fp.

A fun toy memory allocator (not thread safe, that's a future TODO). I also wanted to explain how I approached it, so I also wrote a tutorial blog post (~20 minute read) covering the code which you can find the link to in the README.

Comments

quibono•7h ago

I hate that very often my first reaction to Show HN posts like this is to cynically look for signs of blatant AI code use.

I don't think that's the case here though.

achierius•7h ago

Looks nice! Though I have to say, you should probably avoid sbreak even for small allocations -- obviously it's slow, but even beyond that you have to deal with the fact that it's essentially a global singleton and introduces a lot of subtle failure cases you might not think of + which you can't really solve anyways. It's better to mmap out some chunk of memory and sub-allocate it out yourself.

macintux•6h ago

Can you supply an example of a failure case that can’t be solved (or is at least challenging to solve)?

sweetjuly•5h ago

sbrk grows linearly, and if anything is mapped in the way it fails. mmap can map anywhere there's space as it is not restricted to linear mappings. So, you'd better hope a mapping doesn't randomly land there and run you out of space.

It's not a failure but relatedly as sbrk is linear, you also don't really have a reasonable way to deal with fragmentation. For example, suppose you allocate 1000 page sized objects and then free all but the last one. With an mmap based heap, you can free all 999 other pages back to the OS whereas with sbrk you're stuck with those 999 pages you don't need for the lifetime of that 1000th object (better hope it's not long lived!).

Really, sbrk only exists for legacy reasons.

ori_b•4h ago

> With an mmap based heap, you can free all 999 other pages back to the OS whereas with sbrk you're stuck with those 999 pages you don't need for the lifetime of that 1000th object (better hope it's not long lived!).

Thanks to the wonders of virtual memory, you can madvise(MADV_DONTNEED), and return the memory to the OS, without giving up the address space.

squirrellous•4h ago

Not giving up the address space feels like an anti feature. This would mean, among other things, that access to the DONTNEED memory is no longer a segfault but garbage values instead, which is not ideal.

checker659•5h ago

That project structure is reminding me of claude.

keyle•5h ago

So does half the readme

leecommamichael•3h ago

Which part?

gameman144•3h ago

Could you elaborate? The project structure looks extremely normal to me, but I don't know if I'm overlooking red flags all over the place.

checker659•2h ago

The structure in the README.md (not the actual structure).

leecommamichael•3h ago

Personally I’d not bother with folders, but to each their own. I’m sorry but I just don’t see what you’re onto.

AdieuToLogic•5h ago

Why redeclare the function signatures in allocator.h[0] when they must match what is already defined by the C standard?

Since this is all allocator.h[0] contains aside from other include statements, why have allocator.h at all?

0 - https://github.com/t9nzin/memory/blob/main/include/allocator...

leecommamichael•3h ago

Why write a mini allocator?

Subsentient•4h ago

As soon as I saw mmap(), I knew this wasn't a true native allocator. So yeah, not quite so insightful after all.

vintagedave•4h ago

It’s a ‘minimal’ allocator. Reading the blog it seems to be going in depth into allocator principles, in practice, things like coalescing blocks.

I haven’t read in full so not sure if it discusses using blocks vs other structures (eg stack-based allocators, stack being the data structure not the program stack.) Ie, it’s a set of implementation choices. It still seems to reflect common ways of allocating in far more detail than many blogs I’ve read on the topic do.

leecommamichael•3h ago

Your comment reads as if you believe that simply writing the syscall C wrapper yourself would constitute a meaningful enhancement to the code. We can all apply more effort to be insightful.

canyp•3h ago

I always like me some memory allocator blog/code. Two links in the context of gamedev below, in case you or anyone else is interested.

https://screwjankgames.github.io/engine%20programming/2020/0...

https://www.bytesbeneath.com/p/the-arena-custom-memory-alloc...

I also don't know how much we want to butcher this blog post, but:

> RAM is fundamentally a giant array of bytes, where each byte has a unique address. However, CPUs don’t fetch data one byte at a time. They read and write memory in fixed-size chunks called words which are typically 4 bytes on 32-bit systems or 8 bytes on 64-bit systems.

CPUs these days fetch entire cache lines. Memory is also split into banks. There are many more details involved, and it is viewing memory as a giant array of bytes that is fundamentally broken. It's a useful abstraction up until some point, but it breaks apart once you analyze performance. This part of the blog didn't seem very accurate.

rurban•2h ago

One line: bump sbrk(). Done.

No need to free in short living processes