The Pht Threading Extension

This extension exposes a new approach to threading in PHP.

Quick feature list:

• Classes, functions, and files may be threaded
• The inter-thread communication (ITC) data structures include: hash table, queue, vector
• Threads are reusable for any number of tasks

Requirements:

• A ZTS version of PHP 7.2. The master branch of php-src is not currently compatible

Any Unix-based OS is supported (including OS X), along with Windows. This extension was explicitly tested on OS X (Yosemite and Sierra), Ubuntu 14.04 (32bit), and Windows Server 2012 (the pthreads-win32 library is needed).

Documentation: php.net/pht

Contents:

• Installation
• Pthreads VS pht
• The Basics
• API
• Quick Examples
  • Threading Types
    • Class Threading
    • Function Threading
    • File Threading
  • Inter-Thread Communication Data Structures
    • Queue
    • Vector
    • Hash Table
  • Atomic Values
    • Atomic Integer

This extension was built using a few ideas from the pthreads extension. I'd therefore like to give credit to, as well as thank, Joe Watkins for his great work on the pthreads project!

Installation

If you're using a Unix-based OS, then you can build the extension from source by:

git clone https://github.com/tpunt/pht
cd pht
git checkout tags/v0.0.1
phpize
./configure
make
make install

If you're using Windows, see the release page for the appropriate .dll extension file. The pthreadVC2.dll file (distributed alongside the extension's .dll file) will need to be made available in your PATH environment variable.

Once the .so or .dll file has been acquire, the php.ini file will then need to be updated (to load the extension) with:

extension="path/to/pht_file"

Pthreads vs pht

Both extensions have their own advantages and disadvantages.

Pthreads advantages over pht:

• Uses synchronised blocks over mutex locks (which can be cleaner to use and harder to mess up)
• Easier communication between threads (no knowledge of data structures is required, which PHP developers tend to lack in)
• Maturer

Pht advantages over pthreads:

• No serialisation of properties on threaded objects, meaning:
  • No performance hit when reading from or writing to properties
  • No unfamiliar semantics, such as implicitly casting arrays to Volatile objects or property immutability for some types
  • No limitations on what types of data can be stored as properties on threaded objects
• The ability to thread functions and files (as well as classes)
• Threaded classes implement a Runnable interface, rather than having to inherit (so more flexibility on inheritance)

The Basics

This approach to threading abstracts away the thread itself behind a dedicated object (Thread), where tasks are added to that thread's internal task queue.

Example:

<?php

use pht\{Thread, Runnable};

class Task implements Runnable
{
    public function run() {}
}

$thread = new Thread();

// Thread::addClassTask(string $className, mixed ...$constructorArgs) : void;
$thread->addClassTask(Task::class);

// Thread::addFunctionTask(callable $fn, mixed ...$fnArgs) : void;
$thread->addFunctionTask(function ($zero) {var_dump($zero);}, 0);

// Thread::addFileTask(string $filename, mixed ...$globals) : void;
$thread->addFileTask('some_file.php', 1, 2, 3);

$thread->start();
$thread->join();

some_file.php:

<?php

[$one, $two, $three] = $_THREAD;

The task types have the following properties:

• Class tasks will be instantiated inside of the new thread
• Function tasks will be serialised and passed into the new thread
• File tasks will open the file inside of the new thread

All of these tasks will execute in isolation. In particular, for class tasks, it means the spawned objects cannot be passed around between threads. By keeping the threading contexts completely separate from one-another, we prevent the need to serialise the properties of threaded objects (a necessary evil if such objects had to operate in multiple threads, as seen in pthreads).

Given the isolation of threaded contexts, we have a new problem: how can data be passed between threads for inter-thread communication (ITC)? To solve this problem, threadable data structures have been implemented, where mutex locks have been exposed to the programmer for controlling access to them. Whilst this has increased the complexity a bit for the programmer, it has also increased the flexibility, too.

So far, the following data structures have been implemented: queue, hash table, and vector. These data structures can be safely passed around between threads, and manipulated by multiple threads using the mutex locks that have been packed in with the data structure. They are reference-counted across threads, and so they do not need to be explicitly destroyed.

