← Back to context

Comment by chubot

4 years ago

It seems like this 2019 paper covers this point, and the content in the gist? I was expecting to see a reference to it

A fork() in the road

https://news.ycombinator.com/item?id=19621799

Although it does say that vfork() is difficult to use safely, while the gist recommends it? I think there is still some clarity needed around the use cases.

Fork today is a convenient API for a single-threaded process with a small memory footprint and simple memory layout that requires fine-grained control over the execution environment of its children but does not need to be strongly isolated from them. In other words, a shell. It’s no surprise that the Unix shell was the first program to fork [69], nor that defenders of fork point to shells as the prime example of its elegance [4, 7]. However, most modern programs are not shells. Is it still a good idea to optimise the OS API for the shell’s convenience?

6 comments

chubot

Reply
cryptonector  4 years ago

As u/amaranth pointed out, my gist predates the MSFT paper, which mostly explains why I didn't reference. Though, to be fair, I saw that paper posted here back in 2019, and I commented on it plenty (13 comments) then. I could have edited my gist to reference it, and, really, probably should have. Sometime this week I will add a reference to it, as well as this and that HN post, since they are clearly germane and useful threads.

I vehemently disagree with those who say that vfork() is much more difficult to use correctly than fork(). Neither is particularly easy to use though. Both have issues to do with, e.g., signals. posix_spawn() is not exactly trivial to use, but it is easier to use it correctly than fork() or vfork(). And posix_spawn() is extensible -- it is not a dead end.

My main points are that vfork() has been unjustly vilified, fork() is really not good, vfork() is better than fork(), and we can do better than vfork(). That said, posix_spawn() is the better answer whenever it's applicable.

Note that the MSFT paper uncritically accepts the idea that vfork() is dangerous. I suspect that is because their focus was on the fork-is-terrible side of things. Their preference seems to be for spawn-type APIs, which is reasonable enough, so why bother with vfork() anyways, right? But here's the thing: Windows WSL can probably get a vfork() added easily enough, and replacing fork() with vfork() will generally be a much simpler change than replacing fork() with posix_spawn(), so I think there is value in vfork() for Microsoft.

Use cases for vfork() or afork()? Wherever you're using fork() today to then exec, vfork() will make that code more performant and it generally won't take too much effort to replace the call to fork() with vfork(). afork() is for apps that need to spawn lots of processes quickly -- these are rare apps, but uses for them do arise from time to time. But also, afork() should be easier to use safely than vfork(). And, again, for Microsoft there is value in vfork() as a smaller change to Linux apps so they can run well in WSL.

BTW, see @famzah's popen-noshell issue #11 [0] for a high-perf spawn use case. I linked it from my gist, and, in fact, the discussion there led directly to my writing that gist.

  [0] https://github.com/famzah/popen-noshell/issues/11

  • Macha  4 years ago

    If you are going to edit, the google query links with the #q=xyz format no longer seem to work, so maybe update them to the ?q=xyz format which still works.

    (Also this article and discussions on it now take up many of the top spots, which I guess is the disadvantage to linking to google for a topic)