It appears that Command::spawn is sometimes orders of magnitute slower when using command-fds. This repo contains a benchmark to test it.

Meaning of the values: 10. percentile, 50. percentile, 90. percentile

We can observe that the spawn() execution time without command-fds does not depend on the amount of RAM used, but there is a huge difference in execution time when command-fds is used.

• command-fds calls Command::pre_exec to set the handler that duplicates FDs:
  • https://github.com/google/command-fds/blob/7714f31e5055058b67e94dc5fc23fc9568768a65/src/lib.rs#L103-L129
• This causes the default Command::spawn implementation on Linux to use the fork() syscall instead of posix_spawn():
  • https://github.com/rust-lang/rust/blob/8c7c151a7a03d92cc5c75c49aa82a658ec1fe4ff/library/std/src/sys/pal/unix/process/process_unix.rs#L455-L463
  • https://github.com/rust-lang/rust/blob/8c7c151a7a03d92cc5c75c49aa82a658ec1fe4ff/library/std/src/sys/pal/unix/process/process_unix.rs#L78-L80
  • https://github.com/rust-lang/rust/blob/8c7c151a7a03d92cc5c75c49aa82a658ec1fe4ff/library/std/src/sys/pal/unix/process/process_unix.rs#L201-L203

The fork() syscall is likely much slower than posix_spawn(), escpecially when memory usage is high. fork() sets all memory pages to copy-on-write, while posix_spawn() uses clone() under the hood to share process memory:

• https://man7.org/linux/man-pages/man2/fork.2.html#NOTES
• https://man7.org/linux/man-pages/man3/posix_spawn.3.html