#[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 >= 32);
assert_eq!(addr1 % 32, 0);
assert_eq!(addr2 % 32, 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§

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impl<T> CachePadded<T>

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pub const fn new(t: T) -> CachePadded<T>

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

§Examples
use crossbeam_utils::CachePadded;

let padded_value = CachePadded::new(1);
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pub fn into_inner(self) -> T

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§

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impl<T> Clone for CachePadded<T>
where T: Clone,

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fn clone(&self) -> CachePadded<T>

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source. Read more
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impl<T> Debug for CachePadded<T>
where T: Debug,

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fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
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impl<T> Default for CachePadded<T>
where T: Default,

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fn default() -> CachePadded<T>

Returns the “default value” for a type. Read more
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impl<T> Deref for CachePadded<T>

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type Target = T

The resulting type after dereferencing.
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fn deref(&self) -> &T

Dereferences the value.
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impl<T> DerefMut for CachePadded<T>

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fn deref_mut(&mut self) -> &mut T

Mutably dereferences the value.
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impl<T> From<T> for CachePadded<T>

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fn from(t: T) -> CachePadded<T>

Converts to this type from the input type.
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impl<T> Hash for CachePadded<T>
where T: Hash,

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fn hash<__H>(&self, state: &mut __H)
where __H: Hasher,

Feeds this value into the given Hasher. Read more
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fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher. Read more
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impl<T> PartialEq for CachePadded<T>
where T: PartialEq,

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fn eq(&self, other: &CachePadded<T>) -> bool

This method tests for self and other values to be equal, and is used by ==.
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fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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impl<T> Copy for CachePadded<T>
where T: Copy,

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impl<T> Eq for CachePadded<T>
where T: Eq,

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impl<T> Send for CachePadded<T>
where T: Send,

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impl<T> StructuralPartialEq for CachePadded<T>

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impl<T> Sync for CachePadded<T>
where T: Sync,

Auto Trait Implementations§

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impl<T> RefUnwindSafe for CachePadded<T>
where T: RefUnwindSafe,

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impl<T> Unpin for CachePadded<T>
where T: Unpin,

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impl<T> UnwindSafe for CachePadded<T>
where T: UnwindSafe,

Blanket Implementations§

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impl<T> Any for T
where T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for T
where T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for T
where T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<!> for T

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fn from(t: !) -> T

Converts to this type from the input type.
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for T
where U: From<T>,

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fn into(self) -> U

Calls U::from(self).

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

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impl<T> Pointable for T

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const ALIGN: usize = _

The alignment of pointer.
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type Init = T

The type for initializers.
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unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
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unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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impl<T> ToOwned for T
where T: Clone,

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type Owned = T

The resulting type after obtaining ownership.
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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T, U> TryFrom<U> for T
where U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.