`ulimit -e` 和 `renice` 之间的区别？

Question

我想以低 CPU 和磁盘 I/O 运行备份脚本。

这有什么不同吗：

#!/bin/bash

ulimit -e 19
ionice -c3 -p $$

和这个：

#!/bin/bash

ionice -c3 -p $$
renice -n 19 -p $$

Answer 1

There is big difference between them.

• ulimit -e only set the RLIMIT_NICE, which is a upper bound value to which the process's nice value can be set using setpriority or nice.

• renice alters the priority of running process.

Doing strace:

$ cat test.sh
#!/bin/bash

ulimit -e 19

Then:

$ strace ./test.sh
...................................................
read(255, "#!/bin/bash\n\nulimit -e 19\n", 26) = 26
rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
getrlimit(RLIMIT_NICE, {rlim_cur=0, rlim_max=0}) = 0
getrlimit(RLIMIT_NICE, {rlim_cur=0, rlim_max=0}) = 0
setrlimit(RLIMIT_NICE, {rlim_cur=19, rlim_max=19}) = 0
rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
read(255, "", 26)                       = 0
exit_group(0)

You can see, ulimit only call setrlimit syscall to change the value of RLIMIT_NICE, nothing more.

Note

• man setrlimit
• A good explanation about RLIMIT_NICE