Options for sorting algorithms are severely limited when (a) you can't use malloc, (b) you don't have enough stack to recurse, and (c) you want a stable sort.
(The first two limitations are because this happens very early in the boot process.)
I'm not sure when this sort happens, but early boot may not have a proper virtual memory implementation yet, so the kernel has to use a fixed stack size allocated by the boot loader.
Right, in this case specifically we're running off a limited bootstack allocated in the kernel's .bss (somewhere between 2 to 6 pages, generally); we won't finish initializing VM until shortly after this sort is done (in some of the SYSINITs that we're sorting).
sure, but bubble sort is trivial to write. they may have figured they'd replace it when they needed to. they may have figured the particular usage was so trivial they would never need to.
grabbed the source and git blamed the file before the patch to see when the original code was from.
94e9d7c12 (Peter Wemm 1998-10-09 23:42:47 +0000 157) * The sysinit table itself. Items are checked off as the are run.
94e9d7c12 (Peter Wemm 1998-10-09 23:42:47 +0000 158) * If we want to register new sysinit types, add them to newsysinit.
So the section was written in 98 and had some bits modified in 2001.
The time wasted during boot here was probably irrelevant next to a hundred other things the project has worked on since.
the bubble sort was good enough that 25 years later they replaced it to save only 2ms during boot.
You'd still think it'd be easier to write an insertion sort though, even accidentally! It's almost the same algorithm conceptually, except cutting out an odd pessimization.
Bubble sort is optimal for sorting data stored on tapes (no random access possible).
If you have RAM, insertion sort is strictly superior (both simpler code + better performance).
Because bubble sort is often faster for small sorts and it’s easy to write. This is the kernel, there’s no libc you have to write the sort algo yourself.
It's really because C doesn't have a proper dependency management system like modern languages do. If it did there's no reason you couldn't import a third party sorting algorithm in the kernel. You can do that in Rust for example, although you would just use the built in sort in this case.
It's not because of that. There exist countless BSD compatible code out there, and many are taken and used in projects like this. CRC codes being an obvious example. There is no reason sort code could not have been grabbed from somewhere, no "dependency management system" required. The FreeBSD kernel has its own existing sort libraries already in the code too -- https://cgit.freebsd.org/src/tree/sys/libkern/qsort.c?id=9a7...
The real reason is because this is an early boot environment with exact constraints that are unusual if not unique to FreeBSD where the facilities to support your general language / runtime environment is not up yet.
> the facilities to support your general language / runtime environment is not up yet.
Rust's [T].sort_unstable() is part of core, that is, the code to sort a slice of arbitrary type T doesn't need the allocator, operating system etc.
Only if you want a fast stable sort do you need to wait for such an environment because their fast stable sort needs an allocator, I doubt that FreeBSD needs a stable sort here.
It does need to be stable, which is why the replacement is mergesort.
The SYSINITs need to bring up the system in a particular order. I dunno why there are both explicit ordering (the sort keys) and implicit ordering (the stability requirement), perhaps cperciva can chime in.
Based on when I last looked at this code, I guess the explicit ordering is to do with startup phases, and the implicit ordering is because of the way SYSINITs are (or were?) implemented using linker sets, which have more guarantees about ordering than __attribute__((constructor)) and other similar functionality.
> a number of bugs have happened in the past because people didn't order properly.
This may be impractical, but I think my reaction would be to re-design it so that the ordering was mechanically a full order, even if culturally the people writing these things don't need to specify if they don't want to.
e.g. maybe a macro can turn your existing explicit order into high bits of a value, and then a "noise" factor into low bits, where that "noise" is derived from a date integer like 20230822 or filenames or whatever, and truly order on the entire value.
The idea is, this means any hidden order is the same for everyone, Intel laptop test rig, ARM WiFi cameras at customer site, last week's build, this week's build, Colin's build, the CI system's build, they're all using the same order, because while heisenbugs are strictly worse, "It only happens with my setup" is also extremely frustrating.
When people discover a hidden ordering requirement they can adjust the "real" ordering accordingly, just now there's a deliberate systemic consistency.
Probably not worth all the bother, but I think I couldn't live with the "stable sort to avoid heisenbugs" situation.
I'm not sure what part of my post you are addressing.
If you had a kernel written in Rust, you would have to bring up the rust runtime environment as part of the boot process. You don't just magically get it because that's the language you used, whether it is "core" or not. You can't even execute simple functions before you have set up stack, and in this case apparently there is a constraint on stack usage. And evidently they do need a stable sort.
In any case my main point is that it's not anything to do with dependency management, not quibbling about whether or not some language could support this function at this exact point of the boot.
> you would have to bring up the rust runtime environment as part of the boot process
Like the existing C runtime environment, this just isn't a big deal by the time we've got here. If you're the code for initialising the DRAM controller then sure, that's pretty intimidating. But this is all happening much later, we're part of an operating system, we were already loaded from disk.
> And evidently they do need a stable sort.
How so? They appear to even have a deliberate tie-breaking mechanism in their data structure, which suggests if order actually matters you're expected to specify when creating the data, not rely on stability to do what you expected.
> > you would have to bring up the rust runtime environment as part of the boot process
> Like the existing C runtime environment, this just isn't a big deal by the time we've got here. If you're the code for initialising the DRAM controller then sure, that's pretty intimidating. But this is all happening much later, we're part of an operating system, we were already loaded from disk.
I don't know what you're trying to say. The code which is written here that we are discussing is in a constrained environment where the regular kernel runtime facilities are not all available. This was cited as one of the reasons cited for open coding a very simple algorithm.
Not sure, the qsort is recursive and stack limitations were mentioned. But the point being that the reason they don't use their library functions because it's a constrained environment.
Maybe they could permit some of their sort library functions to be used in such a constrained environment, sure. Don't need rust to do that, just need to be happy that the implementation is suitable for purpose. Which they would have to do regardless of what language and runtime they were using.
