Piscina is a fast, efficient Node.js Worker Threads pool implementation. It allows you to offload tasks to worker threads, helping to manage CPU-intensive tasks in a Node.js environment without blocking the main thread. This can significantly improve the performance of your Node.js applications, especially for tasks that are computationally expensive or I/O bound.
Executing a Task
This demonstrates how to create a Piscina pool and execute a task in a worker thread. The task is defined in a separate file ('worker.js'), and the main thread communicates with the worker asynchronously, awaiting the result.
const Piscina = require('piscina');
const path = require('path');
const piscina = new Piscina({
filename: path.resolve(__dirname, 'worker.js')
});
(async () => {
const result = await piscina.run({ a: 1, b: 2 });
console.log(result); // Output from your worker task
})();Dynamic Task Module
This example shows how to run a dynamic task directly passed as a function to Piscina, without the need for a separate module file. This can be useful for small, on-the-fly tasks.
const Piscina = require('piscina');
const piscina = new Piscina();
(async () => {
const result = await piscina.runTask({
task: () => 'Hello from the worker!'
});
console.log(result); // 'Hello from the worker!'
})();Workerpool is an npm package that offers a similar functionality to Piscina by providing a pool of workers to run JavaScript functions in parallel, in separate processes or threads. Compared to Piscina, workerpool has a simpler API but might not offer the same level of performance optimizations for Node.js Worker Threads.
Threads.js is another alternative that simplifies working with Web Workers and Worker Threads in JavaScript. It provides a high-level API for spawning new threads and communicating with them. While Threads.js focuses on ease of use and compatibility between web and Node.js environments, Piscina is more focused on high-performance and efficient task scheduling in Node.js specifically.
Written in TypeScript.
For Node.js 12.x and higher.
In main.js:
const Piscina = require('piscina');
const piscina = new Piscina({
filename: path.resolve(__dirname, 'worker.js')
});
(async function() {
const result = await piscina.runTask({ a: 4, b: 6 });
console.log(result); // Prints 10
})();
In worker.js:
module.exports = ({ a, b }) => {
return a + b;
};
The worker may also be an async function or may return a Promise:
const { promisify } = require('util');
const sleep = promisify(setTimeout);
module.exports = async ({ a, b } => {
// Fake some async activity
await sleep(100);
return a + b;
})
ESM is also supported for both Piscina and workers:
import { Piscina } from 'piscina';
const piscina = new Piscina({
// The URL must be a file:// URL
filename: new URL('./worker.mjs', import.meta.url).href
});
(async function () {
const result = await piscina.runTask({ a: 4, b: 6 });
console.log(result); // Prints 10
})();
In worker.mjs:
export default ({ a, b }) => {
return a + b;
};
Submitted tasks may be canceled using either an AbortController or
an EventEmitter:
'use strict';
const Piscina = require('piscina');
const { AbortController } = require('abort-controller');
const { resolve } = require('path');
const piscina = new Piscina({
filename: resolve(__dirname, 'worker.js')
});
(async function() {
const abortController = new AbortController();
try {
const task = piscina.runTask({ a: 4, b: 6 }, abortController.signal);
abortController.abort();
await task;
} catch (err) {
console.log('The task was canceled');
}
})();
To use AbortController, you will need to npm i abort-controller
(or yarn add abort-controller).
Alternatively, any EventEmitter that emits an 'abort' event
may be used as an abort controller:
'use strict';
const Piscina = require('piscina');
const EventEmitter = require('events');
const { resolve } = require('path');
const piscina = new Piscina({
filename: resolve(__dirname, 'worker.js')
});
(async function() {
const ee = new EventEmitter();
try {
const task = piscina.runTask({ a: 4, b: 6 }, ee);
ee.emit('abort');
await task;
} catch (err) {
console.log('The task was canceled');
}
})();
A worker thread will not be made available to process tasks until Piscina determines that it is "ready". By default, a worker is ready as soon as Piscina loads it and acquires a reference to the exported handler function.
There may be times when the availability of a worker may need to be delayed
longer while the worker initializes any resources it may need to operate.
To support this case, the worker module may export a Promise that resolves
the handler function as opposed to exporting the function directly:
async function initialize() {
await someAsyncInitializationActivity();
return ({ a, b }) => a + b;
}
module.exports = initialize();
Piscina will await the resolution of the exported Promise before marking the worker thread available.
When the maxQueue option is set, once the Piscina queue is full, no
additional tasks may be submitted until the queue size falls below the
limit. The 'drain' event may be used to receive notification when the
queue is empty and all tasks have been submitted to workers for processing.
