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
表示每个系统调用平均耗时（微秒）