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Comment by klodolph

4 years ago

fork() is not a small tool.

It is a MASSIVE, unwieldy tool that is difficult to use correctly. It happens to have a small interface.

22 comments

klodolph

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adastra22  4 years ago

...is it?

In it is original implementation, fork() was pretty trivial. All it did was create a new process entry in the kernel table, with all the pages and capabilities and such copied from the original process. Then mark all pages as copy-on-write, and return to the caller. Maybe not trivial, but much less complicated than loading an executable file from disk.

My understanding of Linux internals is maybe 20 years out of date, so I am legitimately curious what makes fork() so complicated these days.

  • cryptonector  4 years ago

    fork() is not trivial now. Processes are huge now -- they have huge heaps among other things. Copying all that is expensive. In the 80s we tried COW, but that turns out to be very slow as well. What operating systems do now is immediately copy the resident set, then do COW for the rest of writable memory, but in large, multi-threaded processes, this is still too slow.

    Use vfork() or posix_spawn().

    • adastra22  4 years ago

      Hrm. Googling "fork linux copy-on-write" seems to find a lot of stack overflow answers from 2014-2015 claiming Linux marks pages as copy-on-write when fork() is called. I didn't see anything more recent in the first page of results.

      I could see it being worthwhile to immediately copy a few pages, like the top of the stack, but copying the whole resident set seems excessive. Especially since some of that data might not even be written to.

      7 replies →

    • spc476  4 years ago

      Hmmm, from <https://www.man7.org/linux/man-pages/man3/posix_spawn.3.html>:

          The posix_spawn() and posix_spawnp() functions provide the
    functionality of a combined fork(2) and exec(3), with some
    optional housekeeping steps in the child process before the
    exec(3).  These functions are not meant to replace the fork(2)
    and execve(2) system calls.  In fact, they provide only a subset
    of the functionality that can be achieved by using the system
    calls.

      Also, there's no way to set resource limits in the child process, nor switch user or group ID, using posix_spawn().

      1 reply →

  • turminal  4 years ago

    > Then mark all pages as copy-on-write, and return to the caller.

    Unix actually copied the memory over initially.