thread_local/
thread_id.rs

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1// Copyright 2017 Amanieu d'Antras
2//
3// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
4// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
5// http://opensource.org/licenses/MIT>, at your option. This file may not be
6// copied, modified, or distributed except according to those terms.
7
8use crate::POINTER_WIDTH;
9use once_cell::sync::Lazy;
10use std::cmp::Reverse;
11use std::collections::BinaryHeap;
12use std::sync::Mutex;
13use std::usize;
14
15/// Thread ID manager which allocates thread IDs. It attempts to aggressively
16/// reuse thread IDs where possible to avoid cases where a ThreadLocal grows
17/// indefinitely when it is used by many short-lived threads.
18struct ThreadIdManager {
19    free_from: usize,
20    free_list: BinaryHeap<Reverse<usize>>,
21}
22impl ThreadIdManager {
23    fn new() -> ThreadIdManager {
24        ThreadIdManager {
25            free_from: 0,
26            free_list: BinaryHeap::new(),
27        }
28    }
29    fn alloc(&mut self) -> usize {
30        if let Some(id) = self.free_list.pop() {
31            id.0
32        } else {
33            let id = self.free_from;
34            self.free_from = self
35                .free_from
36                .checked_add(1)
37                .expect("Ran out of thread IDs");
38            id
39        }
40    }
41    fn free(&mut self, id: usize) {
42        self.free_list.push(Reverse(id));
43    }
44}
45static THREAD_ID_MANAGER: Lazy<Mutex<ThreadIdManager>> =
46    Lazy::new(|| Mutex::new(ThreadIdManager::new()));
47
48/// Data which is unique to the current thread while it is running.
49/// A thread ID may be reused after a thread exits.
50#[derive(Clone, Copy)]
51pub(crate) struct Thread {
52    /// The thread ID obtained from the thread ID manager.
53    pub(crate) id: usize,
54    /// The bucket this thread's local storage will be in.
55    pub(crate) bucket: usize,
56    /// The size of the bucket this thread's local storage will be in.
57    pub(crate) bucket_size: usize,
58    /// The index into the bucket this thread's local storage is in.
59    pub(crate) index: usize,
60}
61impl Thread {
62    fn new(id: usize) -> Thread {
63        let bucket = usize::from(POINTER_WIDTH) - id.leading_zeros() as usize;
64        let bucket_size = 1 << bucket.saturating_sub(1);
65        let index = if id != 0 { id ^ bucket_size } else { 0 };
66
67        Thread {
68            id,
69            bucket,
70            bucket_size,
71            index,
72        }
73    }
74}
75
76/// Wrapper around `Thread` that allocates and deallocates the ID.
77struct ThreadHolder(Thread);
78impl ThreadHolder {
79    fn new() -> ThreadHolder {
80        ThreadHolder(Thread::new(THREAD_ID_MANAGER.lock().unwrap().alloc()))
81    }
82}
83impl Drop for ThreadHolder {
84    fn drop(&mut self) {
85        THREAD_ID_MANAGER.lock().unwrap().free(self.0.id);
86    }
87}
88
89thread_local!(static THREAD_HOLDER: ThreadHolder = ThreadHolder::new());
90
91/// Get the current thread.
92pub(crate) fn get() -> Thread {
93    THREAD_HOLDER.with(|holder| holder.0)
94}
95
96#[test]
97fn test_thread() {
98    let thread = Thread::new(0);
99    assert_eq!(thread.id, 0);
100    assert_eq!(thread.bucket, 0);
101    assert_eq!(thread.bucket_size, 1);
102    assert_eq!(thread.index, 0);
103
104    let thread = Thread::new(1);
105    assert_eq!(thread.id, 1);
106    assert_eq!(thread.bucket, 1);
107    assert_eq!(thread.bucket_size, 1);
108    assert_eq!(thread.index, 0);
109
110    let thread = Thread::new(2);
111    assert_eq!(thread.id, 2);
112    assert_eq!(thread.bucket, 2);
113    assert_eq!(thread.bucket_size, 2);
114    assert_eq!(thread.index, 0);
115
116    let thread = Thread::new(3);
117    assert_eq!(thread.id, 3);
118    assert_eq!(thread.bucket, 2);
119    assert_eq!(thread.bucket_size, 2);
120    assert_eq!(thread.index, 1);
121
122    let thread = Thread::new(19);
123    assert_eq!(thread.id, 19);
124    assert_eq!(thread.bucket, 5);
125    assert_eq!(thread.bucket_size, 16);
126    assert_eq!(thread.index, 3);
127}
thread_local/thread_id.rs

thread_local/
thread_id.rs
