Comment by kazinator | Hacker News Reader

Comment by kazinator | Hacker News Reader

Fork is quite excellent, except in cases when the intent is to run a different program or when threads are involved (threads are basically an incompatible, competing model of concurrency).

The use of fork as a concurrency mechanism (creating a new thread of control that executes in a copy of the address space) is very good and useful.

In the POSIX shell language, the subshell syntax (command1; command2; ...) is easily implemented using fork. This is useful: all destructive manipulations in the subshell like assignments to variables or changing the current directory do not affect the parent.

Check out the fork-based Perl solution to the Amb task in Rosetta code: https://rosettacode.org/wiki/Amb#Using_fork

This essentially simulates continuations (in a way). (If the parent process does nothing but wait for the child to finish, fork can be used to perform speculative execution, similar to creating a continuation and immediately invoking it).

Microsoft "researchers" can stuff it and their company's flagship piece of shit OS.

The paper agrees with you that the fork models had a reason to exist and that is is perfect for shells.

They also point out that on modern hardware you often should want to write multithreaded multiprocess application.

Their main criticism of fork is that it does not compose at any level of the OS (as it cannot be implemented over a different primitive)

I understand that a lot of people here dislike Microsoft for good reason (not only historical), but drawbacks in fork() are well known and recognized, here they point out that it is also hard-to-impossible to implement as a compatibility layer if the kernel does not support fork.

Also:

> Microsoft "researchers" can stuff it and their company's flagship piece of shit OS.

Do you have any reason to insult Microsoft researchers? They have plenty of citations in this paper of other researchers that appear to agree with them. This type of comments does not appear constructive to me

It's an idiotic argument. Only functions compose. Though fork is packaged as a function, it's really an operator with a big effect.

Booting a system doesn't compose; let's not have power-on reset and bootloaders.

Everything in this paper could have been cribbed from twenty year or older Usenet postings, mailing lists and other sources. Fork has been dissected ad nausem; anyone who is anyone in the Unix-like world knows this.

Oh, and threads have perpetually been the way to go on current hardware --- every damn year since 1988 and counting.

Also levels of abstraction compose.

> Booting a system doesn't compose;

Actually this is false, virtual machine and hypervisors allow to boot a system inside another system

Virtual machines can be forked processes, and contain operating systems with forked processes, some of which are virtual machiens ... fork composes!

as a function obviously, the point is that it does not compose easily with other abstractions. That is every other library and OS functionality needs to be fork-aware.

spawn do not have this requirement.

The concept of "fork aware" didn't exist until threads. You could argue it's a thread problem. Remember, every library and OS functionality aso needs to be "thread aware" when threads are introduced. The pthread_atfork function can be thought about as "what do we do about thread and thread paraphernalia when we fork" rather than "what do we do about fork when we have threads".

Even the close-on-exec flag race condition is a result of threads. duplicating a file descriptor and setting its close-on-exec flag is a two step process during which a fork can happen, causing a child to inherit the descriptor without close-on-exec flag being yet set. But that can only happen if there are threads. (Or something crazy, like fork being called out of an async signal handler).

> You could argue it's a thread problem

But I explicitly want to not do it :) thread are obviously a good thing to have.

> every library and OS functionality aso needs to be "thread aware"

which is good, because differently from the case with fork thread aware libraries/OS help performance. Fork aware libraries/OS (in the case fork+exec) do not.

"Fork aware" is "thread aware". Hint: see the "pthread" substring in the identifier "pthread_atfork".

Note that this is necessary only because of the broken threading model that was retrofitted into Unix.

How it should work is that fork should clone the threads also. If a process with 17 threads forks, then the child has 17 threads. The thread IDs should be internal, so that all the pthread_t values in the parent space make sense in the child space and refer to the corresponding threads.

It's not fork's fault that the hacky thread design broke it. Fork is supposed to make a faithful replica of a process; of course if that principle is ignored in a major way (like, oops, where are the parent's threads?) then things are less than copacetic.

Threads also break the concept of a current working directory. If one thread makes a relative path access and another calls chdir, the result is a race condition.

Threads also break signals quite substantially; the integration of signal handling with threads is a mess.

Threads are not inherently a good thing to have; they are idiotic, in fact. Fork provides a disciplined form of threading that eliminates problems from the mutation of shared state, and provides fault isolation. It's much better to use forked processes instead of threads. Shared memory can be used for direct data structure access. With fork, you can create a shared anonymous mmap. This is then cloned into child processes as shared memory at the same virtual address.

I realize that "compating" is a misspelling, but I prefer to read it as a portmanteau of "compatible" and "competing" and think it's quite an excellent word for that difficult concept except that it errs slightly too far on the "competing" side.

Yes, "competible" would be good for the opposite situation.

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