Zig allocators (WIP)
This article is not done, sections might be missing, please be patient
This article will attempt to explain the way most of the useful allocators in the Zig standard library work.
In Zig, allocators are explicit and ubiquitous. This means that in order to allocate anything, you have to know roughly how they work and how to use them. If you don't care about the details and just want to get going, you should do use GPA:
const std = @import("std");
pub fn main() !void {
var gpa = std.heap.GeneralPurposeAllocator(.{}){};
defer std.debug.assert(!gpa.deinit());
const allocator = gpa.allocator();
// Pass allocator to functions that want one
}
However, if you do want to know more about when to pick which allocator, what their differences are, and how to implement your own allocator, you can continue reading.
Preface
In order to properly explain Zig allocators and how to use them, we will need to first explain a couple of details about how memory works, if you're comfortable with manual memory management, you can skip this part.
But before we even do that, we should explain what we mean by allocation conceptually.
What is allocation?
Memory pages
The first important detail to know about memory, is that your operating system actually doesn't let you just ask for memory of any size. Operating systems organize memory into "pages", and pages may vary in size (but they usually don't, the page size on your system is most likely 4096 bytes). There are some articles that advocate the use of larger pages (2MB or even 1GB), but for the purposes of this article we won't focus on large pages.
Resources on large pages:
- https://easyperf.net/blog/2022/09/01/Utilizing-Huge-Pages-For-Code
- https://rigtorp.se/hugepages/
- https://mazzo.li/posts/check-huge-page.html
- https://pointersgonewild.com/2023/03/12/memory-pages-mmap-and-linear-address-spaces/
- https://www.evanjones.ca/hugepages-are-a-good-idea.html
Memory alignment
Alignment is in simple terms the concept that certain datatypes should sit
within addresses that are divisible by a certain minimum size, for example
u64's should sit in addresses divisible by 8.
The reason for this is that CPU's can optimize better when there are more constraints on where in memory words (TODO: ALT TEXT: explain words) can be, and many CPU's will simply not work with misaligned data at all. This is also why padding exists inside C structs.
Good:
[------][------][------][------]
[------][------][------][------]
^ ^ ^ ^
0 8 16 32
Bad:
[------][------][------][------]
[------][------][------][------]
^ ^ ^ ^
1 9 17 33
CPU cachelines
You might have seen code like the following Zig or C:
var x: *u8 = ...;
std.debug.print("{}\n", .{ x.* });
char *x = ...;
printf("%d\n", *x);
This code will effectively load a single byte from memory. But this is only a half-truth, because CPU's are actually incapable of loading only one byte. For every load from main memory, CPU's generally have to load 64 consecutive bytes (8x u64's). After that, the entire 64 bytes are cached in L1, L2, L3 caches, subsequent loads then consult the cache before resorting to loading from main memory. Cache lines also generally exist on 64-byte alignments, so addresses 0..64, 64..128, etc would each be their own cachelines.