在大多数情况下,我倾向于在 bash 脚本中使用管道而不是进程替换,特别是在使用多组命令的情况下,因为这样做似乎更具... | ... | ...
可读性... < <(... < <(...))
。
我想知道为什么在某些情况下使用进程替换比使用管道要快得多。
为了测试这一点,我time
使用相同附加命令的迭代创建了两个脚本10000
,其中一个使用管道,另一个使用进程替换。
脚本:
pipeline.bash
:
for i in {1..10000}; do
echo foo bar |
while read; do
echo $REPLY >/dev/null
done
done
proc-sub.bash
for i in {1..10000}; do
while read; do
echo $REPLY >/dev/null
done < <(echo foo bar)
done
结果:
~$ time ./pipeline.bash
real 0m17.678s
user 0m14.666s
sys 0m14.807s
~$ time ./proc-sub.bash
real 0m8.479s
user 0m4.649s
sys 0m6.358s
我知道管道创建子进程,而进程替换创建命名管道或某些文件/dev/fd
,但不清楚这些差异如何影响性能。
答案1
做同样的事情strace
,你可以看到差异:
和pipe
:
$ strace -c ./pipe.sh
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
57.89 0.103005 5 20000 clone
40.81 0.072616 2 30000 10000 wait4
0.58 0.001037 0 120008 rt_sigprocmask
0.40 0.000711 0 10000 pipe
和proc-sub
:
$ strace -c ./procsub.sh
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
85.08 0.045502 5 10000 clone
3.25 0.001736 0 90329 322 read
2.12 0.001133 0 20009 open
2.03 0.001086 0 50001 dup2
通过以上统计数据,您可以看到pipe
创建了更多的子进程(clone
系统调用)并花费了很多时间等待子进程(wait4
系统调用)完成以便父进程继续执行。
Process substitution
不是。它可以直接从子进程读取。Process substitution
与参数和变量扩展同时执行,该命令Process Substitution
在后台运行。从bash manpage
:
Process Substitution
Process substitution is supported on systems that support named pipes
(FIFOs) or the /dev/fd method of naming open files. It takes the form
of <(list) or >(list). The process list is run with its input or out‐
put connected to a FIFO or some file in /dev/fd. The name of this file
is passed as an argument to the current command as the result of the
expansion. If the >(list) form is used, writing to the file will pro‐
vide input for list. If the <(list) form is used, the file passed as
an argument should be read to obtain the output of list.
When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.
更新
对子进程的统计数据进行 strace:
和pipe
:
$ strace -fqc ./pipe.sh
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
70.76 0.215739 7 30000 10000 wait4
28.04 0.085490 4 20000 clone
0.78 0.002374 0 220008 rt_sigprocmask
0.17 0.000516 0 110009 20000 close
0.15 0.000456 0 10000 pipe
和proc-sub
:
$ strace -fqc ./procsub.sh
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
52.38 0.033977 3 10000 clone
32.24 0.020913 0 96070 6063 read
5.24 0.003398 0 20009 open
2.34 0.001521 0 110003 10001 fcntl
1.87 0.001210 0 100009 close