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Fix allocator waste assessment in docs (#9167)

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* Fix allocator comment.

* Add explanations where this comes from.

* Clarify absolute values.
This commit is contained in:
Tomasz Drwięga
2021-06-22 13:20:29 +02:00
committed by GitHub
parent 77a4b980ae
commit 8be60821bb
1 changed files with 5 additions and 2 deletions
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substrate/primitives/allocator/src/freeing_bump.rs
+5 -2
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@@ -60,8 +60,11 @@
//! fail.
//!
//! - Sizes of allocations are rounded up to the nearest order. That is, an allocation of 2,00001 MiB
//! will be put into the bucket of 4 MiB. Therefore, typically more than half of the space in allocation
//! will be wasted. This is more pronounced with larger allocation sizes.
//! will be put into the bucket of 4 MiB. Therefore, any allocation of size `(N, 2N]` will take
//! up to `2N`, thus assuming a uniform distribution of allocation sizes, the average amount in use
//! of a `2N` space on the heap will be `(3N + ε) / 2`. So average utilisation is going to be around
//! 75% (`(3N + ε) / 2 / 2N`) meaning that around 25% of the space in allocation will be wasted.
//! This is more pronounced (in terms of absolute heap amounts) with larger allocation sizes.
use crate::Error;
use sp_std::{mem, convert::{TryFrom, TryInto}, ops::{Range, Index, IndexMut}};