#[repr(align(128))]
pub struct CachePadded<T> { /* private fields */ }
Expand description

Pads and aligns a value to the length of a cache line.

In concurrent programming, sometimes it is desirable to make sure commonly accessed pieces of data are not placed into the same cache line. Updating an atomic value invalidates the whole cache line it belongs to, which makes the next access to the same cache line slower for other CPU cores. Use CachePadded to ensure updating one piece of data doesn’t invalidate other cached data.

Size and alignment

Cache lines are assumed to be N bytes long, depending on the architecture:

  • On x86-64, aarch64, and powerpc64, N = 128.
  • On arm, mips, mips64, and riscv64, N = 32.
  • On s390x, N = 256.
  • On all others, N = 64.

Note that N is just a reasonable guess and is not guaranteed to match the actual cache line length of the machine the program is running on. On modern Intel architectures, spatial prefetcher is pulling pairs of 64-byte cache lines at a time, so we pessimistically assume that cache lines are 128 bytes long.

The size of CachePadded<T> is the smallest multiple of N bytes large enough to accommodate a value of type T.

The alignment of CachePadded<T> is the maximum of N bytes and the alignment of T.

Examples

Alignment and padding:

use crossbeam_utils::CachePadded;

let array = [CachePadded::new(1i8), CachePadded::new(2i8)];
let addr1 = &*array[0] as *const i8 as usize;
let addr2 = &*array[1] as *const i8 as usize;

assert!(addr2 - addr1 >= 64);
assert_eq!(addr1 % 64, 0);
assert_eq!(addr2 % 64, 0);

When building a concurrent queue with a head and a tail index, it is wise to place them in different cache lines so that concurrent threads pushing and popping elements don’t invalidate each other’s cache lines:

use crossbeam_utils::CachePadded;
use std::sync::atomic::AtomicUsize;

struct Queue<T> {
    head: CachePadded<AtomicUsize>,
    tail: CachePadded<AtomicUsize>,
    buffer: *mut T,
}

Implementations

Pads and aligns a value to the length of a cache line.

Examples
use crossbeam_utils::CachePadded;

let padded_value = CachePadded::new(1);

Returns the inner value.

Examples
use crossbeam_utils::CachePadded;

let padded_value = CachePadded::new(7);
let value = padded_value.into_inner();
assert_eq!(value, 7);

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Returns the “default value” for a type. Read more

The resulting type after dereferencing.

Dereferences the value.

Mutably dereferences the value.

Performs the conversion.

Feeds this value into the given Hasher. Read more

Feeds a slice of this type into the given Hasher. Read more

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Performs the conversion.

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

🔬 This is a nightly-only experimental API. (toowned_clone_into)

Uses borrowed data to replace owned data, usually by cloning. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.