Comment by pizlonator | Hacker News Reader

Comment by pizlonator | Hacker News Reader

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You're referring to something else, and maybe I'm using the term "zygote" incorrectly.

In all uses of zygotes that I have seen, here's what's really happening:

- `fork` is being used to reduce the cost of starting a process that has a high start-up cost. So, you start one process, run it through the expensive initialization, and then fork it from there to start new processes.

- To make this even faster, you have a pool of pre-forked processes sit around.

- Having pre-forked processes sitting around ready to be used is not expensive because of the CoW property and the fact that a process that forks and then immediately pauses will not have triggered any significant CoW yet.

So, the zygote optimization you speak of is in practice only meaningful on top of systems that are using an optimization uniquely enabled by `fork` (avoiding process initialization costs by cloning a process), and that zygote optimization is further optimized by another property of `fork` (memory sharing of forked processes that haven't done anything else yet).

Oh I see. I guess your zygotes have developed more than mine. I think Google may have coined or at least popularized the term zygote for this in Chrome and Android, Chrome documentation [1] says:

> A zygote process is one that listens for spawn requests from a main process and forks itself in response. Generally they are used because forking a process after some expensive setup has been performed can save time and share extra memory pages.

I think reading the first sentance and stopping covers my zygote, but adding the second sentance covers yours. So I think we're both right!

I think both paths are useful. If your children need time to startup and become ready, spawn one that does start up work, and then it (pre)forks at the ready state to have processes ready to handle requests (your zygote). This does require a traditional fork() to avoid duplication of work.

But if forking is expensive at runtime because you have a million FDs open and a whole lot of memory allocations, spawn spawners before you start doing work (my zygote). This could be unnecessary with a inexpensive way to spawn a new process from an process that has lots of resources in use.

Of course, you can also use my zygotes to spawn your zygotes. Zygoteception.

[1] https://chromium.googlesource.com/chromium/src/+/HEAD/docs/l...

I quite like the idea. I’m using OpenBSD on an oldish laptop, and fork-exec is expensive enough that it conflicts with the usb subsystem. Isochronous transfers have a 1ms realtime requirement and it seem that the fork-exec system calls hold the giant lock long enough to mess with it (audio stutters).

While I’ve not bothered to profile it, but it seems that process that have lot of mapped pages is the issue (firefox, emacs,…). In the emacs case, the issue is when the main process trying to fork-exec, if I start a shell session (with shell-mode or term-mode), it works fine.

> Oh I see. I guess your zygotes have developed more than mine. I think Google may have coined or at least popularized the term zygote for this in Chrome and Android, Chrome documentation [1] says:

Google may have popularized the term, but this approach was already in use by KDE developers in the KDE 2.x timeframe, where it was used as part of a system called kdeinit.

In this scheme, launching KDE apps from a KDE desktop could bypass much of the startup cost of dynamic linking by forking from a long-running kdeinit process (with kdeinit itself deliberately linked to all large dependency libs like Qt and kdelibs), dynamically loading the application logic (stored as a .so) and then launching the app.

This was more to save startup time due to how long it took to dynamically resolve a multitude of C++-based symbols back then, all the common logic came before the app's own main() would ever be called. But it did also save a bit of memory as well.

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