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It is somewhat interesting that the most widely used "big" OS that doesn't use fork, i.e. Windows, has dog slow process creation... I agree that there should b
by anarazel 4mo ago
It is somewhat interesting that the most widely used "big" OS that doesn't use fork, i.e. Windows, has dog slow process creation...
I agree that there should be non-fork primitives, I'm just not that sure that performance is the best argument.
- pjmlp 4mo agoBecause that OS best practices is to use threads. Traditionally Windows applications that create processes all the time come from UNIX heritage. Contrary to UNIX, Windows NT was designed with threads first mentality, from the get go. While on UNIX they were added after fact, and to this day there are gotchas mixing posix threads with signals, fork and exec.
- zozbot234 4mo agoWindows was designed with threads-first mentality because on pre-386 machines you don't have viable process memory protection, so your tasks share memory by necessity. This is not a great argument.
- JdeBP 4mo agoWindows NT was never designed with pre-386 machines in mind. That was the territory of the old DOS+Windows. Windows NT from the get-go was for machines with page-based virtual memory. * https://computernewb.com/~lily/files/Documents/NTDesignWorkbook/vm.pdf https://computernewb.com/~lily/files/Documents/NTDesignWorkb...
- pstuart 4mo agoWinNT 3.5 was a solid offering.
- epcoa 4mo agoThis is not true. NT never had fork, was always based on the assumption of an MMU and Dave Cutler was a well known fork hater in the 80s long before this paper came out and made it cool to be so. By the time Windows 95 was out, the baseline was 386 with an MMU. CreateThread was initially designed for NT in 1993 though (which didn’t support pre-386 CPUs).
- JdeBP 4mo agoAs mentioned elsewhere on this page, Windows NT had fork from the start. Vide NtCreateProcess and what happens if an image file is not explicitly supplied. * https://computernewb.com/~lily/files/Documents/NTDesignWorkbook/proc.pdf https://computernewb.com/~lily/files/Documents/NTDesignWorkb...
- dcrazy 4mo agoNtCreateProcess doesn’t accept an image file parameter.
- JdeBP 4mo agoYou haven't read the doco. I did point to some. The image file is supplied (or not) via the section object. Think it through. Windows NT supported fork from the start in its POSIX subsystem, that subsystem was layered on top of the Native API, and this is the Native API mechanism that the POSIX subsystem employed. Although it took until Gary Nebbett for someone to publicly show how, even though people knew informally back in 1993.
- epcoa 4mo agoNtCreateProcess was not a public Windows API. NT was flexible, that’s not what was being discussed, which should have been clear from the context.
- keitmo 4mo agoNT performed unnatural acts to implement fork semantics for the POSIX subsystem.
- deleted 4mo ago[deleted]
- pjmlp 4mo agoWindows NT! Misread on purpose to make a point?
- dcrazy 4mo agoNT was designed to be platform-agnostic, and its original target was the DEC Alpha. Its process model owes nothing to pre-386 CPUs. The WinAPI CreateProcess function is a layer atop NtCreateProcess, so that is where the pre-386 heritage lives. But even the WinAPI process model changed significantly with 32-bit Windows.
- peterfirefly 4mo agoNo. https://en.wikipedia.org/wiki/Windows_NT#Development https://en.wikipedia.org/wiki/Windows_NT#Development Windows NT was developed on various different CPUs before the Alpha was a thing. When it was released in 1993, it was released for three CPUs: IA-32, MIPS, and Alpha.
- dcrazy 4mo agoSorry, I had conflated Windows NT development with development of 64-bit Windows as told by Raymond Chen: https://learn.microsoft.com/en-us/previous-versions/technet-magazine/cc718978(v=msdn.10) https://learn.microsoft.com/en-us/previous-versions/technet-... Raymond also says elsewhere that most WinNT engineers did development on i386, but doesn’t explicitly say what time period he is describing: https://devblogs.microsoft.com/oldnewthing/20250513-00/?p=111176 https://devblogs.microsoft.com/oldnewthing/20250513-00/?p=11...
- PaulDavisThe1st 4mo agoA more accurate way to describe this is that Windows' (NT onward) core execution context model is a bunch of threads that by default share memory, whereas Unixen have a core task context model of a bunch of threads that by default do not share memory. Both systems are implemented using threads as the execution context, but in Unix, the history means that that you fork+exec most of the time, resulting in a two tasks that do not share memory any more. By contrast, on Windows (NT onward) the common case when creating a new execution context is to create a thread that shares memory with others in its process. Both systems allow the easy use of the other's core abstraction. On Unix, you can either code like its 1986 and use fork without exec, or use clone(3) or any of its higher level abstractions like pthreads. You're right that POSIX semantics get tangled when using threads.
- snozolli 4mo agowhereas Unixen have a core task context model of a bunch of threads that by default do not share memory. How are those not simply child processes? I don't understand your use of the word 'threads' here. Does the Unix world not distinguish between threads and processes? In Win32, threads exist within processes, and you can create new threads or child processes.
