performance.measureUserAgentSpecificMemory

[emilk]

Motivation

window.performance.memory in an API for accessing the memory use of JavaScript (and by extension, wasm).

Example JS: window.performance.memory.usedJSHeapSize

Proposed Solution

Add a Memory feature flag to expose window.performance.memory with three fields:

• js_heap_size_limit
• total_js_heap_size
• used_js_heap_size

Alternatives

WASM apps can install a custom #[global_allocator] that does its own bookkeeping of amount of RAM used.

Additional Context

Knowing the memory used by a WASM application is useful for it to know whether or not to try free up some memory.

[Liamolucko]

performance.memory is a non-standard, deprecated API, at least according to MDN. performance.measureUserAgentSpecificMemory seems to be the more modern version, with an actual spec (although not actually standardised yet). Would adding (unstable) support for that instead be acceptable?

[emilk]

Yes, performance.measureUserAgentSpecificMemory sounds perfect!