0x7ffff7fd6710 (LWP 7307)] [Switching to Thread 0x7ffff7fd6710 (LWP 7307)] Breakpoint 1, print_message_function (ptr=0x4007e8) at pthread1.c:40 40 printf ( "%s \n" , message); Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.7.el6.x86_64 (gdb) info threads [New Thread 0x7ffff77d5710 (LWP 7308)] 3 Thread 0x7ffff77d5710 (LWP 7308) 0x0000003eba0e14f1 in clone () from /lib64/libc.so.6 * 2 Thread 0x7ffff7fd6710 (LWP 7307) print_message_function (ptr=0x4007e8) at pthread1.c:40 1 Thread 0x7ffff7fd8700 (LWP 7304) 0x0000003ebac05bf0 in __nptl_create_event () from /lib64/libpthread.so.0 (gdb) thread 3 [Switching to thread 3 (Thread 0x7ffff77d5710 (LWP 7308))]#0 0x0000003eba0e14f1 in clone () from /lib64/libc.so.6 (gdb) info reg rsp rsp 0x7ffff77d4ff0 0x7ffff77d4ff0 (gdb) thread 2 [Switching to thread 2 (Thread 0x7ffff7fd6710 (LWP 7307))]#0 print_message_function ( ptr=0x4007e8) at pthread1.c:40 40 printf ( "%s \n" , message); (gdb) info reg rsp rsp 0x7ffff7fd5e90 0x7ffff7fd5e90 (gdb) thread 1 [Switching to thread 1 (Thread 0x7ffff7fd8700 (LWP 7304))]#0 0x0000003ebac05bf0 in __nptl_create_event () from /lib64/libpthread.so.0 (gdb) info reg rsp rsp 0x7fffffffe3c8 0x7fffffffe3c8 (gdb) |
可以看到三个线程的栈指针是不一样的。另外,我在上面的代码基础上做了一下改动,创建5个相同的线程,查看它们的rsp分别为:
rsp 0x7fffffffe450 0x7fffffffe450 rsp 0x7ffff7fd5ca0 0x7ffff7fd5ca0 rsp 0x7ffff77d4ca0 0x7ffff77d4ca0 rsp 0x7ffff6fd3ca0 0x7ffff6fd3ca0 rsp 0x7ffff67d2ca0 0x7ffff67d2ca0 rsp 0x7ffff5fd1ca0 0x7ffff5fd1ca0 |
上面第一行为线程1(即主线程)的rsp,然后依次为线程2、3、4、5、6的rsp,可以看到,它们各自的可用栈空间是不重叠的,每个栈大小为0×801000,即8196KB,这刚好匹配我的系统设置(各个栈之间应该是留了一页内存作为分割屏障):
[root@localhost pthread]# ulimit -s 8192 |
设置栈大小为1024后,测试各个线程之间的rsp相差:
[root@localhost pthread]# ulimit -s 1024 [root@localhost pthread]# ulimit -s 1024 |
0x7ffff7fd5ca0 – 0x7ffff7ed4ca0 = 0×101000
即1028KB。
[root@localhost pthread]# cat t.c #include <pthread.h> #include <stdio.h> int main( int argc, char *argv[]) { size_t stacksize; pthread_attr_t attr; pthread_attr_init(&attr); pthread_attr_getstacksize (&attr, &stacksize); printf ( "Default stack size = %d\n" , stacksize); } [root@localhost pthread]# gcc t.c -lpthread [root@localhost pthread]# ./a.out Default stack size = 1048576 |
新开终端用ps命令查看:
1 2 3 4 5 6 7 8 | [root@localhost linux-2.6.38.8]# ps -eL -f -F | grep PSR | grep -v grep UID PID PPID LWP C NLWP SZ RSS PSR STIME TTY TIME CMD [root@localhost linux-2.6.38.8]# ps -eL -f -F | grep pthread root 7302 27742 7302 0 1 58006 15672 0 04:43 pts/2 00:00:00 gdb ./pthread1 root 7304 7302 7304 0 3 5641 308 0 04:43 pts/2 00:00:00 /home/work/pthread/pthread1 root 7304 7302 7307 0 3 5641 308 1 04:43 pts/2 00:00:00 /home/work/pthread/pthread1 root 7304 7302 7308 0 3 5641 308 1 04:43 pts/2 00:00:00 /home/work/pthread/pthread1 root 7487 28643 7487 0 1 25800 804 0 04:51 pts/3 00:00:00 grep pthread |
PID和PPID相同,而LWP为thread ID,是各不相同的,NLWP表示number of threads,都为3,PSR列表示线程当前所在的cpu号。
4,NPTL的辅助调试工具
GDB:http://www.sourceware.org/gdb/onlinedocs/gdb/Threads.html
另外两个专门工具貌似很久没更新了,不知道现在是否还有用,如下:
POSIX Thread Trace Tool (PTT):http://nptltracetool.sourceforge.net/
Open POSIX Test Suite (OPTS):http://posixtest.sourceforge.net/
参考:
1,http://www.ibm.com/developerworks/cn/linux/kernel/l-thread/
2,http://zh.wikipedia.org/wiki/Native_POSIX_Thread_Library
3,http://xanpeng.github.com/linux/2012/03/28/linux-pthread.html
4,http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html
5,http://stackoverflow.com/questions/807506/threads-vs-processes-in-linux
6,http://www.thegeekstuff.com/2012/03/linux-threads-intro/
7,http://www.linuxquestions.org/questions/linux-newbie-8/default-stack-size-on-linux-glibc-pthreads-358438/
8,http://stackoverflow.com/questions/807506/threads-vs-processes-in-linux
9,http://stackoverflow.com/questions/2340093/how-is-stack-size-of-process-on-linux-related-to-pthread-fork-and-exec
10,http://xanpeng.github.com/linux/2012/05/15/more-linux-pthreads.html
联系客服