cat | grep vs grep

gistfile1.txt

# strace -cf sh -c "cat 101Mb_file | grep string"
strace: Process 5094 attached
strace: Process 5095 attached
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 92.34    0.148169       74085         2           wait4
  6.02    0.009665          12       804           write
  1.64    0.002630           2      1664           read
  0.00    0.000000           0        18         4 open
  0.00    0.000000           0        24         1 close
  0.00    0.000000           0        14        10 stat
  0.00    0.000000           0        17           fstat
  0.00    0.000000           0        41           mmap
  0.00    0.000000           0        18           mprotect
  0.00    0.000000           0        10           munmap
  0.00    0.000000           0         9           brk
  0.00    0.000000           0         9           rt_sigaction
  0.00    0.000000           0         1           rt_sigprocmask
  0.00    0.000000           0         2           rt_sigreturn
  0.00    0.000000           0         2         1 ioctl
  0.00    0.000000           0        12        12 access
  0.00    0.000000           0         1           pipe
  0.00    0.000000           0         2           dup2
  0.00    0.000000           0         1           getpid
  0.00    0.000000           0         2           clone
  0.00    0.000000           0         3           execve
  0.00    0.000000           0         1           getrlimit
  0.00    0.000000           0         1           getuid
  0.00    0.000000           0         1           getgid
  0.00    0.000000           0         2           geteuid
  0.00    0.000000           0         1           getegid
  0.00    0.000000           0         1           getppid
  0.00    0.000000           0         3           arch_prctl
  0.00    0.000000           0         1           futex
  0.00    0.000000           0         1           set_tid_address
  0.00    0.000000           0         1           fadvise64
  0.00    0.000000           0         1           set_robust_list
------ ----------- ----------- --------- --------- ----------------
100.00    0.160464                  2670        28 total

# strace -cf sh -c "grep string 101Mb_file"
strace: Process 5099 attached
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 61.94    0.060000       60000         1           wait4
 38.06    0.036868         450        82           read
  0.00    0.000000           0         9           open
  0.00    0.000000           0        12           close
  0.00    0.000000           0         8         5 stat
  0.00    0.000000           0        11           fstat
  0.00    0.000000           0         2           lseek
  0.00    0.000000           0        32           mmap
  0.00    0.000000           0        14           mprotect
  0.00    0.000000           0         8           munmap
  0.00    0.000000           0         6           brk
  0.00    0.000000           0         9           rt_sigaction
  0.00    0.000000           0         1           rt_sigprocmask
  0.00    0.000000           0         1           rt_sigreturn
  0.00    0.000000           0         2         1 ioctl
  0.00    0.000000           0         9         9 access
  0.00    0.000000           0         1           getpid
  0.00    0.000000           0         1           clone
  0.00    0.000000           0         2           execve
  0.00    0.000000           0         1           getrlimit
  0.00    0.000000           0         1           getuid
  0.00    0.000000           0         1           getgid
  0.00    0.000000           0         2           geteuid
  0.00    0.000000           0         1           getegid
  0.00    0.000000           0         1           getppid
  0.00    0.000000           0         2           arch_prctl
  0.00    0.000000           0         1           futex
  0.00    0.000000           0         1           set_tid_address
  0.00    0.000000           0         1           openat
  0.00    0.000000           0         1           set_robust_list
------ ----------- ----------- --------- --------- ----------------
100.00    0.096868                   224        15 total