github.com
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.
> This part of the blog didn't seem very accurate.
It was a sufficient amount of understanding to produce this allocator :-). I think that if we have beginner[0] projects posted and upvoted, we must understand that the author's understanding may be lacking some nuance.
[0] author might be a very good programmer, just not familiar with this particular area!
I think this is good work anyway.
Or even, they may be familiar, but challenging their understanding or using simplifying assumptions to reduce complexity.
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.
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.
Actually... you can free those 999 sbrk() pages using munmap() on Linux and Darwin (so most likely the BSDs too). You can also change the mappings within the sbrk()-allocated range, much like any other mmap.
This feature is not well known, nor particularly useful :-)
A couple of minor C points:
- The code seems to completely lack use of `static` for things that should be local to the implementation, such as `META_SIZE`, `find_free_block()` and others.
- The header includes `<stdbool.h>` but the interface doesn't use it so it could be included in the C file instead (which, in fact, it is!).
- Could do with more `const` for clarity, but that is quite personal.
- Interesting to use explicit comparison to check for integers being zero, but treat pointers being NULL as implicit boolean. I prefer comparing in both cases.
My remarks:
1) in calloc() you correctly check size*n against SIZE_MAX, but in multiple other spots you don't check size+META_SIZE.
2) the field is_mmap is useless: it can be replaced by (size>=MMAP_THRESHOLD) practically everywhere. The only corner case where this doesn't work is a large block initially backed by mmap() that's then shrunk via realloc() to under the threshold. But realloc() has other inconsistencies anyway, see next point.
3) realloc() shows the signs of a refactoring gone wild. The first if on block->size lacks a test on is_mmap, as split_block() doesn't seem to do the right thing with mmapped blocks...
4) free_list does not in fact track free nodes, as its name suggests, but all nodes, whether they are free or not. Wouldn't it be better to add a node to the list only when it's freed? I leave to you to iron out all the corner cases!
https://pubs.opengroup.org/onlinepubs/7908799/xsh/brk.html
> The behaviour of brk() and sbrk() is unspecified if an application also uses any other memory functions (such as malloc(), mmap(), free()). Other functions may use these other memory functions silently.
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...
Lots of reasons.
Leveraging platform-specific functionality, enabling use in embedded systems, as a learning exercise, etc.
What is not needed is a header file which redeclares the same C standard function signatures defined by the replacement allocator.
It's interesting to brainstorm new memory allocation interfaces. Some cool ideas:
https://nullprogram.com/blog/2023/12/17/
https://gist.github.com/o11c/6b08643335388bbab0228db763f9921...
I'm in a position to do this in my programming language project. Wrote my own allocator for it. Maybe it's time to reinvent a better wheel.
The referenced header file is defined thusly:
#include <stddef.h>
#include <stdbool.h>
void *malloc(size_t size);
void *calloc(size_t n, size_t size);
void *realloc(void *ptr, size_t size);
void free(void *ptr);
> I'm in a position to do this in my programming language project. Wrote my own allocator for it. Maybe it's time to reinvent a better wheel.
Wonderful.
But if your intent is to replace the aforementioned C standard library memory allocation functions, then they would have to have the same signatures of the functions being replaced. Which leads back to the original assertion that there is no need for a header file which declares the same C functions defined by the C standard library for which they replace.
I don't think that's the case here though.
No need to free in short living processes
Perfectly usable in many applications. Unfortunately, since it depends on assumptions about the application, it's not really suited for a general purpose library.