Scalable Concurrent Containers

A collection of high-performance containers and utilities for concurrent and asynchronous programming.

Features

• Asynchronous counterparts of blocking and synchronous methods.
• Equivalent, Loom and Serde support: features = ["equivalent", "loom", "serde"].
• Near-linear scalability.
• No spin-locks and no busy loops.
• SIMD lookup to scan multiple entries in parallel: require RUSTFLAGS='-C target_feature=+avx2' on x86_64.

Concurrent and Asynchronous Containers

• HashMap is a concurrent and asynchronous hash map.
• HashSet is a concurrent and asynchronous hash set.
• HashIndex is a read-optimized concurrent and asynchronous hash map.
• HashCache is a 32-way associative cache backed by HashMap.
• TreeIndex is a read-optimized concurrent and asynchronous B-plus tree.

Utilities for Concurrent Programming

• LinkedList is a type trait implementing a lock-free concurrent singly linked list.
• Queue is a concurrent lock-free first-in-first-out container.
• Stack is a concurrent lock-free last-in-first-out container.
• Bag is a concurrent lock-free unordered opaque container.

HashMap

HashMap is a concurrent hash map optimized for highly parallel write-heavy workloads. HashMap is structured as a lock-free stack of entry bucket arrays. The entry bucket array is managed by sdd, thus enabling lock-free access to it and non-blocking container resizing. Each bucket is a fixed-size array of entries, protected by a read-write lock that simultaneously provides blocking and asynchronous methods.

Locking behavior

Entry access: fine-grained locking

Read/write access to an entry is serialized by the read-write lock in the bucket containing the entry. There are no container-level locks; therefore, the larger the container gets, the lower the chance of the bucket-level lock being contended.

Resize: lock-free

Resizing of a HashMap is entirely non-blocking and lock-free; resizing does not block any other read/write access to the container or resizing attempts. Resizing is analogous to pushing a new bucket array into a lock-free stack. Each entry in the old bucket array will be incrementally relocated to the new bucket array upon future access to the container, and the old bucket array will eventually be dropped after it becomes empty.

Examples

If the key is unique, an entry can be inserted. The inserted entry can be updated, read, and removed synchronously or asynchronously.

use scc::HashMap;

let hashmap: HashMap<u64, u32> = HashMap::default();

assert!(hashmap.insert(1, 0).is_ok());
assert!(hashmap.insert(1, 1).is_err());
assert_eq!(hashmap.upsert(1, 1).unwrap(), 0);
assert_eq!(hashmap.update(&1, |_, v| { *v = 3; *v }).unwrap(), 3);
assert_eq!(hashmap.read(&1, |_, v| *v).unwrap(), 3);
assert_eq!(hashmap.remove(&1).unwrap(), (1, 3));

hashmap.entry(7).or_insert(17);
assert_eq!(hashmap.read(&7, |_, v| *v).unwrap(), 17);

let future_insert = hashmap.insert_async(2, 1);
let future_remove = hashmap.remove_async(&1);

The Entry API of HashMap is helpful if the workflow is complicated.

use scc::HashMap;

let hashmap: HashMap<u64, u32> = HashMap::default();

hashmap.entry(3).or_insert(7);
assert_eq!(hashmap.read(&3, |_, v| *v), Some(7));

hashmap.entry(4).and_modify(|v| { *v += 1 }).or_insert(5);
assert_eq!(hashmap.read(&4, |_, v| *v), Some(5));

HashMap does not provide an Iterator since it is impossible to confine the lifetime of Iterator::Item to the Iterator. The limitation can be circumvented by relying on interior mutability, e.g., letting the returned reference hold a lock. However, it may lead to a deadlock if not correctly used, and frequent acquisition of locks may impact performance. Therefore, Iterator is not implemented; instead, HashMap provides several methods to iterate over entries synchronously or asynchronously: any, any_async, first_entry, first_entry_async, prune, prune_async, retain, retain_async, scan, scan_async, OccupiedEntry::next, and OccupiedEntry::next_async.

use scc::HashMap;

let hashmap: HashMap<u64, u32> = HashMap::default();

assert!(hashmap.insert(1, 0).is_ok());
assert!(hashmap.insert(2, 1).is_ok());

// Entries can be modified or removed via `retain`.
let mut acc = 0;
hashmap.retain(|k, v_mut| { acc += *k; *v_mut = 2; true });
assert_eq!(acc, 3);
assert_eq!(hashmap.read(&1, |_, v| *v).unwrap(), 2);
assert_eq!(hashmap.read(&2, |_, v| *v).unwrap(), 2);

// `any` returns `true` when an entry satisfying the predicate is found.
assert!(hashmap.insert(3, 2).is_ok());
assert!(hashmap.any(|k, _| *k == 3));

// Multiple entries can be removed through `retain`.
hashmap.retain(|k, v| *k == 1 && *v == 2);

