Improve accuracy and change semantics of metric wasm_memory_bytes#5131
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gefjon wants to merge 4 commits into
Open
Improve accuracy and change semantics of metric wasm_memory_bytes#5131gefjon wants to merge 4 commits into
wasm_memory_bytes#5131gefjon wants to merge 4 commits into
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Prior to this commit, the metric `wasm_memory_bytes` had several problems: 1. Despite its name, it was used for both Wasmtime and V8 modules. For V8 modules, it was the same value as `v8_used_heap_size_bytes`. 2. It stored only the value for a single instance at any given time, so it under-reported a database's memory usage. 3. The same row (set of label values) was written concurrently by all instances of a particular database, with each one clobbering the previously written value. In this commit, we change the metric so that: 1. It is recorded only for Wasmtime instances, not V8 instances. For V8 instances, instead directly check `v8_used_heap_size_bytes`, or one of the other V8 heap metrics. This change involved moving the recording of this metric from `module_host_actor.rs` to `wasmtime/wasm_instance_env.rs` 2. Similar to the V8 heap metrics, all the instances cooperatively share the metric entry, updating it by incrementing and decrementing rather than `set`ting. Note that this metric is used for billing, and so we will need to update our billing code (elsewhere) to account for the change. In particular, our billing code should now charge for the sum of `wasm_memory_bytes` and `v8_used_heap_size_bytes`. We also should expect with this change for each database's recorded usage to increase, as we are now accurately recording the usage for all instances, not just one.
joshua-spacetime
approved these changes
May 28, 2026
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Marked as draft until I write its private companion; this PR will probably pass internal tests but still requires private changes.
Description of Changes
Prior to this commit, the metric
wasm_memory_byteshad several problems:v8_used_heap_size_bytes.In this commit, we change the metric so that:
v8_used_heap_size_bytes, or one of the other V8 heap metrics. This change involved moving the recording of this metric frommodule_host_actor.rstowasmtime/wasm_instance_env.rssetting.Note that this metric is used for billing, and so we will need to update our billing code (elsewhere) to account for the change. In particular, our billing code should now charge for the sum of
wasm_memory_bytesandv8_used_heap_size_bytes. We also should expect with this change for each database's recorded usage to increase, as we are now accurately recording the usage for all instances, not just one.API and ABI breaking changes
Billing metric semantics changed.
Expected complexity level and risk
3: billing metric semantics changed.
Testing
I do not know how to test metrics.