With this approach to threading, only the given built-in data structures need to be safely passed around between threads.

This means that the serialisation points to be aware of are:

• The arguments being passed to Thread::addClassTask(), Thread::addFunctionTask(), and Thread::addFileTask()
• The values being placed into the ITC-based data structures

API

<?php

namespace pht;

class Thread
{
    public function addClassTask(string $className, mixed ...$ctorArgs) : void;
    public function addFunctionTask(callable $fn, mixed ...$fnArgs) : void;
    public function addFileTask(string $filename, mixed ...$globals) : void;
    public function taskCount(void) : int;
    public function start(void) : void;
    public function join(void) : void;
}

interface Runnable
{
    public function run(void) : void;
}

// internal interface, not implementable by userland PHP classes
interface Threaded
{
    public function lock(void) : void;
    public function unlock(void) : void;
}

final class Queue implements Threaded
{
    public function push(mixed $value) : void;
    public function pop(void) : mixed;
    public function front(void) : mixed;
    public function lock(void) : void;
    public function unlock(void) : void;
    public function size(void) : int;
}

final class HashTable implements Threaded
{
    public function lock(void) : void;
    public function unlock(void) : void;
    public function size(void) : int;
    // ArrayAccess API is enabled, but the userland interface is not explicitly implemented
}

final class Vector implements Threaded
{
    public function __construct([int $size = 0 [, mixed $defaultValue = 0]]);
    public function resize(int $size [, mixed $defaultValue = 0]) : void;
    public function push(mixed $value) : void;
    public function pop(void) : mixed;
    public function shift(void) : mixed;
    public function unshift(mixed $value) : void;
    public function insertAt(mixed $value, int $index) : void;
    public function updateAt(mixed $value, int $index) : void;
    public function deleteAt(int $index) : void;
    public function lock(void) : void;
    public function unlock(void) : void;
    public function size(void) : int;
    // ArrayAccess API is enabled, but the userland interface is not explicitly implemented
}

final class AtomicInteger implements Threaded
{
    public function __construct([int $value = 0]);
    public function get(void) : int;
    public function set(int $value) : void;
    public function inc(void) : void;
    public function dec(void) : void;
    public function lock(void) : void;
    public function unlock(void) : void;
}

Quick Examples

This section demonstrates some quick examples of the basic features. For generic examples, see the examples folder instead. And for further information on the classes and their methods, checkout the documentation.

Threading Types

Class Threading

Classes that will be threaded need to implement the Runnable interface. The implemented Runnable::run method acts as the entry point of execution for when the class task is executed.

<?php

use pht\{Thread, Runnable};

class Task implements Runnable
{
    private $one;

    public function __construct(int $one)
    {
        $this->one = $one;
    }

    public function run()
    {
        var_dump($this->one);
    }
}

$thread = new Thread();

$thread->addClassTask(Task::class, 1);

$thread->start();
$thread->join();

All $ctorArgs being passed into the Thread::addClassTask() method will be serialised.

Function Threading

Functions must not refer to $this (it will become null in the threaded context), and must not import variables from their outer scope (via the use statement). They should be completely standalone.

<?php

use pht\Thread;

class Test
{
    public static function run(){var_dump(5);}
    public static function run2(){var_dump(6);}
}

function aFunc(){var_dump(3);}

$thread = new Thread();

$thread->addFunctionTask(static function($one) {var_dump($one);}, 1);
$thread->addFunctionTask(function() {var_dump(2);});
$thread->addFunctionTask('aFunc');
$thread->addFunctionTask('array_map', function ($n) {var_dump($n);}, [4]);
$thread->addFunctionTask(['Test', 'run']);
$thread->addFunctionTask([new Test, 'run2']);

$thread->start();
$thread->join();

All $fnArgs being passed into the Thread::addFunctionTask() method will be serialised.

File Threading

To pass data to the file being threaded, pass them as additional arguments to the Thread::addFileTask() method. They will then become available in a special $_THREAD superglobals array inside of the threaded file.