// `hash_map::OccupiedEntry` also can return the next closest occupied entry.
let first_entry = hashmap.first_entry();
assert_eq!(*first_entry.as_ref().unwrap().key(), 1);
let second_entry = first_entry.and_then(|e| e.next());
assert!(second_entry.is_none());

fn is_send<T: Send>(f: &T) -> bool {
    true
}

// Asynchronous iteration over entries using `scan_async`.
let future_scan = hashmap.scan_async(|k, v| println!("{k} {v}"));
assert!(is_send(&future_scan));

// Asynchronous iteration over entries using the `Entry` API.
let future_iter = async {
    let mut iter = hashmap.first_entry_async().await;
    while let Some(entry) = iter {
        // `OccupiedEntry` can be sent across awaits and threads.
        assert!(is_send(&entry));
        assert_eq!(*entry.key(), 1);
        iter = entry.next_async().await;
    }
};
assert!(is_send(&future_iter));

HashSet

HashSet is a special version of HashMap where the value type is ().

Examples

Most HashSet methods are identical to that of HashMap except that they do not receive a value argument, and some HashMap methods for value modification are not implemented for HashSet.

use scc::HashSet;

let hashset: HashSet<u64> = HashSet::default();

assert!(hashset.read(&1, |_| true).is_none());
assert!(hashset.insert(1).is_ok());
assert!(hashset.read(&1, |_| true).unwrap());

let future_insert = hashset.insert_async(2);
let future_remove = hashset.remove_async(&1);

HashIndex

HashIndex is a read-optimized version of HashMap. In a HashIndex, not only is the memory of the bucket array managed by sdd, but also that of entry buckets is protected by sdd, enabling lock-free-read access to individual entries.

Entry lifetime

HashIndex does not drop removed entries immediately; instead, they are dropped when one of the following conditions is met.

1. Epoch reaches the next generation since the last entry was removed in a bucket, and the bucket is write-accessed.
2. HashIndex is cleared, or resized.
3. Buckets full of removed entries occupy 50% of the capacity.

Those conditions do not guarantee that the removed entry will be dropped within a definite period of time; therefore, HashIndex would not be an optimal choice if the workload is write-heavy and the entry size is large.

Examples

The peek and peek_with methods are completely lock-free.

use scc::HashIndex;

let hashindex: HashIndex<u64, u32> = HashIndex::default();

assert!(hashindex.insert(1, 0).is_ok());

// `peek` and `peek_with` are lock-free.
assert_eq!(hashindex.peek_with(&1, |_, v| *v).unwrap(), 0);

let future_insert = hashindex.insert_async(2, 1);
let future_remove = hashindex.remove_if_async(&1, |_| true);

The Entry API of HashIndex can update an existing entry.

use scc::HashIndex;

let hashindex: HashIndex<u64, u32> = HashIndex::default();
assert!(hashindex.insert(1, 1).is_ok());

if let Some(mut o) = hashindex.get(&1) {
    // Create a new version of the entry.
    o.update(2);
};

if let Some(mut o) = hashindex.get(&1) {
    // Update the entry in place.
    unsafe { *o.get_mut() = 3; }
};

An Iterator is implemented for HashIndex because any derived references can survive as long as the associated ebr::Guard lives.

use scc::ebr::Guard;
use scc::HashIndex;

let hashindex: HashIndex<u64, u32> = HashIndex::default();

assert!(hashindex.insert(1, 0).is_ok());

// Existing values can be replaced with a new one.
hashindex.get(&1).unwrap().update(1);

let guard = Guard::new();

// An `Guard` has to be supplied to `iter`.
let mut iter = hashindex.iter(&guard);

// The derived reference can live as long as `guard`.
let entry_ref = iter.next().unwrap();
assert_eq!(iter.next(), None);

drop(hashindex);

// The entry can be read after `hashindex` is dropped.
assert_eq!(entry_ref, (&1, &1));

HashCache

HashCache is a 32-way associative concurrent cache based on the HashMap implementation. HashCache does not keep track of the least recently used entry in the entire cache. Instead, each bucket maintains a doubly linked list of occupied entries, which is updated on entry access.

Examples

The LRU entry in a bucket is evicted when a new entry is inserted, and the bucket is full.

use scc::HashCache;

let hashcache: HashCache<u64, u32> = HashCache::with_capacity(100, 2000);

/// The capacity cannot exceed the maximum capacity.
assert_eq!(hashcache.capacity_range(), 128..=2048);

/// If the bucket corresponding to `1` or `2` is full, the LRU entry will be evicted.
assert!(hashcache.put(1, 0).is_ok());
assert!(hashcache.put(2, 0).is_ok());

/// `1` becomes the most recently accessed entry in the bucket.
assert!(hashcache.get(&1).is_some());

/// An entry can be normally removed.
assert_eq!(hashcache.remove(&2).unwrap(), (2, 0));

TreeIndex

TreeIndex is a B-plus tree variant optimized for read operations. sdd protects the memory used by individual entries, thus enabling lock-free read access to them.

Locking behavior

Read access is always lock-free and non-blocking. Write access to an entry is lock-free and non-blocking as long as no structural changes are required. However, when nodes are split or merged by a write operation, other write operations on keys in the affected range are blocked.