- pjmlp 4mo agoActually on Windows a process is a thread with additional information. The unit of execution is the thread. On the UNIX world it depends on which UNIX you are talking about. Linux has a similar model to Windows NT nowadays, hence clone() as key primitive. Other UNIXes have different approaches.
- PaulDavisThe1st 4mo agoI worked on the kernel of DEC Ultrix, Mach/BSD and a couple of other early Unixen. The approach in all the ones I worked on was broadly the same.
- trumpdong 4mo ago
- knome 4mo agothe only difference between a thread and a process on linux is how many structures they share. the function is identical.
- pjmlp 4mo agoAgreed, however not all UNIXes are like Linux.
- sunshowers 4mo agoThe problem is that threads are not fault boundaries but processes are. So they're not interchangeable when you care about resilience and misbehaving code.
- pjmlp 4mo agoTrue, but on Windows the approach is then to use COM servers, which have a faster IPC model, and can even serve multiple clients, depending on how the appartement space is configured.
- mort96 4mo ago"Faster IPC model" than what? Faster than writing to and reading from a pipe? Faster than POSIX shared memory?
- pjmlp 4mo agoThan UNIX fork/exec model, or calling into Create Process all the time. Windows has a more rich set of IPC stuff than POSIX, especially since it has a microkernel like design. If you are going to say it is everything on the same memory space anyway, it isn't. Optional on Windows 10, and enforced on Windows 11, Hyper-V is always running, and several components including kernel and driver modules are sandboxed into their little worlds. Several additional sandboxing changes were announced at BUILD.
- nine_k 4mo agoPOSIX threads having problems with signals is, imho, mostly the problem with signals in general. They are pretty poorly designed: https://lwn.net/Articles/414618/ https://lwn.net/Articles/414618/
- deleted 4mo ago[deleted]
- nvme0n1p1 4mo agoThat's not the reason for the performance difference. Windows does have a fork primitive (ZwCreateProcess) and it's still slower than Linux's equivalent.
- dcrazy 4mo agoAgain, NtCreateProcess does not implement fork(). The fundamental characteristic of fork is that the child is an exact replica of the parent, down to the instruction pointer. Windows does not have a way to create a process object with such a configuration. Also, using the Zw prefix doesn’t make you look more knowledgeable, it makes you look like you’re trying way too hard to borrow credibility.
- nvme0n1p1 4mo agoOkay but people don't claim that copying the instruction pointer (a single machine register) is the reason for any speed difference. They claim it's due to the memory sharing. And that's easily disproven since you can share pages, just like on Linux, simply by passing null for the section handle, yet there's still a performance difference. Why does it matter which prefix I used? They both point to the same routine so my point applies either way.
- netbsdusers 4mo agoIt's a completely uncontroversial fact that NT does implement fork(). Turn to page 183 of Helen Custer's "Inside Windows NT" and you will read about it.
- aseipp 4mo agoI suspect it's a long tail sort of thing; it mostly doesn't matter except when it really matters. It's interesting that the stated motivation for the patch is in the context of agentic tools spawning subcommands. There's some related prior art in this area where the payoffs could be much greater, like fuzzing: https://gts3.org/assets/papers/2017/xu:os-fuzz.pdf https://gts3.org/assets/papers/2017/xu:os-fuzz.pdf is an example. It would be very interesting to see this patch applied to e.g. AFL++
- mort96 4mo agoThe problem with fork isn't really that it's slow. The problem is that if you want it to be not-slow, it locks you into a bunch of OS design decisions: you more or less need a memory subsystem where all writable pages are refcounted and copy-on-write when the refcount is bigger than 1, and you need overcommit. Now these decisions aren't objectively bad, but they have significant trade-offs and it's probably not a good idea that they're forced simply because we use fork()+exec() for process creation.
- theK 4mo agoDidn't he just say that fork turns out to be comparatively faster to the non-fork samples we get? Ie Linux spawns processes faster than Microsoft's kernels?
- nvme0n1p1 4mo agoWe don't have any broadly used non-fork samples. Windows, macOS, and Linux all have fork. So the presence of fork can't be the reason for the performance difference. (Windows's fork is called ZwCreateProcess)
- dcrazy 4mo agoNtCreateProcess does not implement a forking model. It is analogous to posix_spawn.
- nvme0n1p1 4mo agoIf you pass null for the section handle, it shares pages with the calling process, thus implementing a forking model. Or at least the parts of a forking model that some people erroneously believe are responsible for performance differences.
- Someone 4mo agoMacOS has posix_spawn. See https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/posix_spawn.2.html https://developer.apple.com/library/archive/documentation/Sy... (yes, that’s an iOS man page. MacOS has the call, too, but I couldn’t find the man page online and it looks identical to me) I don’t know how they implemented it, though. Under the hood, it could do the equivalent of a fork/exec pair.