# Over-Allocation with the Page Allocator

**URL:** <https://ziggit.dev/t/over-allocation-with-the-page-allocator/3159>\
**Category:** Docs\
**Tags:** memory-management, footgun\
**Created:** [February 6, 2024, 2:00pm UTC](https://ziggit.dev/t/over-allocation-with-the-page-allocator/3159 "2024-02-06T14:00:33Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dude\_the\_builder](https://ziggit.dev/user_avatar/ziggit.dev/dude_the_builder/32/557_2.png) [@dude\_the\_builder](https://ziggit.dev/u/dude_the_builder)\
**Post date:** [February 6, 2024, 2:00pm UTC](https://ziggit.dev/t/over-allocation-with-the-page-allocator/3159/1 "2024-02-06T14:00:33Z")

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The Zig standard library has a variety of useful memory allocators, each carefully designed to address a specific need by employing a specific allocation strategy. The Language Reference has an excellent section on [choosing an allocator](https://ziglang.org/documentation/master/#Choosing-an-Allocator), which is highly recommended as your go-to reference for this crucial step in Zig software development.

One of these allocators is the page allocator (`std.heap.page_allocator`), which is often used as a backing-allocator for other higher-level allocators. The page allocator couldn’t have a more descriptive name given it does exactly what it says on the label: it allocates pages of memory. A [page of memory](https://en.wikipedia.org/wiki/Page_(computer_memory)) can vary in size among processor architectures, and for example can range from 4,096 bytes to several mebibytes (MiB).

## The Footgun

Each call to the page allocator’s allocation methods will allocate at least one page of memory, no matter how small the actual requested allocation size:

```zig
// Actually allocates 4 KiB on most architectures.
var two_bytes = try std.heap.page_allocator.alloc(u8, 2);

```

So if your intent was to just allocate 2 bytes, the page allocator is not the best option. There can be situations where you indeed want to allocate full pages of memory\[1\], and in those cases using the page allocator is fine; but the footgun here is not being aware of the true workings of the chosen allocator and, in this case, over-allocating memory.

One additional footgun when using the page allocator directly is not knowing that each allocation will result in a system call to the operating system, requesting at least a page of memory. System calls can be expensive in terms of performance\[2\], so it’s also important to be aware of this and avoid it if not required.

## The Workaround

If you’re not actually interested in allocating pages of memory, avoid using `std.heap.page_allocator` directly. There’s absolutely no problem in using the page allocator as a backing-allocator for other allocators, of which the General Purpose Allocator should be your preferred choice if no other specific allocator is appropriate:

```zig
var gpa = std.heap.GeneralPurposeAllocator(.{}){};
defer _ = gpa.deinit();
const allocator = gpa.allocator();

```

## In Summary

Don’t use `std.heap.page_allocator` directly unless you want to allocate at least a full page of memory (and make a system call) with each allocation. There’s an excellent discussion about this topic [here](https://ziggit.dev/t/is-it-ok-to-use-std-heap-page-allocator-directly/2589), specifically, [this post](https://ziggit.dev/t/is-it-ok-to-use-std-heap-page-allocator-directly/2589/7) by @squeek502 that provides sample code to demonstrate the behavior of the page allocator in different use cases.

* * *

1. Maybe you’re developing your own allocator. 

2. A system call requires a context switch between user space and kernel space.

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<div class="post-metadata">

**Author:** ![Sze](https://ziggit.dev/user_avatar/ziggit.dev/sze/32/496_2.png) [@Sze](https://ziggit.dev/u/Sze)\
**Post date:** [February 6, 2024, 2:29pm UTC](https://ziggit.dev/t/over-allocation-with-the-page-allocator/3159/2 "2024-02-06T14:29:47Z")

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> [@dude\_the\_builder](#):
>
> The page alligator

Sorry totally off topic, just thought that was a funny typo 😉

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<div class="post-metadata">

**Author:** ![dude\_the\_builder](https://ziggit.dev/user_avatar/ziggit.dev/dude_the_builder/32/557_2.png) [@dude\_the\_builder](https://ziggit.dev/u/dude_the_builder)\
**Post date:** [February 6, 2024, 2:53pm UTC](https://ziggit.dev/t/over-allocation-with-the-page-allocator/3159/3 "2024-02-06T14:53:18Z")

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LOL It eats up your memory! 😸 🐊