You can do a lot with an empty file

Re:You can do a lot with an empty file by adren123 on Thursday April 07, 2022 @03:56AM (#30941855)

What is also interesting is that while strace seems to report much less overhead in "./empty" compared to /bin/true

   $ strace -c ./empty 
   strace: exec: Erreur de format pour exec()
   % time     seconds  usecs/call     calls    errors syscall
   ------ ----------- ----------- --------- --------- ----------------
     0,00    0,000000           0         1         1 execve
   ------ ----------- ----------- --------- --------- ----------------
   100,00    0,000000           0         1         1 total


   $ strace -c /bin/true
   % time     seconds  usecs/call     calls    errors syscall
   ------ ----------- ----------- --------- --------- ----------------
    54,05    0,000060          15         4           mprotect
    24,32    0,000027          27         1           set_tid_address
    10,81    0,000012          12         1           set_robust_list
     8,11    0,000009           9         1           munmap
     2,70    0,000003           3         1           prlimit64
     0,00    0,000000           0         1           read
     0,00    0,000000           0         2           close
     0,00    0,000000           0         8           mmap
     0,00    0,000000           0         1           brk
     0,00    0,000000           0         4           pread64
     0,00    0,000000           0         1         1 access
     0,00    0,000000           0         1           execve
     0,00    0,000000           0         2         1 arch_prctl
     0,00    0,000000           0         2           openat
     0,00    0,000000           0         2           newfstatat
   ------ ----------- ----------- --------- --------- ----------------
   100,00    0,000111           3        32         2 total

the multiple execution seems to be much slower with the empty executable than with /bin/true using the multitime tool https://tratt.net/laurie/src/multitime/

   $ multitime -n 10 ./empty
   ===> multitime results
   1: ./empty
               Mean        Std.Dev.    Min         Median      Max
   real        0.006       0.000       0.005       0.006       0.006       
   user        0.001       0.001       0.000       0.002       0.002       
   sys         0.000       0.001       0.000       0.000       0.002       
   
   $ multitime -n 10 /bin/true
   ===> multitime results
   1: /bin/true
               Mean        Std.Dev.    Min         Median      Max
   real        0.002       0.000       0.001       0.002       0.003       
   user        0.001       0.000       0.001       0.001       0.001       
   sys         0.000       0.000       0.000       0.000       0.000

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Re:You can do a lot with an empty file by faho on Thursday April 07, 2022 @06:25AM (#30942719)

> What is also interesting is that while strace seems to report much less overhead in "./empty" compared to /bin/true

That's because it reports an error and does not actually execute the file.

>strace: exec: Erreur de format pour exec()

`strace` uses one of the exec()-style functions that don't automatically invoke a shell here. And if it did, it would invoke an entire shell (/bin/sh, typically), which might have more overhead.

----

You need to be very careful when measuring this, because what happens is that a shell executes the shebangless empty file.

If you try launching it from e.g. a C program using execlp(), it will have to first start that shell.

If you try launching it from e.g. bash, it might directly run that in-process or at least after a fork() or similar, skipping some of the shell setup.

So the overhead might depend on context.

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