Crate scc

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Scalable Concurrent Containers

Cargo GitHub Workflow Status

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

  • Asynchronous counterparts to blocking and synchronous methods.
  • Formally verified EBR implementation.
  • Near-linear scalability.
  • No spin-locks and no busy loops.
  • SIMD lookups to scan multiple entries in parallel 1.
  • Zero dependencies on other crates.
  • Serde support: features = ["serde"].
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 sampling-based LRU cache backed by HashMap.
  • TreeIndex is a read-optimized concurrent and asynchronous B-plus tree.
Utilities for Concurrent Programming
  • EBR implements lock-free epoch-based reclamation.
  • 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 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 EBR, thus enabling lock-free access to it and non-blocking container resizing. Each bucket is a fixed-size array of entries, and it is protected by a special read-write lock which provides both 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 completely 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 on future access to the container, and the old bucket array gets dropped eventually after it becomes empty.


An entry can be inserted if the key is unique. 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_eq!(hashmap.update(&1, |_, v| { *v = 2; *v }).unwrap(), 2);
assert_eq!(, |_, v| *v).unwrap(), 2);
assert_eq!(hashmap.remove(&1).unwrap(), (1, 2));

assert_eq!(, |_, 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 useful if the workflow is complicated.

use scc::HashMap;

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

assert_eq!(, |_, v| *v), Some(7));

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

let future_entry = hashmap.entry_async(3);

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., let the returned reference hold a lock, however it will easily lead to a deadlock if not correctly used, and frequent acquisition of locks may impact performance. Therefore, Iterator is not implemented, instead, HashMap provides a number of methods to iterate over entries synchronously or asynchronously: any, any_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!(, |_, v| *v).unwrap(), 2);
assert_eq!(, |_, v| *v).unwrap(), 2);

// `any` returns `true` as soon as 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();
let second_entry = first_entry.and_then(|e|;

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


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


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!(, |_| true).is_none());
assert!(, |_| true).unwrap());

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


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


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 be used to update an entry in-place.

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.

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.

let guard = Guard::new();

// An `ebr::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 =;
assert_eq!(, None);


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


HashCache is a concurrent sampling-based LRU cache that is 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 access to entries in order to keep track of the least recently used entry within the bucket.


The LRU entry in a bucket is evicted when a new entry is being 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.

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


TreeIndex is a B-plus tree variant optimized for read operations. EBR 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 also lock-free and non-blocking as long as no structural changes are required. However, when nodes are being split or merged by a write operation, other write operations on keys in the affected range are blocked.


An entry can be inserted if the key is unique, and it can be read, and removed afterwards. 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);

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

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();

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

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 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)


use scc::Bag;

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

assert_eq!(bag.pop(), Some(1));


Queue is an EBR backed concurrent lock-free first-in-first-out container.


use scc::Queue;

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

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));


Stack is an EBR backed concurrent lock-free last-in-first-out container.


use scc::Stack;

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

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


The ebr module implements epoch-based reclamation and various types of auxiliary data structures to make use of it safely. Its epoch-based reclamation algorithm is similar to that implemented in crossbeam_epoch, however users may find it easier to use as the lifetime of an instance is safely managed. For instance, ebr::AtomicOwned and ebr::Owned automatically retire the contained instance and ebr::AtomicShared and ebr::Shared hold a reference-counted instance which is retired when the last strong reference is dropped.

Memory Overhead

Retired instances are stored in intrusive queues in thread-local storage, and therefore additional 16-byte space for Option<NonNull<dyn Collectible>> is allocated per instance.


The ebr module can be used without an unsafe block.

use scc::ebr::{suspend, AtomicOwned, AtomicShared, Guard, Ptr, Shared, Tag};

use std::sync::atomic::Ordering::Relaxed;

// `atomic_shared` holds a strong reference to `17`.
let atomic_shared: AtomicShared<usize> = AtomicShared::new(17);

// `atomic_owned` owns `19`.
let atomic_owned: AtomicOwned<usize> = AtomicOwned::new(19);

// `guard` prevents the garbage collector from dropping reachable instances.
let guard = Guard::new();

// `ptr` cannot outlive `guard`.
let mut ptr: Ptr<usize> = atomic_shared.load(Relaxed, &guard);
assert_eq!(*ptr.as_ref().unwrap(), 17);

// `atomic_shared` can be tagged.
atomic_shared.update_tag_if(Tag::First, |p| p.tag() == Tag::None, Relaxed, Relaxed);

// `ptr` is not tagged, so CAS fails.
    (Some(Shared::new(18)), Tag::First),

// `ptr` can be tagged.

// The return value of CAS is a handle to the instance that `atomic_shared` previously owned.
let prev: Shared<usize> = atomic_shared.compare_exchange(
    (Some(Shared::new(18)), Tag::Second),
assert_eq!(*prev, 17);

// `17` will be garbage-collected later.

// `ebr::AtomicShared` can be converted into `ebr::Shared`.
let shared: Shared<usize> = atomic_shared.into_shared(Relaxed).unwrap();
assert_eq!(*shared, 18);

// `18` and `19` will be garbage-collected later.

// `17` is still valid as `guard` keeps the garbage collector from dropping it.
assert_eq!(*ptr.as_ref().unwrap(), 17);

// Execution of a closure can be deferred until all the current readers are gone.
guard.defer_execute(|| println!("deferred"));

// If the thread is expected to lie dormant for a while, call `suspend()` to allow other threads
// to reclaim its own retired instances.


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


use scc::ebr::{AtomicShared, Guard, Shared};
use scc::LinkedList;

use std::sync::atomic::Ordering::Relaxed;

struct L(AtomicShared<L>, usize);
impl LinkedList for L {
    fn link_ref(&self) -> &AtomicShared<L> {

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());

// Users can mark a flag on an entry.

// `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.
assert!(head.next_ptr(Relaxed, &guard).is_null());


HashMap and HashIndex

Comparison with DashMap.


  • The average time taken to enter and exit a protected region: 2.1 nanoseconds on Apple M1.


  1. Advanced SIMD instructions are used only when respective target features are added, e.g., -C target_feature=+avx2




  • Bag is a lock-free concurrent unordered instance container.
  • Entry stores an instance of T and a link to the next entry.
  • Queue is a lock-free concurrent first-in-first-out container.
  • Stack is a lock-free concurrent last-in-first-out container.