Scalable Concurrent Containers

A collection of concurrent data structures and building blocks for concurrent programming.

• ebr implements epoch-based reclamation.
• LinkedList is a type trait implementing a wait-free concurrent singly linked list.
• HashMap is a concurrent hash map.
• HashSet is a concurrent hash set based on HashMap.
• HashIndex is a concurrent hash index allowing lock-free read and scan.
• TreeIndex is a concurrent B+ tree allowing lock-free read and scan.

See Performance for benchmark results.

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::{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);

LinkedList

LinkedList is a type trait that implements wait-free concurrent singly linked list operations, backed by EBR. It additionally provides support for marking an entry of a linked list to indicate that the entry is in 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());

HashMap

HashMap is a scalable in-memory unique key-value container that is targeted at highly concurrent heavy workloads. It applies EBR to its entry array management, thus enabling it to avoid container-level locking and data sharding.

Examples

A unique key can be inserted along with its corresponding value, and then it can be updated, read, and removed.

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));

It supports upsert as in database management software; it tries to insert the given key-value pair, and if it fails, 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);

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 two methods that allow a HashMap to iterate over its entries: for_each, and retain.

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(|key, value| *key == 1 && *value == 0), (1, 2));

Asynchronous methods can be used in asynchronous code blocks.

use scc::HashMap;

let hashmap: HashMap<u64, u32> = HashMap::default();
let future_insert = hashmap.insert_async(11, 17);
let result = future_insert.await;

HashSet

HashSet is a variant 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());

The capacity of a HashSet can be specified.

use scc::HashSet;
use std::collections::hash_map::RandomState;

let hashset: HashSet<u64, RandomState> = HashSet::new(1000000, RandomState::new());
assert_eq!(hashset.capacity(), 1048576);

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 acquiring locks.

Examples

Its read method 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);

An Iterator is implemented for HashIndex, because 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.

use scc::TreeIndex;

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

assert!(treeindex.insert(1, 10).is_ok());
assert_eq!(treeindex.read(&1, |_, value| *value).unwrap(), 10);
assert!(treeindex.remove(&1));

Key-value pairs can be scanned.

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);

Asynchronous methods can be used in asynchronous code blocks.

use scc::TreeIndex;

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

let future_insert = treeindex.insert_async(11, 17);
let result = future_insert.await;

Performance

Setup

• OS: SUSE Linux Enterprise Server 15 SP2
• CPU: Intel(R) Xeon(R) CPU E7-8880 v4 @ 2.20GHz x 4
• RAM: 1TB
• Rust: 1.60.0
• SCC: 0.6.8

Workload

• A disjoint range of 16M usize integers is assigned to each thread.
• Insert: each thread inserts its own records.
• Read: each thread reads its own records in the container.
• Scan: each thread scans the entire container once.
• Remove: each thread removes its own records from the container.
• InsertR, RemoveR: each thread additionally operates using keys belonging to a randomly chosen remote thread.
• MixedR: each thread performs InsertR -> ReadR -> RemoveR.

Results

• HashMap

• HashIndex

• TreeIndex

Performance Comparison with DashMap and flurry

• HashMap outperforms the others if the workload is highly concurrent or write-heavy.
• The benchmark tests are based on conc-map-bench.
• Results on Apple M1.
• Results on Intel Xeon.

Changelog

0.6.8

• Fix wait queue performance issues with asynchronous TreeIndex methods.

0.6.7

• Fix ebr API, ebr::AtomicArc::swap: return the previous ebr::Tag along with the pointer.
• Fix ebr API, ebr::AtomicArc::compare_exchange: receive a reference to ebr::Barrier.
• Fix #49 for synchronous TreeIndex methods.

0.6.6

• Add {HashMap, HashSet}::{scan, scan_async} for a read-only scan.
• Minor HashMap optimization.

0.6.5

• Add more asynchronous methods to HashMap, HashIndex, and HashSet.
• TreeIndex remove performance improvement.