<?php

use pht\Thread;

$thread = new Thread();

$thread->addFileTask('file.php', 1, 2, 3);

$thread->start();
$thread->join();

file.php

<?php

[$one, $two, $three] = $_THREAD;

var_dump($one, $two, $three);

All $globals being passed into the Thread::addFileTask() method will be serialised.

Inter-Thread Communication Data Structures

The inter-thread communication (ITC) data structures enable for a two-way communication style between threads.

They are:

• safe to pass around between threads
• reference-counted across threads
• nestable within one-another (such as using a Vector in a Vector)

Things to note:

• cyclic references will leak (due to the simplistic nature of reference counting)
• all values placed into these data structures will be serialised
• the mutexes exposed by these data structures are not reentrant

Queue

<?php

use pht\{Thread, Queue};

$thread = new Thread();
$queue = new Queue();
$queueItemCount = 5;

$thread->addFunctionTask(function ($queue, $queueItemCount) {
    for ($i = 0; $i < $queueItemCount; ++$i) {
        $queue->lock();
        $queue->push($i);
        $queue->unlock();
    }
}, $queue, $queueItemCount);

$thread->start();

while (true) {
    $queue->lock();

    if ($queue->size() === $queueItemCount) {
        $queue->unlock();
        break;
    }

    $queue->unlock();
}

$thread->join();

// since we are no longer using $queue in multiple threads, we don't need to lock it
while ($queue->size()) {
    var_dump($queue->pop());
}

Vector

<?php

use pht\{Thread, Vector};

$thread = new Thread();
$vector = new Vector();
$vectorItemCount = 5;

for ($i = 0; $i < $vectorItemCount; ++$i) {
    $thread->addFunctionTask(function ($vector, $i) {
        $vector->lock();
        $vector->push($i);
        $vector->unlock();
    }, $vector, $i);
}

$thread->start();

while (true) {
    $vector->lock();

    if ($vector->size() === $vectorItemCount) {
        $vector->unlock();
        break;
    }

    $vector->unlock();
}

$thread->join();

// since we are no longer using $vector in multiple threads, we don't need to lock it
for ($i = 0; $i < $vectorItemCount; ++$i) {
    var_dump($vector[$i]);
}

Hash Table

<?php

use pht\{Thread, HashTable};

$thread = new Thread();
$hashTable = new HashTable();
$hashTableItemCount = 5;

for ($i = 0; $i < $hashTableItemCount; ++$i) {
    $thread->addFunctionTask(function ($hashTable, $i) {
        $hashTable->lock();
        $hashTable[chr(ord('a') + $i)] = $i;
        $hashTable->unlock();
    }, $hashTable, $i);
}

$thread->start();

while (true) {
    $hashTable->lock();

    if ($hashTable->size() === $hashTableItemCount) {
        $hashTable->unlock();
        break;
    }

    $hashTable->unlock();
}

$thread->join();

// since we are no longer using $hashTable in multiple threads, we don't need to lock it
for ($i = 0; $i < $hashTableItemCount; ++$i) {
    var_dump($hashTable[chr(ord('a') + $i)]);
}

Atomic Values

Atomic values are classes that wrap simple values. These values are safe to update without acquiring mutex locks, but they also pack with them mutex locks should multiple operations need to be performed together. The mutex locks, for this reason, are reentrant.

Atomic Integer

<?php

use pht\{Thread, AtomicInteger};

$thread = new Thread();
$atomicInteger = new AtomicInteger();
$max = 100000;

$thread->addFunctionTask(function ($atomicInteger, $max) {
    for ($i = 0; $i < $max; ++$i) {
        $atomicInteger->inc();
    }
}, $atomicInteger, $max);

$thread->start();

// safe
for ($i = 0; $i < $max; ++$i) {
    $atomicInteger->inc();
}

// safe
while ($atomicInteger->get() !== $max * 2);

// requires mutex locking since we need to perform multiple operations together
$atomicInteger->lock();
$atomicInteger->set($atomicInteger->get() * 2);
$atomicInteger->unlock();

$thread->join();

var_dump($atomicInteger->get()); // int(400000)