Scalable Concurrent Containers

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

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.
• TreeIndex is a read-optimized concurrent and asynchronous B+ tree.
• Queue is a concurrent lock-free first-in-first-out queue.

Utilities for Concurrent Programming

• EBR implements epoch-based reclamation.
• LinkedList is a type trait implementing a lock-free concurrent singly linked list.

See Performance for benchmark results for the containers.

HashMap

HashMap is a scalable in-memory unique key-value container that is targeted at highly concurrent write-heavy workloads. It uses EBR for its hash table memory management in order to implement non-blocking resizing and fine-granular locking without static data sharding; it is not a lock-free data structure, and each access to a single key is serialized by a bucket-level mutex. HashMap is optimized for frequently updated large data sets, such as the lock table in database management software.

Examples

A unique key can be inserted along with its corresponding value, and then 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!(hashmap.read(&1, |_, v| *v).unwrap(), 2);
assert_eq!(hashmap.remove(&1).unwrap(), (1, 2));

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

It supports upsert as in database management software; it tries to insert the given key-value pair, and if the key exists, it updates the value field with the supplied closure.

use scc::HashMap;

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

hashmap.upsert(1, || 2, |_, v| *v = 2);
assert_eq!(hashmap.read(&1, |_, v| *v).unwrap(), 2);
hashmap.upsert(1, || 2, |_, v| *v = 3);
assert_eq!(hashmap.read(&1, |_, v| *v).unwrap(), 3);

let future_upsert = hashmap.upsert_async(2, || 1, |_, v| *v = 3);

There is no method to confine the lifetime of references derived from an Iterator to the Iterator, and it is illegal to let them live as long as the HashMap. Therefore Iterator is not implemented, instead, it provides a number of methods as substitutes for Iterator: for_each, for_each_async, scan, scan_async, retain, and retain_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());

// Inside `for_each`, an `ebr::Barrier` protects the entry array.
let mut acc = 0;
hashmap.for_each(|k, v_mut| { acc += *k; *v_mut = 2; });
assert_eq!(acc, 3);

// `for_each` can modify the entries.
assert_eq!(hashmap.read(&1, |_, v| *v).unwrap(), 2);
assert_eq!(hashmap.read(&2, |_, v| *v).unwrap(), 2);

assert!(hashmap.insert(3, 2).is_ok());

// Inside `retain`, an `ebr::Barrier` protects the entry array.
assert_eq!(hashmap.retain(|k, v| *k == 1 && *v == 0), (1, 2));

// It is possible to scan the entries asynchronously.
let future_scan = hashmap.scan_async(|k, v| println!("{k} {v}"));
let future_for_each = hashmap.for_each_async(|k, v_mut| { *v_mut = *k; });

HashSet

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

Examples

All the HashSet methods do not receive a value argument.

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. It applies EBR to its entry management as well, enabling it to perform read operations without blocking or being blocked.

Examples

Its read method is completely lock-free and does not modify any shared data.

use scc::HashIndex;

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

assert!(hashindex.insert(1, 0).is_ok());
assert_eq!(hashindex.read(&1, |_, v| *v).unwrap(), 0);

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

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

use scc::ebr::Barrier;
use scc::HashIndex;

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

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

let barrier = Barrier::new();

// An `ebr::Barrier` has to be supplied to `iter`.
let mut iter = hashindex.iter(&barrier);

// The derived reference can live as long as `barrier`.
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, &0));

TreeIndex

TreeIndex is a B+ tree variant optimized for read operations. The ebr module enables it to implement lock-free read and scan methods.

Examples

Key-value pairs can be inserted, read, and removed, and the read method is lock-free.

use scc::TreeIndex;

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

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

Key-value pairs can be scanned and the scan method is lock-free.

use scc::ebr::Barrier;
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 barrier = Barrier::new();

let mut visitor = treeindex.iter(&barrier);
assert_eq!(visitor.next().unwrap(), (&1, &10));
assert_eq!(visitor.next().unwrap(), (&2, &11));
assert_eq!(visitor.next().unwrap(), (&3, &13));
assert!(visitor.next().is_none());

Key-value pairs in a specific range can be scanned.

use scc::ebr::Barrier;
use scc::TreeIndex;

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

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

let barrier = Barrier::new();

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

Queue

Queue is a concurrent lock-free first-in-first-out queue.

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());

EBR

The ebr module implements epoch-based reclamation and various types of auxiliary data structures to make use of it. 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::AtomicArc and ebr::Arc hold a strong reference to the underlying instance, and the instance is automatically passed to the garbage collector when the reference count drops to zero.

Examples

The ebr module can be used without an unsafe block.

use scc::ebr::{suspend, Arc, AtomicArc, Barrier, Ptr, Tag};

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

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

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

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

// `atomic_arc` can be tagged.
atomic_arc.update_tag_if(Tag::First, |t| t == Tag::None, Relaxed);

// `ptr` is not tagged, so CAS fails.
assert!(atomic_arc.compare_exchange(
    ptr,
    (Some(Arc::new(18)), Tag::First),
    Relaxed,
    Relaxed,
    &barrier).is_err());

// `ptr` can be tagged.
ptr.set_tag(Tag::First);

// The return value of CAS is a handle to the instance that `atomic_arc` previously owned.
let prev: Arc<usize> = atomic_arc.compare_exchange(
    ptr,
    (Some(Arc::new(18)), Tag::Second),
    Relaxed,
    Relaxed,
    &barrier).unwrap().0.unwrap();
assert_eq!(*prev, 17);

// `17` will be garbage-collected later.
drop(prev);

// `ebr::AtomicArc` can be converted into `ebr::Arc`.
let arc: Arc<usize> = atomic_arc.try_into_arc(Relaxed).unwrap();
assert_eq!(*arc, 18);

// `18` will be garbage-collected later.
drop(arc);

// `17` is still valid as `barrier` 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.
barrier.defer_execute(|| println!("deferred"));
drop(barrier);

// The closure will be repeatedly invoked until it returns `true`.
let barrier = Barrier::new();
let mut data = 3;
barrier.defer_incremental_execute(move || {
    if data == 0 {
        return true;
    }
    data -= 1;
    false
});
drop(barrier);

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

LinkedList

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

Examples

use scc::ebr::{Arc, AtomicArc, Barrier};
use scc::LinkedList;

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

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

let barrier = Barrier::new();

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

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

Performance

Comparison with DashMap.

• Results on Apple M1 (8 cores).
• Results on Intel Xeon (88 cores).
• Interpret the results cautiously as benchmarks do not represent real world workloads.
• HashMap and HashIndex outperform the others according to the benchmark test under highly concurrent workloads.
• The benchmark test a fork of conc-map-bench.

Changelog