Example: Using a Node.js stream to feed a Piscina worker pool:
'use strict';
const { resolve } = require('path');
const Pool = require('../..');
const pool = new Pool({
filename: resolve(__dirname, 'worker.js'),
maxQueue: 'auto'
});
const stream = getStreamSomehow();
stream.setEncoding('utf8');
pool.on('drain', () => {
if (stream.isPaused()) {
console.log('resuming...', counter, pool.queueSize);
stream.resume();
}
});
stream
.on('data', (data) => {
pool.runTask(data);
if (pool.queueSize === pool.options.maxQueue) {
console.log('pausing...', counter, pool.queueSize);
stream.pause();
}
})
.on('error', console.error)
.on('end', () => {
console.log('done');
});
Additional examples can be found in the GitHub repo at https://github.com/jasnell/piscina/tree/master/examples
PiscinaPiscina works by creating a pool of Node.js Worker Threads to which one or more tasks may be dispatched. Each worker thread executes a single exported function defined in a separate file. Whenever a task is dispatched to a worker, the worker invokes the exported function and reports the return value back to Piscina when the function completes.
This class extends EventEmitter from Node.js.
new Piscina([options])filename: (string | null) Provides the default source for the code that
runs the tasks on Worker threads. This should be an absolute path or an
absolute file:// URL to a file that exports a JavaScript function or
async function as its default export or module.exports. ES modules
are supported.minThreads: (number) Sets the minimum number of threads that are always
running for this thread pool. The default is based on the number of
available CPUs.maxThreads: (number) Sets the maximum number of threads that are
running for this thread pool. The default is based on the number of
available CPUs.idleTimeout: (number) A timeout in milliseconds that specifies how long
a Worker is allowed to be idle, i.e. not handling any tasks, before it is
shut down. By default, this is immediate.maxQueue: (number | string) The maximum number of tasks that may be
scheduled to run, but not yet running due to lack of available threads, at
a given time. By default, there is no limit. The special value 'auto'
may be used to have Piscina calculate the maximum as the square of maxThreads.
When 'auto' is used, the calculated maxQueue value may be found by checking
the options.maxQueue property.concurrentTasksPerWorker: (number) Specifies how many tasks can share
a single Worker thread simultaneously. The default is 1. This generally
only makes sense to specify if there is some kind of asynchronous component
to the task. Keep in mind that Worker threads are generally not built for
handling I/O in parallel.useAtomics: (boolean) Use the Atomics API for faster communication
between threads. This is on by default.resourceLimits: (object) See Node.js new Worker options
maxOldGenerationSizeMb: (number) The maximum size of each worker threads
main heap in MB.maxYoungGenerationSizeMb: (number) The maximum size of a heap space for
recently created objects.codeRangeSizeMb: (number) The size of a pre-allocated memory range used
for generated code.env: (object) If set, specifies the initial value of process.env inside
the worker threads. See Node.js new Worker options for details.argv: (any[]) List of arguments that will be stringified and appended to
process.argv in the worker. See Node.js new Worker options for details.execArgv: (string[]) List of Node.js CLI options passed to the worker.
See Node.js new Worker options for details.workerData: (any) Any JavaScript value that can be cloned and made
available as require('piscina').workerData. See Node.js new Worker options
for details. Unlike regular Node.js Worker Threads, workerData must not
specify any value requiring a transferList. This is because the workerData
will be cloned for each pooled worker.taskQueue : (TaskQueue) By default, Piscina uses a first-in-first-out
queue for submitted tasks. The taskQueue option can be used to provide an
alternative implementation. See Custom Task Queues for additional detail.Use caution when setting resource limits. Setting limits that are too low may
result in the Piscina worker threads being unusable.
runTask(task[, transferList][, filename][, abortSignal])Schedules a task to be run on a Worker thread.
task: Any value. This will be passed to the function that is exported from
filename.transferList: An optional lists of objects that is passed to
[postMessage()] when posting task to the Worker, which are transferred
rather than cloned.filename: Optionally overrides the filename option passed to the
constructor for this task. If no filename was specified to the constructor,
this is mandatory.abortSignal: An [AbortSignal][] instance. If passed, this can be used to
cancel a task. If the task is already running, the corresponding Worker
thread will be stopped.
(More generally, any EventEmitter or EventTarget that emits 'abort'
events can be passed here.) Abortable tasks cannot share threads regardless
of the concurrentTasksPerWorker options.This returns a Promise for the return value of the (async) function call
made to the function exported from filename. If the (async) function throws
an error, the returned Promise will be rejected with that error.
If the task is aborted, the returned Promise is rejected with an error
as well.
destroy()Stops all Workers and rejects all Promises for pending tasks.
This returns a Promise that is fulfilled once all threads have stopped.
'error'An 'error' event is emitted by instances of this class when:
All other errors are reported by rejecting the Promise returned from
runTask(), including rejections reported by the handler function itself.
