138 lines
8 KiB
XML
138 lines
8 KiB
XML
// This module exists only to provide a separate page for the implementation
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// documentation.
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//! Notes on `sharded-slab`'s implementation and design.
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//!
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//! # Design
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//!
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//! The sharded slab's design is strongly inspired by the ideas presented by
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//! Leijen, Zorn, and de Moura in [Mimalloc: Free List Sharding in
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//! Action][mimalloc]. In this report, the authors present a novel design for a
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//! memory allocator based on a concept of _free list sharding_.
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//!
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//! Memory allocators must keep track of what memory regions are not currently
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//! allocated ("free") in order to provide them to future allocation requests.
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//! The term [_free list_][freelist] refers to a technique for performing this
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//! bookkeeping, where each free block stores a pointer to the next free block,
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//! forming a linked list. The memory allocator keeps a pointer to the most
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//! recently freed block, the _head_ of the free list. To allocate more memory,
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//! the allocator pops from the free list by setting the head pointer to the
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//! next free block of the current head block, and returning the previous head.
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//! To deallocate a block, the block is pushed to the free list by setting its
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//! first word to the current head pointer, and the head pointer is set to point
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//! to the deallocated block. Most implementations of slab allocators backed by
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//! arrays or vectors use a similar technique, where pointers are replaced by
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//! indices into the backing array.
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//!
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//! When allocations and deallocations can occur concurrently across threads,
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//! they must synchronize accesses to the free list; either by putting the
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//! entire allocator state inside of a lock, or by using atomic operations to
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//! treat the free list as a lock-free structure (such as a [Treiber stack]). In
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//! both cases, there is a significant performance cost — even when the free
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//! list is lock-free, it is likely that a noticeable amount of time will be
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//! spent in compare-and-swap loops. Ideally, the global synchronzation point
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//! created by the single global free list could be avoided as much as possible.
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//!
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//! The approach presented by Leijen, Zorn, and de Moura is to introduce
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//! sharding and thus increase the granularity of synchronization significantly.
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//! In mimalloc, the heap is _sharded_ so that each thread has its own
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//! thread-local heap. Objects are always allocated from the local heap of the
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//! thread where the allocation is performed. Because allocations are always
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//! done from a thread's local heap, they need not be synchronized.
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//!
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//! However, since objects can move between threads before being deallocated,
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//! _deallocations_ may still occur concurrently. Therefore, Leijen et al.
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//! introduce a concept of _local_ and _global_ free lists. When an object is
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//! deallocated on the same thread it was originally allocated on, it is placed
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//! on the local free list; if it is deallocated on another thread, it goes on
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//! the global free list for the heap of the thread from which it originated. To
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//! allocate, the local free list is used first; if it is empty, the entire
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//! global free list is popped onto the local free list. Since the local free
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//! list is only ever accessed by the thread it belongs to, it does not require
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//! synchronization at all, and because the global free list is popped from
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//! infrequently, the cost of synchronization has a reduced impact. A majority
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//! of allocations can occur without any synchronization at all; and
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//! deallocations only require synchronization when an object has left its
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//! parent thread (a relatively uncommon case).
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//!
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//! [mimalloc]: https://www.microsoft.com/en-us/research/uploads/prod/2019/06/mimalloc-tr-v1.pdf
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//! [freelist]: https://en.wikipedia.org/wiki/Free_list
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//! [Treiber stack]: https://en.wikipedia.org/wiki/Treiber_stack
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//!
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//! # Implementation
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//!
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//! A slab is represented as an array of [`MAX_THREADS`] _shards_. A shard
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//! consists of a vector of one or more _pages_ plus associated metadata.
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//! Finally, a page consists of an array of _slots_, head indices for the local
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//! and remote free lists.
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//!
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//! ```text
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//! ┌─────────────┐
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//! │ shard 1 │
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//! │ │ ┌─────────────┐ ┌────────┐
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//! │ pages───────┼───▶│ page 1 │ │ │
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//! ├─────────────┤ ├─────────────┤ ┌────▶│ next──┼─┐
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//! │ shard 2 │ │ page 2 │ │ ├────────┤ │
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//! ├─────────────┤ │ │ │ │XXXXXXXX│ │
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//! │ shard 3 │ │ local_head──┼──┘ ├────────┤ │
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//! └─────────────┘ │ remote_head─┼──┐ │ │◀┘
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//! ... ├─────────────┤ │ │ next──┼─┐
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//! ┌─────────────┐ │ page 3 │ │ ├────────┤ │
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//! │ shard n │ └─────────────┘ │ │XXXXXXXX│ │
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//! └─────────────┘ ... │ ├────────┤ │
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//! ┌─────────────┐ │ │XXXXXXXX│ │
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//! │ page n │ │ ├────────┤ │
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//! └─────────────┘ │ │ │◀┘
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//! └────▶│ next──┼───▶ ...
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//! ├────────┤
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//! │XXXXXXXX│
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//! └────────┘
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//! ```
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//!
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//!
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//! The size of the first page in a shard is always a power of two, and every
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//! subsequent page added after the first is twice as large as the page that
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//! preceeds it.
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//!
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//! ```text
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//!
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//! pg.
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//! ┌───┐ ┌─┬─┐
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//! │ 0 │───▶ │ │
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//! ├───┤ ├─┼─┼─┬─┐
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//! │ 1 │───▶ │ │ │ │
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//! ├───┤ ├─┼─┼─┼─┼─┬─┬─┬─┐
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//! │ 2 │───▶ │ │ │ │ │ │ │ │
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//! ├───┤ ├─┼─┼─┼─┼─┼─┼─┼─┼─┬─┬─┬─┬─┬─┬─┬─┐
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//! │ 3 │───▶ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │
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//! └───┘ └─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘
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//! ```
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//!
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//! When searching for a free slot, the smallest page is searched first, and if
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//! it is full, the search proceeds to the next page until either a free slot is
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//! found or all available pages have been searched. If all available pages have
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//! been searched and the maximum number of pages has not yet been reached, a
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//! new page is then allocated.
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//!
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//! Since every page is twice as large as the previous page, and all page sizes
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//! are powers of two, we can determine the page index that contains a given
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//! address by shifting the address down by the smallest page size and
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//! looking at how many twos places necessary to represent that number,
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//! telling us what power of two page size it fits inside of. We can
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//! determine the number of twos places by counting the number of leading
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//! zeros (unused twos places) in the number's binary representation, and
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//! subtracting that count from the total number of bits in a word.
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//!
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//! The formula for determining the page number that contains an offset is thus:
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//!
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//! ```rust,ignore
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//! WIDTH - ((offset + INITIAL_PAGE_SIZE) >> INDEX_SHIFT).leading_zeros()
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//! ```
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//!
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//! where `WIDTH` is the number of bits in a `usize`, and `INDEX_SHIFT` is
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//!
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//! ```rust,ignore
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//! INITIAL_PAGE_SIZE.trailing_zeros() + 1;
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//! ```
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//!
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//! [`MAX_THREADS`]: https://docs.rs/sharded-slab/latest/sharded_slab/trait.Config.html#associatedconstant.MAX_THREADS
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