← Back to context

Comment by garethrowlands

4 years ago

"The received wisdom suggests that Unix’s unusual combination of fork() and exec() for process creation was an inspired design. In this paper, we argue that fork was a clever hack for machines and programs of the 1970s that has long outlived its usefulness and is now a liability. We catalog the ways in which fork is a terrible abstraction for the modern programmer to use, describe how it compromises OS implementations, and propose alternatives."

from A Fork in the Road, <https://www.microsoft.com/en-us/research/uploads/prod/2019/0...>

38 comments

garethrowlands

Reply
wongarsu  4 years ago

fork() and exec() is a reflection of the unix philosophy: small tools for separate tasks, and as a result you have great composability. You can set up right etc for your new process by calling any number of system calls between fork() and exec().

Windows is a great example of the alternative (following "Windows philosophy"): There are about three different API calls for creating a new process, each with a heap of complicated optional arguments. The API becomes more complex, less composable, less extensible and less powerful. But also easier to reason about and easier for the kernel to provide, and arguably with fewer footguns.

  • _0w8t  4 years ago

    Windows approach is not the only alternative. Simply provide API to create a process in a suspended state, then adjust its properties based on pid/handle and then start the process execution.

    • ncmncm  4 years ago

      A huge number of tricky thread problems go away if the child thread is blocked at startup, and allowed to run only after the parent allows it. To retrofit this, it is easiest to lock a mutex before spawning and have the child block on that. Then the parent unlocks it to let the child run.

      1 reply →

    • Blikkentrekker  4 years ago

      I always felt that the way to create new processes and new threads should initially start the same. A new process is simply a thread that after creation does a syscall to isolate itself from the other thread and receive a unique copy of all resources.

      This could also solve the issue with forking from multithreaded programs since we can ensure we own all shared resources when we isolate our thread, to effectively thus become a new process.

      So instead of fork we have clone/isolate.

      A new thread can also of course immediately suspend itself, allow other threads to work on it's data in some way, who then give it a signal to resume itself and then execute if need be.

  • 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.

    • 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.

      20 replies →

  • phendrenad2  4 years ago

    Has Linux gained syscalls equivalent to those Windows API calls yet? Or is linux too different from the windows kernel to make that happen? (In that case, what does WINE do?)

indrora  4 years ago

To present some contrast:

Windows doesn't have fork(). It has a real, fully mature thread and process model. In Windows NT, every process consists of a handle that is a "Process", which in turn points to a structure containing a list of "Threads". A process is done when its main thread exits or all threads exit, whichever is defined by the main process. Fork/Exec is replaced with CreateProcess (or ShellExecute, your choice).

For a very zen-like example of the fork/exec and pipe management that you'd do on a POSIX system done in Windows, the [MSDN Docs](https://docs.microsoft.com/en-us/windows/win32/procthread/cr...) are quite informative.

  • skissane  4 years ago

    > Windows doesn't have fork()

    It does (or maybe did, not sure if it still works) have the literal equivalent of fork() - NtCreateProcess() with NULL SectionHandle argument creates a new process which is a clone of the caller. However, it never really worked. The Win32 API did not support forking, and so any process which forked itself and then tried to invoke any Win32 API calls would soon crash or fail mysteriously. In theory forking did work for pure NT API applications, but those are rather limited in their abilities.

  • btilly  4 years ago

    When I see what most people try to do with "a real, fully mature thread and process model", I wind up shaking my head about bad engineering processes.

    • jiggawatts  4 years ago

      Bad... how?

      From my perspective watching the various techniques used by a multitude of operating systems over many decades, the Windows-style process+thread model seems to be winning out over the UNIX fork() model.

      For example, PostgreSQL seems to suffer greatly from the "forked process per connection" model, necessitating front-ends that do connection pooling. Database engine after database engine seems to go through this phase and then "upgrade" to either a single-process thread pool model, or start using async IO in some way. (Web servers also.)

      For reference, Microsoft SQL Server back in the 1990s could on Windows NT 4 could handle more connections than PostgreSQL in 2020...