'drain'A 'drain' event is emitted whenever the queueSize reaches 0.
completed (readonly)The current number of completed tasks.
duration (readonly)The length of time (in milliseconds) since this Piscina instance was
created.
options (readonly)A copy of the options that are currently being used by this instance. This object has the same properties as the options object passed to the constructor.
runTime (readonly)A histogram summary object summarizing the collected run times of completed tasks. All values are expressed in milliseconds.
runTime.average {number} The average run time of all tasksrunTime.mean {number} The mean run time of all tasksrunTime.stddev {number} The standard deviation of collected run timesrunTime.min {number} The fastest recorded run timerunTime.max {number} The slowest recorded run timeAll properties following the pattern p{N} where N is a number (e.g. p1, p99)
represent the percentile distributions of run time observations. For example,
p99 is the 99th percentile indicating that 99% of the observed run times were
faster or equal to the given value.
{
average: 1880.25,
mean: 1880.25,
stddev: 1.93,
min: 1877,
max: 1882.0190887451172,
p0_001: 1877,
p0_01: 1877,
p0_1: 1877,
p1: 1877,
p2_5: 1877,
p10: 1877,
p25: 1877,
p50: 1881,
p75: 1881,
p90: 1882,
p97_5: 1882,
p99: 1882,
p99_9: 1882,
p99_99: 1882,
p99_999: 1882
}
threads (readonly)An Array of the Worker instances used by this pool.
queueSize (readonly)The current number of tasks waiting to be assigned to a Worker thread.
utilization (readonly)A point-in-time ratio comparing the approximate total mean run time of completed tasks to the total runtime capacity of the pool.
A pools runtime capacity is determined by multiplying the duration
by the options.maxThread count. This provides an absolute theoretical
maximum aggregate compute time that the pool would be capable of.
The approximate total mean run time is determined by multiplying the mean run time of all completed tasks by the total number of completed tasks. This number represents the approximate amount of time the pool as been actively processing tasks.
The utilization is then calculated by dividing the approximate total
mean run time by the capacity, yielding a fraction between 0 and 1.
waitTime (readonly)A histogram summary object summarizing the collected times tasks spent waiting in the queue. All values are expressed in milliseconds.
waitTime.average {number} The average wait time of all taskswaitTime.mean {number} The mean wait time of all taskswaitTime.stddev {number} The standard deviation of collected wait timeswaitTime.min {number} The fastest recorded wait timewaitTime.max {number} The longest recorded wait timeAll properties following the pattern p{N} where N is a number (e.g. p1, p99)
represent the percentile distributions of wait time observations. For example,
p99 is the 99th percentile indicating that 99% of the observed wait times were
faster or equal to the given value.
{
average: 1880.25,
mean: 1880.25,
stddev: 1.93,
min: 1877,
max: 1882.0190887451172,
p0_001: 1877,
p0_01: 1877,
p0_1: 1877,
p1: 1877,
p2_5: 1877,
p10: 1877,
p25: 1877,
p50: 1881,
p75: 1881,
p90: 1882,
p97_5: 1882,
p99: 1882,
p99_9: 1882,
p99_99: 1882,
p99_999: 1882
}
isWorkerThread (readonly)Is true if this code runs inside a Piscina threadpool as a Worker.
version (readonly)Provides the current version of this library as a semver string.
move(value)By default, any value returned by a worker function will be cloned when
returned back to the Piscina pool, even if that object is capable of
being transfered. The Piscina.move() method can be used to wrap and
mark transferable values such that they will by transfered rather than
cloned.
The value may be any object supported by Node.js to be transferable
(e.g. ArrayBuffer, any TypedArray, or MessagePort), or any object
implementing the Transferable interface.
const { move } = require('piscina');
module.exports = () => {
return move(new ArrayBuffer(10));
}
The move() method will throw if the value is not transferable.
The object returned by the move() method should not be set as a
nested value in an object. If it is used, the move() object itself
will be cloned as opposed to transfering the object it wraps.
TransferableObjects may implement the Transferable interface to create their own
custom transferable objects. This is useful when an object being
passed into or from a worker contains a deeply nested transferable
object such as an ArrayBuffer or MessagePort.
Transferable objects expose two properties inspected by Piscina
to determine how to transfer the object. These properties are
named using the special static Piscina.transferableSymbol and
Piscina.valueSymbol properties:
The Piscina.transferableSymbol property provides the object
(or objects) that are to be included in the transferList.
The Piscina.valueSymbol property provides a surrogate value
to transmit in place of the Transferable itself.
Both properties are required.
