Shared Memory Objects

A library to try to make making a multi-threaded game in Javascript possible. This package is to provide a wrapper to create objects and data structures that are backed by a SharedArrayBuffer and can be shared between multiple threads. The end result is a package that has all of the slowness of Javascript with all of the baggage of dealing with manual memory allocations. If you need to multi-thread you are probably better of just using a different language and compiling to WebAssembly. But if you, like me, just want to use Javascript/Typescript and are willing to deal with dealing with manual memory allocations then this library could save you some time.

A demo can be found at https://daneren2005.github.io/ecs-sharedarraybuffer-playground/#/shared-memory-objects
The code is at https://github.com/daneren2005/ecs-sharedarraybuffer-playground/tree/dev/src/shared-memory-objects

Basics

The core of this package is the MemoryHeap. You should usually just have a single heap that is shared between all of your different threads. Each heap can have multiple MemoryBuffers. By default each buffer is only 8KB but it can be configured up to 1MB, and you can have up to 4k buffers for a total of 4GB. When you allocate memory, if there is not enough space it will allocate another buffers automatically. When allocating memory, you will get a AllocatedMemory object that is a wrapper around the allocated memory by calling heap.allocUI32({count of 32 bit numbers}). By default AllocatedMemory is backed by a Uint32Array but you can get any type of array from AllocatedMemory.getArray(Int32Array);.

Each allocated memory location can be stored as an int pointer. You can use getPointer(int) to get the bufferPosition (ie: buffer index in the heap) and bufferByteOffset that the memory location points to. You can also convert a bufferPosition/bufferByteOffset pair to an int pointer with createPointer(bufferPosition, bufferByteOffset). The pointer format is uses 12 bits for the buffer index and the remaining 20 bits for the byte offset in that buffer for a total of 1MB per buffer and 4GB total of memory. Each allocated memory object can return either a pointer via allocatedMemory.pointer or the raw position/byte offset via allocatedMemory.getSharedMemory().

When passing memory to another thread you can either pass a pointer or a serialized version of the buffer position/byte offset in order to re-create the object in the other thread.

Getting Started

npm install @daneren2005/shared-memory-objects

Example to update blocks of memory from a thread.

let heap = new MemoryHeap();
let memory = heap.allocUI32(4);

// Pass memory to another thread
thread.postMessage({
	heap: heap.getSharedMemory(),
	memory: memory.getSharedMemory()
});

// From worker thread re-construct memory and change it
self.onmessage = (e) => {
	let heap = new MemoryHeap(e.data.heap);
	let memory = new AllocatedMemory(heap, e.data.memory);
	memory.data[2] = 5;
};

// Example to work with data structures from a thread. When constructing a new structure you just pass the heap. When re-creating a structure from an already initialized memory location pass the heap and the shared memory location for it.

let heap = new MemoryHeap();
let list = new SharedList(heap);

// Pass memory to another thread
thread.postMessage({
	heap: heap.getSharedMemory(),
	list: list.getSharedMemory()
});

// From worker thread re-construct memory and change it
self.onmessage = (e) => {
	let heap = new MemoryHeap(e.data.heap);
	let list = new SharedList(heap, e.data.list);

	list.push(5);
};

let mainList = new SharedList(memory); let secondList = new SharedList(memory, mainList.getSharedMemory());

Data Structures

• SharedList
• SharedVector
• SharedMap
• SharedString

Thread Safety

• Memory allocations is thread safe as long as it does not need to create a new buffer. Right now that can only be done from the main thread.
• SharedList, SharedVector, and SharedMap are all not thread safe.
• SharedString is thread safe with a lock on read/write with a cached version of the string so it doesn't need to lock after the first read unless the string has changed.

TODO

• Make creating new buffers from allocations possible from multiple threads
• Make data structures thread safe
• Add basic thread safe object example

Credit

The entire core of this library is based on a fork of @thi.ng/malloc found at https://github.com/thi-ng/umbrella/blob/develop/packages/malloc. The only big difference between our MemoryBuffer and their MemPool is making allocations/freeing memory thread safe.