Path: blob/main/docs/examples-fast-instantiation.md
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Tuning Wasmtime for Fast Instantiation
Before a WebAssembly module can begin execution, it must first be compiled and then instantiated. Compilation can happen ahead of time, which removes compilation from the critical path. That leaves just instantiation on the critical path. This page documents methods for tuning Wasmtime for fast instantiation.
Enable the Pooling Allocator
By enabling the pooling allocator, you are configuring Wasmtime to up-front and ahead-of-time allocate a large pool containing all the resources necessary to run the configured maximum number of concurrent instances. Creating a new instance doesn't require allocating new Wasm memories and tables on demand, it just takes pre-allocated memories and tables from the pool, which is generally much faster. Deallocating an instance returns its memories and tables to the pool.
See wasmtime::PoolingAllocationConfig, wasmtime::InstanceAllocationStrategy, and wasmtime::Config::allocation_strategy for more details.
Enable Copy-on-Write Heap Images
Initializing a WebAssembly linear memory via a copy-on-write mapping can drastically improve instantiation costs because copying memory is deferred from instantiation time to when the data is first mutated. When the Wasm module only reads the initial data, and never overwrites it, then the copying is completely avoided.
See the API docs for wasmtime::Config::memory_init_cow for more details.
Use InstancePre
To instantiate a WebAssembly module or component, Wasmtime must look up each of the module's imports and check that they are of the expected types. If the imports are always the same, this work can be done ahead of time, before instantiation. A wasmtime::InstancePre represents an instance just before it is instantiated, after all type-checking and imports have been resolved. The only thing left to do for this instance is to actually allocate its memories, tables, and internal runtime context, initialize its state, and run its start function, if any.
See the API docs for wasmtime::InstancePre, wasmtime::Linker::instantiate_pre, wasmtime::component::InstancePre, and wasmtime::component::Linker::instantiate_pre for more details.