For example,
const {
move,
transferableSymbol,
valueSymbol
} = require('piscina');
module.exports = () => {
const obj = {
a: { b: new Uint8Array(5); },
c: { new Uint8Array(10); },
get [transferableSymbol]() {
// Transfer the two underlying ArrayBuffers
return [this.a.b.buffer, this.c.buffer];
}
get [valueSymbol]() {
return { a: { b: this.b }, c: this.c };
}
};
return move(obj);
};
By default, Piscina uses a simple array-based first-in-first-out (fifo) task queue. When a new task is submitted and there are no available workers, tasks are pushed on to the queue until a worker becomes available.
If the default fifo queue is not sufficient, user code may replace the
task queue implementation with a custom implementation using the
taskQueue option on the Piscina constructor.
Custom task queue objects must implement the TaskQueue interface,
described below using TypeScript syntax:
interface Task {
readonly [Piscina.queueOptionsSymbol] : object | null;
}
interface TaskQueue {
readonly size : number;
shift () : Task | null;
remove (task : Task) : void;
push (task : Task) : void;
}
An example of a custom task queue that uses a shuffled priority queue
is available in examples/task-queue;
The special symbol Piscina.queueOptionsSymbol may be set as a property
on tasks submitted to runTask() as a way of passing additional options
on to the custom TaskQueue implementation. (Note that because the
queue options are set as a property on the task, tasks with queue
options cannot be submitted as JavaScript primitives).
Workers are generally optimized for offloading synchronous, compute-intensive operations off the main Node.js event loop thread. While it is possible to perform asynchronous operations and I/O within a Worker, the performance advantages of doing so will be minimal.
Specifically, it is worth noting that asynchronous operations within Node.js, including I/O such as file system operations or CPU-bound tasks such as crypto operations or compression algorithms, are already performed in parallel by Node.js and libuv on a per-process level. This means that there will be little performance impact on moving such async operations into a Piscina worker (see examples/scrypt for example).
Piscina provides the ability to configure the minimum and
maximum number of worker threads active in the pool, as well as
set limits on the number of tasks that may be queued up waiting
for a free worker. It is important to note that setting the
maxQueue size too high relative to the number of worker threads
can have a detrimental impact on performance and memory usage.
Setting the maxQueue size too small can also be problematic
as doing so could cause your worker threads to become idle and
be shutdown. Our testing has shown that a maxQueue size of
approximately the square of the maximum number of threads is
generally sufficient and performs well for many cases, but this
will vary significantly depending on your workload. It will be
important to test and benchmark your worker pools to ensure you've
effectively balanced queue wait times, memory usage, and worker
pool utilization.
The thread pool maintained by Piscina has both a minimum and maximum
limit to the number of threads that may be created. When a Piscina
instance is created, it will spawn the minimum number of threads
immediately, then create additional threads as needed up to the
limit set by maxThreads. Whenever a worker completes a task, a
check is made to determine if there is additional work for it to
perform. If there is no additional work, the thread is marked idle.
By default, idle threads are shutdown immediately, with Piscina
ensuring that the pool always maintains at least the minimum.
When a Piscina pool is processing a stream of tasks (for instance, processing http server requests as in the React server-side rendering example in examples/react-ssr), if the rate in which new tasks are received and queued is not sufficient to keep workers from going idle and terminating, the pool can experience a thrashing effect -- excessively creating and terminating workers that will cause a net performance loss. There are a couple of strategies to avoid this churn:
Strategy 1: Ensure that the queue rate of new tasks is sufficient to keep workers from going idle. We refer to this as "queue pressure". If the queue pressure is too low, workers will go idle and terminate. If the queue pressure is too high, tasks will stack up, experience increased wait latency, and consume additional memory.
Strategy 2: Increase the idleTimeout configuration option. By
default, idle threads terminate immediately. The idleTimeout option
can be used to specify a longer period of time to wait for additional
tasks to be submitted before terminating the worker. If the queue
pressure is not maintained, this could result in workers sitting idle
but those will have less of a performance impact than the thrashing
that occurs when threads are repeatedly terminated and recreated.
Strategy 3: Increase the minThreads configuration option. This has
the same basic effect as increasing the idleTimeout. If the queue
pressure is not high enough, workers may sit idle indefinitely but
there will be less of a performance hit.
In applications using Piscina, it will be most effective to use a combination of these three approaches and tune the various configuration parameters to find the optimum combination both for the application workload and the capabilities of the deployment environment. There are no one set of options that are going to work best.
Piscina.move()maxQueue = 'auto' to autocalculate the maximum queue size.'drain' eventenv, argv, execArgv, and workerData optionsresourceLimitsPiscina development is sponsored by NearForm Research.
FAQs
A fast, efficient Node.js Worker Thread Pool implementation
We found that piscina demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 6 open source maintainers collaborating on the project.
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