beamr 0.6.4

A Rust runtime with the BEAM's execution model, targeting Gleam
Documentation 
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
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141

//! Work-stealing logic.
//!
//! Idle scheduler threads deterministically scan other scheduler queues in
//! round-robin order and steal approximately half of a victim's work. A queue
//! with a single process is never stolen from.

use crate::scheduler::run_queue::{PriorityStealers, RunQueue};

/// Result of a steal attempt.
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum StealResult {
    /// Work was stolen from `victim_index`.
    Stolen {
        /// Number of process IDs stolen.
        count: usize,
        /// Victim scheduler index.
        victim_index: usize,
    },
    /// No queue had stealable work.
    Empty,
}

/// Attempt to steal from other scheduler queues in deterministic round-robin
/// order. `last_victim` is the previously attempted victim and the returned
/// `usize` should be fed into the next call.
pub fn try_steal(
    my_queue: &RunQueue,
    my_index: usize,
    stealers: &[PriorityStealers],
    last_victim: usize,
) -> (StealResult, usize) {
    let thread_count = stealers.len();
    if thread_count <= 1 {
        return (StealResult::Empty, last_victim);
    }

    let mut victim = last_victim;
    for _ in 0..thread_count - 1 {
        victim = next_victim(victim, my_index, thread_count);
        let count = my_queue.steal_half_from(&stealers[victim]);
        if count > 0 {
            return (
                StealResult::Stolen {
                    count,
                    victim_index: victim,
                },
                victim,
            );
        }
    }

    (StealResult::Empty, victim)
}

fn next_victim(current: usize, my_index: usize, thread_count: usize) -> usize {
    let mut victim = (current + 1) % thread_count;
    if victim == my_index {
        victim = (victim + 1) % thread_count;
    }
    victim
}

#[cfg(test)]
mod tests {
    use super::{StealResult, try_steal};
    use crate::scheduler::run_queue::RunQueue;

    #[test]
    fn empty_thread_steals_from_thread_with_processes() {
        let queues: Vec<_> = (0..4).map(|_| RunQueue::new()).collect();
        for pid in 0..10 {
            queues[2].push(pid);
        }
        let stealers: Vec<_> = queues.iter().map(RunQueue::stealer).collect();

        let (result, _) = try_steal(&queues[0], 0, &stealers, 0);

        match result {
            StealResult::Stolen {
                count,
                victim_index,
            } => {
                assert_eq!(victim_index, 2);
                assert!((4..=6).contains(&count), "stole {count}");
                assert!(!queues[0].is_empty());
            }
            StealResult::Empty => panic!("expected work stealing"),
        }
    }

    #[test]
    fn queue_with_one_process_is_not_stolen_from() {
        let queues: Vec<_> = (0..2).map(|_| RunQueue::new()).collect();
        queues[1].push(99);
        let stealers: Vec<_> = queues.iter().map(RunQueue::stealer).collect();

        let (result, _) = try_steal(&queues[0], 0, &stealers, 0);

        assert_eq!(result, StealResult::Empty);
        assert_eq!(queues[1].pop(), Some(99));
    }

    #[test]
    fn round_robin_victim_selection_visits_all_threads_before_repeating() {
        let queues: Vec<_> = (0..4).map(|_| RunQueue::new()).collect();
        for pid in 0..10 {
            queues[3].push(pid);
        }
        let stealers: Vec<_> = queues.iter().map(RunQueue::stealer).collect();

        let (result, next) = try_steal(&queues[0], 0, &stealers, 0);
        assert!(matches!(
            result,
            StealResult::Stolen {
                victim_index: 3,
                ..
            }
        ));
        assert_eq!(next, 3);

        for pid in 100..110 {
            queues[1].push(pid);
        }
        let (result, _) = try_steal(&queues[0], 0, &stealers, next);
        assert!(matches!(
            result,
            StealResult::Stolen {
                victim_index: 1,
                ..
            }
        ));
    }

    #[test]
    fn all_empty_returns_empty() {
        let queues: Vec<_> = (0..3).map(|_| RunQueue::new()).collect();
        let stealers: Vec<_> = queues.iter().map(RunQueue::stealer).collect();

        assert_eq!(try_steal(&queues[0], 0, &stealers, 0).0, StealResult::Empty);
    }
}