Zero-Pool: Consistent High-Performance Thread Pool

When microseconds matter and allocation is the enemy.

This is an experimental thread pool implementation focused on exploring lock-free FIFO MPMC queue techniques. Consider this a performance playground rather than a production-ready library.

Key Features:

• Zero locks - lock-free
• Zero queue limit - unbounded
• Zero channels - no std/crossbeam channel overhead
• Zero virtual dispatch - function pointer dispatch avoids vtable lookups
• Zero core spinning - event based
• Zero result transport cost - tasks write directly to caller-provided memory
• Zero per worker queues - single global queue structure = perfect workload balancing
• Zero external dependencies - standard library only and stable rust

Using a result-via-parameters pattern means workers place results into caller provided memory, removing thread transport overhead. The single global queue structure ensures optimal load balancing without the complexity of work-stealing or load redistribution algorithms.

Since the library uses raw pointers, you must ensure parameter structs remain valid until TaskFuture::wait() completes, result pointers remain valid until task completion, and that your task functions are thread-safe. The library provides type-safe methods like submit_task and submit_batch_uniform for convenient usage.

Note: TaskFuture uses a small Mutex + Condvar to efficiently block waiting threads. Core pool operations remain lock-free.

Benchmarks

test bench_heavy_compute_rayon             ... bench:   4,612,748.55 ns/iter (+/- 650,510.12)
test bench_heavy_compute_zeropool          ... bench:   4,433,491.25 ns/iter (+/- 325,246.06)
test bench_indexed_computation_rayon       ... bench:      37,142.34 ns/iter (+/- 13,770.56)
test bench_indexed_computation_zeropool    ... bench:      35,483.85 ns/iter (+/- 2,157.35)
test bench_individual_tasks_rayon_empty    ... bench:      46,815.76 ns/iter (+/- 1,467.04)
test bench_individual_tasks_zeropool_empty ... bench:     154,867.12 ns/iter (+/- 38,962.00)
test bench_task_overhead_rayon             ... bench:      34,408.66 ns/iter (+/- 13,369.08)
test bench_task_overhead_zeropool          ... bench:      33,548.53 ns/iter (+/- 1,711.41)

Note: the pool fares poorly for workloads dominated by millions of tiny, individual task submissions (see bench_individual_tasks_zeropool_empty). This is mainly due to per-submission allocation; prefer batching or larger tasks for best throughput.

Example Usage

Submitting a Single Task

use zero_pool::{ZeroPool, zp_define_task_fn, zp_write};

struct CalculationParams {
    iterations: usize,
    result: *mut u64,
}

zp_define_task_fn!(calculate_task, CalculationParams, |params| {
    let mut sum = 0u64;
    for i in 0..params.iterations {
        sum += i as u64;
    }
    zp_write!(params.result, sum);
});

let pool = ZeroPool::new();
let mut result = 0u64;
let task = CalculationParams { iterations: 1000, result: &mut result };

let future = pool.submit_task(calculate_task, &task);
future.wait();

println!("Result: {}", result);

Submitting Uniform Batches

Submits multiple tasks of the same type to the thread pool.

use zero_pool::{ZeroPool, zp_define_task_fn, zp_write};

struct ComputeParams {
    work_amount: usize,
    result: *mut u64,
}

zp_define_task_fn!(compute_task, ComputeParams, |params| {
    let mut sum = 0u64;
    for i in 0..params.work_amount {
        sum += i as u64;
    }
    zp_write!(params.result, sum);
});

let pool = ZeroPool::new();
let mut results = vec![0u64; 100];

let tasks: Vec<_> = results.iter_mut().enumerate().map(|(i, result)| {
    ComputeParams { work_amount: 1000 + i * 10, result }
}).collect();

let future = pool.submit_batch_uniform(compute_task, &tasks);
future.wait();

println!("First result: {}", results[0]);

Submitting Multiple Independent Tasks

You can submit individual tasks and uniform batches in parallel:

use zero_pool::{ZeroPool, zp_define_task_fn, zp_write};

// Define first task type
struct ComputeParams {
    work_amount: usize,
    result: *mut u64,
}

zp_define_task_fn!(compute_task, ComputeParams, |params| {
    let mut sum = 0u64;
    for i in 0..params.work_amount {
        sum += i as u64;
    }
    zp_write!(params.result, sum);
});

// Define second task type
struct MultiplyParams { x: u64, y: u64, result: *mut u64 }

zp_define_task_fn!(multiply_task, MultiplyParams, |params| {
    zp_write!(params.result, params.x * params.y);
});

let pool = ZeroPool::new();

// Individual task - separate memory location
let mut single_result = 0u64;
let single_task_params = ComputeParams { work_amount: 1000, result: &mut single_result };

// Uniform batch - separate memory from above
let mut batch_results = vec![0u64; 50];
let batch_task_params: Vec<_> = batch_results.iter_mut().enumerate()
    .map(|(i, result)| ComputeParams { work_amount: 500 + i, result })
    .collect();

// Submit all batches
let future1 = pool.submit_task(compute_task, &single_task_params);
let future2 = pool.submit_batch_uniform(compute_task, &batch_task_params);

// Wait on them in any order; completion order is not guaranteed
future1.wait();
future2.wait(); 

println!("Single: {}", single_result);
println!("Batch completed: {} tasks", batch_results.len());

zp_define_task_fn!

Defines a task function that safely dereferences the parameter struct.

use zero_pool::{zp_define_task_fn, zp_write};

struct SumParams {
    iterations: usize,
    result: *mut u64,
}

zp_define_task_fn!(sum_task, SumParams, |params| {
    let mut sum = 0u64;
    for i in 0..params.iterations {
        sum += i as u64;
    }
    zp_write!(params.result, sum);
});

zp_write!

Eliminates explicit unsafe blocks when writing to result pointers.

use zero_pool::{zp_define_task_fn, zp_write};

struct TaskParams {
    value: u64,
    result: *mut u64,
}

zp_define_task_fn!(my_task, TaskParams, |params| {
    let computed_value = params.value * 2;
    zp_write!(params.result, computed_value);
});

zp_write_indexed!

Safely writes a value to a specific index in a Vec or array via raw pointer, useful for batch processing where each task writes to a different index.

use zero_pool::{ZeroPool, zp_define_task_fn, zp_write_indexed};

struct BatchParams {
    index: usize,
    work_size: usize,
    results: *mut Vec<u64>,
}

zp_define_task_fn!(batch_task, BatchParams, |params| {
    let mut sum = 0u64;
    for i in 0..params.work_size {
        sum += i as u64;
    }
    zp_write_indexed!(params.results, params.index, sum);
});

// Usage with a pre-allocated vector
let pool = ZeroPool::new();
let mut results = vec![0u64; 100];
let task_params: Vec<_> = (0..100).map(|i| {
    BatchParams { index: i, work_size: 1000, results: &mut results }
}).collect();

let future = pool.submit_batch_uniform(batch_task, &task_params);
future.wait();