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Comment by dmitrygr | Hacker News Reader

VDSO is designed for things that should be fast and are called often. There is absolutely no reason to call those functions often.

The only time your PID would change from a previous value obtained from that very same function is in the child, after a call to fork(), which is itself so heavy, that it would dwarf your calls to getpid

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Is it a scarce resource? For what it’s worth when the caching was removed from glibc it caused a performance problem in libsystemd which had a tight loop to detect when a process was forked. So while this may be true in many cases, there’s clearly cases where it’s not and your answer doesn’t explain why getpid isn’t a vdso.

> which had a tight loop to detect when a process was forked.

This sounds so mad it needs more context. You don't need to detect when a process is forked, it's an action issued from inside the process?

Maybe the problem is that it is "lib"systemd, meaning it doesn't really have control over when the process forks, it's the application code's responsibility. Although I would probably also avoid this design, but that would probably make the library interface a bit unfriendly, with the application needing to pass its own pid to library functions where appropriate.

The audacity of the design to detect a pid change by looking at it in a loop, is interesting. I don't think I could have come up with something like that. Feels very "sync the web DOM by traversing it and comparing with a shadow DOM".

If only we had an API to tell us when a fork happened. We could place it into the usual library for threading stuff: pthread, and could call it, oh I don't know, maybe pthread_atfork()

https://man7.org/linux/man-pages/man3/pthread_atfork.3.html

It is mentioned in the glibc bug tracker entry as a replacement to the polling. systemd didn't use it because they thought it would be impossible for a library using to be dlclose'd as there is no way to unregister the callback. But apparently glibc has (undocumented) magic under the hood to take care of it.

Memory is always a scarce resource. However, a big you make the vdso, that amount of memory, multiplied by the number of processes is the amount wasted. In reality, it will be more, because of the required additional page tables to map it at an address range that is not commonly used.

Also, what sort of raving lunatic had a busy loop checking the current process PID? Who allowed such a person access to a compiler? That seems like the bigger problem.

I would assume good faith on the part of systemd developers. Afaik they’re as talented as any other top tier group of developers.

Somehow I doubt the code for getpid as a vdso would meaningfully impact the amount of memory used on a running system nor would I expect any meaningful impact to the number of page table entries. Do you have any supporting evidence for your claim otherwise?

Good faith, yes. But even talented people get in strange waters sometimes.

No, libsystemd did this because it’s a library. They went with pthread_at_fork as an alternative to detect when the application code forked, but it’s not without its own set of risks. Of course they could have required the user of the API supply the PID everywhere to avoid this but it’s possible even that wouldn’t solve the problem and it complicates the UI and the UI of the library API is equally important as other technical details - it’s a design tradeoff and judging it as some kind of correctness thing without having a clear picture and insulting the authors out of hand is weird behavior.

I agree with everything you said except that "systemd" and "insulting" don't go together like peanut butter and jelly. Insults seem to be the fuel the project is run on.

>multiplied by the number of processes

Why aren't the pages shared like other dynamic libraries?

If we put the PID into the vDSO, this data page can no longer be shared among different processes (at least not in the same container).

We could use the rseq area for PID and TID.

Because you just asked to put data there that would differ per process?

That doesn't mean that code pages couldn't be shared. The data that differs per process is a single number in this case.

Having said that I agree that it's probably not worth it to put getpid() into vDSO.

A number here, a number there, before you know it, it is a lot.

If we're going to add all sorts of dross to the vdso, like getpid(), who knows what else will end up there?

> The only time your PID would change from a previous value obtained from that very same function is in the child, after a call to fork()

Or clone3, but with PID namespaces the answer to "what is the PID of a process" is a trickier question.

Library code sometimes needs to detect fork without being in control of when the application calls fork. Think things like CSPRNGs. The state of the art here is to put the CSPRNG state in a memory region that is set to zero on fork (madvise MADV_WIPEONFORK), instead of using getpid.

Well, would you look at that, yet another solution that doesn’t require a busy loop burning watts of energy.

Nothing busy looped getpid. A library used getpid on each entry to check if the process had forked. The library was called many times.

getpid was a fairly common way to check for fork prior to mechanisms like madvise WIPEONFORK (Linux 4.14, released in 2017). systemd existed prior to 2017 and likely needed to support older kernels past 4.14's initial release. And the getpid cache was something glibc had prior to April 2017, likely to support this kind of use case. It was even documented behavior[1].

So, anyway, what systemd was doing wasn't totally stupid. It became a lot more expensive when glibc broke it on them and then it needed to be improved. But before that it wasn't as objectionable as people in this thread are suggesting.

(Also: pthread_atfork requires linking pthreads, which is or at least historically was seen as a significant burden on applications which do not use pthreads.)

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1469670#c9

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