Entry lifetime

TreeIndex does not drop removed entries immediately. Instead, they are dropped when the leaf node is cleared or split, and this makes TreeIndex a sub-optimal choice if the workload is write-heavy.

Examples

If the key is unique, an entry can be inserted, read, and removed afterward. Locks are acquired or awaited only when internal nodes are split or merged.

use scc::TreeIndex;

let treeindex: TreeIndex<u64, u32> = TreeIndex::new();

assert!(treeindex.insert(1, 2).is_ok());

// `peek` and `peek_with` are lock-free.
assert_eq!(treeindex.peek_with(&1, |_, v| *v).unwrap(), 2);
assert!(treeindex.remove(&1));

let future_insert = treeindex.insert_async(2, 3);
let future_remove = treeindex.remove_if_async(&1, |v| *v == 2);

Entries can be scanned without acquiring any locks.

use scc::TreeIndex;
use sdd::Guard;

let treeindex: TreeIndex<u64, u32> = TreeIndex::new();

assert!(treeindex.insert(1, 10).is_ok());
assert!(treeindex.insert(2, 11).is_ok());
assert!(treeindex.insert(3, 13).is_ok());

let guard = Guard::new();

// `iter` iterates over entries without acquiring a lock.
let mut iter = treeindex.iter(&guard);
assert_eq!(iter.next().unwrap(), (&1, &10));
assert_eq!(iter.next().unwrap(), (&2, &11));
assert_eq!(iter.next().unwrap(), (&3, &13));
assert!(iter.next().is_none());

A specific range of keys can be scanned.

use scc::ebr::Guard;
use scc::TreeIndex;

let treeindex: TreeIndex<u64, u32> = TreeIndex::new();

for i in 0..10 {
    assert!(treeindex.insert(i, 10).is_ok());
}

let guard = Guard::new();

assert_eq!(treeindex.range(1..1, &guard).count(), 0);
assert_eq!(treeindex.range(4..8, &guard).count(), 4);
assert_eq!(treeindex.range(4..=8, &guard).count(), 5);

Bag

Bag is a concurrent lock-free unordered container. Bag is completely opaque, disallowing access to contained instances until they are popped. Bag is especially efficient if the number of contained instances can be maintained under ARRAY_LEN (default: usize::BITS / 2)

Examples

use scc::Bag;

let bag: Bag<usize> = Bag::default();

bag.push(1);
assert!(!bag.is_empty());
assert_eq!(bag.pop(), Some(1));
assert!(bag.is_empty());

Queue

Queue is a concurrent lock-free first-in-first-out container backed by sdd.

Examples

use scc::Queue;

let queue: Queue<usize> = Queue::default();

queue.push(1);
assert!(queue.push_if(2, |e| e.map_or(false, |x| **x == 1)).is_ok());
assert!(queue.push_if(3, |e| e.map_or(false, |x| **x == 1)).is_err());
assert_eq!(queue.pop().map(|e| **e), Some(1));
assert_eq!(queue.pop().map(|e| **e), Some(2));
assert!(queue.pop().is_none());

Stack

Stack is a concurrent lock-free last-in-first-out container backed by sdd.

Examples

use scc::Stack;

let stack: Stack<usize> = Stack::default();

stack.push(1);
stack.push(2);
assert_eq!(stack.pop().map(|e| **e), Some(2));
assert_eq!(stack.pop().map(|e| **e), Some(1));
assert!(stack.pop().is_none());

LinkedList

LinkedList is a type trait that implements lock-free concurrent singly linked list operations backed by sdd. It additionally provides a method for marking a linked list entry to denote a user-defined state.

Examples

use scc::ebr::{AtomicShared, Guard, Shared};
use scc::LinkedList;
use std::sync::atomic::Ordering::Relaxed;

#[derive(Default)]
struct L(AtomicShared<L>, usize);
impl LinkedList for L {
    fn link_ref(&self) -> &AtomicShared<L> {
        &self.0
    }
}

let guard = Guard::new();

let head: L = L::default();
let tail: Shared<L> = Shared::new(L(AtomicShared::null(), 1));

// A new entry is pushed.
assert!(head.push_back(tail.clone(), false, Relaxed, &guard).is_ok());
assert!(!head.is_marked(Relaxed));

// Users can mark a flag on an entry.
head.mark(Relaxed);
assert!(head.is_marked(Relaxed));

// `next_ptr` traverses the linked list.
let next_ptr = head.next_ptr(Relaxed, &guard);
assert_eq!(next_ptr.as_ref().unwrap().1, 1);

// Once `tail` is deleted, it becomes invisible.
tail.delete_self(Relaxed);
assert!(head.next_ptr(Relaxed, &guard).is_null());

Performance

HashMap Tail Latency

The expected tail latency of a distribution of latencies of 1048576 insertion operations (K = u64, V = u64) ranges from 180 microseconds to 200 microseconds on Apple M2 Max.

HashMap and HashIndex Throughput

• Results on Apple M2 Max (12 cores).
• Results on Intel Xeon (32 cores, avx2).

Changelog