#concurrent #hashmap #map #tree #ebr


A collection of building blocks for concurrent programming

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#77 in Data structures

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7.5K SLoC

Scalable Concurrent Containers

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

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


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 automatically managed. For instance, ebr::AtomicArc and ebr::Arc hold a strong reference to the underlying instance, and the instance is passed to the garbage collector when the reference count drops to zero.


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.
    (Some(Arc::new(18)), Tag::First),

// `ptr` can be tagged.

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

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

// `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.

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


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 special state.


use scc::ebr::{Arc, AtomicArc, Barrier};
use scc::LinkedList;
use std::sync::atomic::Ordering::Relaxed;

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

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());

// Users can mark a flag on an entry.

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


HashMap is a scalable in-memory unique key-value store that is targeted at highly concurrent heavy workloads. It applies EBR to its entry array management, thus allowing it to reduce the number of locks and avoid data sharding.


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> = Default::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 of an existing entry corresponding to the key.

use scc::HashMap;

let hashmap: HashMap<u64, u32> = Default::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 for them to live as long as the HashMap due to the lack of global lock inside it. Therefore Iterator is not implemented, instead, it provides two methods that enable a HashMap to iterate over its entries: for_each, and retain.

use scc::HashMap;

let hashmap: HashMap<u64, u32> = Default::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`, a `ebr::Barrier` protects the entry array.
assert_eq!(hashmap.retain(|key, value| *key == 1 && *value == 0), (1, 2));


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.


Its read methods does not modify any shared data.

use scc::HashIndex;

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

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

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

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

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


// The entry can be read after `hashindex` is dropped.
assert_eq!(entry_ref, (&1, &0));


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


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);

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));

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);


Test setup

  • OS: SUSE Linux Enterprise Server 15 SP1
  • CPU: Intel(R) Xeon(R) CPU E7-8880 v4 @ 2.20GHz x 4
  • RAM: 1TB
  • Rust: 1.54.0
  • SCC: 0.5.0 (HashMap, and HashIndex), 0.5.1 (TreeIndex)
  • HashMap<usize, usize, RandomState> = Default::default()
  • HashIndex<usize, usize, RandomState> = Default::default()
  • TreeIndex<usize, usize> = Default::default()

Test data

  • Each thread is assigned a disjoint range of usize integers.
  • The performance test code asserts the expected outcome of each operation, and the post state of the container.
  • The number of records in the test data dedicated to a thread for HashMap tests is 128M, and 4M for HashIndex and TreeIndex tests.

Test workload: local

  • Each test is run twice in a single process in order to minimize the effect of page faults as the overhead is unpredictable.
  • 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.
  • The read/scan/remove data is populated by the insert test.

Test workload: local-remote

  • Insert, remove: each thread additionally tries to perform operations using records belonging to a randomly chosen remote thread.
  • Mixed: each thread performs insert-local -> insert-remote -> read-local -> read-remote -> remove-local -> remove-remote.

Test Results

11 threads 22 threads 44 threads
InsertL 186.564s 192.279s 258.713s
ReadL 102.086s 108.321s 117.399s
ScanL 35.874s 63.281s 134.607s
RemoveL 118.931s 127.682s 135.774s
InsertR 334.535s 342.914s 359.023s
MixedR 363.165s 384.775s 396.604s
RemoveR 260.543s 277.702s 302.858s
11 threads 22 threads 44 threads
InsertL 4.172s 4.693s 5.947s
ReadL 2.291s 2.337s 2.647s
ScanL 0.703s 1.459s 3.04s
RemoveL 2.719s 2.877s 3.706s
InsertR 7.201s 7.853s 9.105s
MixedR 12.343s 13.367s 15.063s
RemoveR 8.726s 9.357s 10.94s
11 threads 22 threads 44 threads
InsertL 6.433s 11.117s 16.817s
ReadL 1.045s 1.106s 1.737s
ScanL 11.779s 26.093s 57.284s
RemoveL 5.221s 9.460s 18.489s
InsertR 16.531s 17.092s 24.066s
MixedR 20.575s 20.187s 23.871s
RemoveR 6.016s 6.422s 7.34s




  • Add TreeIndex::remove_if.
  • Fix TreeIndex issues.


  • Fix ebr::Arc.



  • Own EBR implementation.
  • Substantial API changes.