1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
// MIT License
//
// Copyright (c) 2020 Gregory Meyer
//
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation files
// (the "Software"), to deal in the Software without restriction,
// including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software,
// and to permit persons to whom the Software is furnished to do so,
// subject to the following conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
// BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
// ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//! Segmented lockfree hash tables.
//!
//! Segmented hash tables divide their entries between a number of smaller
//! logical hash tables, or segments. Each segment is entirely independent from
//! the others, and entries are never relocated across segment boundaries.
//!
//! In the context of this crate, a segment refers specifically to an array of
//! bucket pointers. The number of segments in a hash table is rounded up to the
//! nearest power of two; this is so that selecting the segment for a key is no
//! more than a right shift to select the most significant bits of a hashed key.
//!
//! Each segment is entirely independent from the others, all operations can be
//! performed concurrently by multiple threads. Should a set of threads be
//! operating on disjoint sets of segments, the only synchronization between
//! them will be destructive interference as they access and update the bucket
//! array pointer and length for each segment.
//!
//! Compared to the unsegmented hash tables in this crate, the segmented hash
//! tables have higher concurrent write throughput for disjoint sets of keys.
//! However, the segmented hash tables have slightly lower read and
//! single-threaded write throughput. This is because the segmenting structure
//! adds another layer of indirection between the hash table and its buckets.
//!
//! The idea for segmenting hash tables was inspired by the
//! [`ConcurrentHashMap`] from OpenJDK 7, which consists of a number of
//! separately-locked segments. OpenJDK 8 introduced a striped concurrent hash
//! map that stripes a set of bucket locks across the set of buckets using the
//! least significant bits of hashed keys.
//!
//! [`ConcurrentHashMap`]: https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/share/classes/java/util/concurrent/ConcurrentHashMap.java
pub use HashMap;