如题，请问下面结果中的usecs/call是什么意思？

strace -c ls -R
Entries  Repository  Root
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 67.09    0.412153          14     29664           getdents64
 27.70    0.170168          11     14849        14 open
  4.24    0.026043           0    123740           write
  0.72    0.004443           0     14837           close
  0.20    0.001204           0     14836           fstat
  0.05    0.000285         285         1           execve
  0.00    0.000000           0        12           read
  0.00    0.000000           0         4         3 stat
  0.00    0.000000           0        33           mmap
  0.00    0.000000           0        18           mprotect
  0.00    0.000000           0         4           munmap
  0.00    0.000000           0        12           brk
  0.00    0.000000           0         2           rt_sigaction
  0.00    0.000000           0         1           rt_sigprocmask
  0.00    0.000000           0         2           ioctl
  0.00    0.000000           0         1         1 access
  0.00    0.000000           0         3           mremap
  0.00    0.000000           0         1           fcntl
  0.00    0.000000           0         1           getrlimit
  0.00    0.000000           0         1           statfs
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         3         1 futex
  0.00    0.000000           0         1           set_tid_address
  0.00    0.000000           0         8           fadvise64
  0.00    0.000000           0         1           set_robust_list
------ ----------- ----------- --------- --------- ----------------
100.00    0.614296                198036        19 total