问题
I have got results for strace -c on RHEL 7 and RHEL 6
for this command:
strace -c /bin/sleep 20
and I don't understand why the seconds column for nanosleep is equal to 0. I had expected it to be 20.
0.00 0.000000 0 1 nanosleep
Here is a full strace report:
$ strace -c /bin/sleep 20
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00 0.000019 1 15 12 open
0.00 0.000000 0 1 read
0.00 0.000000 0 5 close
0.00 0.000000 0 8 6 stat
0.00 0.000000 0 3 fstat
0.00 0.000000 0 9 mmap
0.00 0.000000 0 3 mprotect
0.00 0.000000 0 1 munmap
0.00 0.000000 0 3 brk
0.00 0.000000 0 1 1 access
0.00 0.000000 0 1 nanosleep
0.00 0.000000 0 1 execve
0.00 0.000000 0 1 arch_prctl
------ ----------- ----------- --------- --------- ----------------
100.00 0.000019 52 19 total
And there is a call to nanosleep in a detailed report:
nanosleep({20, 0}, NULL) = 0
So seconds must be 20, not 0. What do you think?
回答1:
From the manual page of strace(1):
-cOn Linux, this attempts to show system time (CPU time spent running in the kernel)
I think that:
when a process calls nanosleep(), it asks the kernel to be suspended for a period of time. The kernel sets up a few things (like some flag, a timer, a timestamp...), suspends the calling process, and goes to do something else. strace(1) reports the time spent by the kernel to do this, not the time the process stays suspended.
May be that this -c strace option can be thought as "-cost": how much time this syscall costs?
回答2:
In order to understand this question I run strace for strace -c /bin/sleep:
This is how it looked:
$ strace -T -o syscalls.txt -v strace -c /bin/sleep 20
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
31.54 0.000429 29 15 12 open
13.68 0.000186 23 8 mmap
13.60 0.000185 46 4 mprotect
11.62 0.000158 20 8 6 stat
6.91 0.000094 19 5 close
5.96 0.000081 81 1 munmap
4.63 0.000063 16 4 brk
3.38 0.000046 46 1 arch_prctl
3.16 0.000043 43 1 nanosleep
2.21 0.000030 30 1 read
1.47 0.000020 20 1 1 access
1.32 0.000018 6 3 fstat
0.51 0.000007 7 1 execve
------ ----------- ----------- --------- --------- ----------------
100.00 0.001360 53 19 total
Below is some lines from sycalls.txt related to the nanosleep syscall:
ptrace(PTRACE_SYSCALL, 6498, 0, SIG_0) = 0 <0.000028>
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 <0.000017>
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == 133}], __WALL, {ru_utime={0, 0}, ru_stime={0, 3706}, ru_maxrss=616, ru_ixrss=0, ru_idrss=0, ru_isrss=0, ru_minflt=205, ru_majflt=0, ru_nswap=0, ru_inblock=0, ru_oublock=0, ru_msgsnd=0, ru_msgrcv=0, ru_nsignals=0, ru_nvcsw=108, ru_nivcsw=1}) = 6498 <20.000423>
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=6498, si_status=SIGTRAP, si_utime=0, si_stime=0} ---
So, nanosleep itself lasts for 20 seconds as it is shown in the end of the line: <20.000423>. However wait4 returns for it:
{ru_utime={0, 0}, ru_stime={0, 3706}
So it takes 3 microseconds to do nanosleep according to the report. So the seconds column is likely to mean (user_time + sys_time + some-unclear-overhead) spent by OS to handle a syscall. It is not meant to be wall time for a syscall.
来源：https://stackoverflow.com/questions/42535397/strace-c-does-not-show-correct-seconds