What are the implications of a maximum memory limits in WebAssembly?

Question

When I declare a memory section in a WebAssembly then I have to set the initial size and I can set the optional maximum size.

Does it have any advantages if I set the maximum to the same value as the initial value? What are the implications of this value for WebAssembly runtime?

Background: I write a Java to WebAssembly compiler and want to use the coming GC feature for my data. I does not need to grow the memory. I would use it only for constant values.

Answer 1

Allocating a large memory (especially when it's gigabytes) may fail. Failed to allocate the initial memory is a fatal error, while failed to grow the memory later is not. So it is a good idea to start with a smaller and safe initial size.

The WebAssembly communities already provides pretty good documentations:

• Linear Memory Resizing in WebAssembly Design Rationale. (Highly recommended if you are writing a compiler.)
• Resizing memory in Modules page.

I will sum up the information on how WebAssembly memory works here.

Why we need the optional maximum size

The underlying WebAssembly Memory is a JS ArrayBuffer object. ArrayBuffer is not a dynamic array, meaning it cannot be resized. However, Wasm Memory is a special ArrayBuffer that can be resized by Memory.grow() call, which is correspondent to grow_memory instruction in Wasm. Still, implementation of resizing ArrayBuffer costs a lot - It is the same as realloc(), which allocate a new buffer with the new size then deallocate the old buffer. You may avoid the overhead of reallocating the buffer by allocating a large initial memory but it causes another problem that the operation may fail and failed to do so means the Wasm engine failed to load the Wasm binary.

The optional maximum size solves those problems. When the maximum size is defined, the Wasm Memory tries to pre-allocate the maximum size of the buffer. By pre-allocating the buffer you can resize the buffer later without an expensive realloc() operation. It is okay even if the pre-allocating operation failed - you can try reallocte later when you need.

WebAssembly Memory Resizing Scenarios

• grow_memory without setting the maximum size: The Wasm engine tries to reallocate the whole buffer, which is very expensive and increases possibilities to fail.
• Allocating a big initial memory: It may fail and its failure is a fatal error.
• grow_memory with the maximum size: The engine will use pre-allocated buffer instantly. Even if it failed to grow it is not a fatal error.
• Setting initial size == maximum size: You won't benefit from any of them. It may experience a fatal error, and you cannot resize it later neither.