Lazily populate a store's trampoline map#3742
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alexcrichton merged 3 commits intoFeb 2, 2022
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This commit is another installment of "how fast can we make instantiation". Currently when instantiating a module with many function imports each function, typically from the host, is inserted into the store. This insertion process stores the `VMTrampoline` for the host function in a side table so it can be looked up later if the host function is called through the `Func` interface. This insertion process, however, involves a hash map insertion which can be relatively expensive at the scale of the rest of the instantiation process. The optimization implemented in this commit is to avoid inserting trampolines into the store at `Func`-insertion-time (aka instantiation time) and instead only lazily populate the map of trampolines when needed. The theory behind this is that almost all `Func` instances that are called indirectly from the host are actually wasm functions, not host-defined functions. This means that they already don't need to go through the map of host trampolines and can instead be looked up from the module they're defined in. With the assumed rarity of host functions making `lookup_trampoline` a bit slower seems ok. The `lookup_trampoline` function will now, on a miss from the wasm modules and `host_trampolines` map, lazily iterate over the functions within the store and insert trampolines into the `host_trampolines` map. This process will eventually reach something which matches the function provided because it should at least hit the same host function. The relevant `lookup_trampoline` now sports a new documentation block explaining all this as well for future readers. Concretely this commit speeds up instantiation of an empty module with 100 imports and ~80 unique signatures from 10.6us to 6.4us, a 40% improvement.
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Thanks for the detailed comments! Very helpful. I nitpicked a bunch, but I hope my nitpicks should collectively make reading this documentation even more helpful for future spelunkers.
| .skip(self.host_func_trampolines_registered) | ||
| { | ||
| self.host_func_trampolines_registered += 1; | ||
| self.host_trampolines.insert(f.sig_index(), f.trampoline()); |
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| self.host_trampolines.insert(f.sig_index(), f.trampoline()); | |
| let old_entry = self.host_trampolines.insert(f.sig_index(), f.trampoline()); | |
| debug_assert!(old_entry.is_none()); |
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Ah actually doing this causes a test failure because we might find duplicates of signatures we're not looking for as the array of functions is scanned.
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* Lazily populate a store's trampoline map This commit is another installment of "how fast can we make instantiation". Currently when instantiating a module with many function imports each function, typically from the host, is inserted into the store. This insertion process stores the `VMTrampoline` for the host function in a side table so it can be looked up later if the host function is called through the `Func` interface. This insertion process, however, involves a hash map insertion which can be relatively expensive at the scale of the rest of the instantiation process. The optimization implemented in this commit is to avoid inserting trampolines into the store at `Func`-insertion-time (aka instantiation time) and instead only lazily populate the map of trampolines when needed. The theory behind this is that almost all `Func` instances that are called indirectly from the host are actually wasm functions, not host-defined functions. This means that they already don't need to go through the map of host trampolines and can instead be looked up from the module they're defined in. With the assumed rarity of host functions making `lookup_trampoline` a bit slower seems ok. The `lookup_trampoline` function will now, on a miss from the wasm modules and `host_trampolines` map, lazily iterate over the functions within the store and insert trampolines into the `host_trampolines` map. This process will eventually reach something which matches the function provided because it should at least hit the same host function. The relevant `lookup_trampoline` now sports a new documentation block explaining all this as well for future readers. Concretely this commit speeds up instantiation of an empty module with 100 imports and ~80 unique signatures from 10.6us to 6.4us, a 40% improvement. * Review comments * Remove debug assert
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Mar 17, 2022
* Lazily populate a store's trampoline map This commit is another installment of "how fast can we make instantiation". Currently when instantiating a module with many function imports each function, typically from the host, is inserted into the store. This insertion process stores the `VMTrampoline` for the host function in a side table so it can be looked up later if the host function is called through the `Func` interface. This insertion process, however, involves a hash map insertion which can be relatively expensive at the scale of the rest of the instantiation process. The optimization implemented in this commit is to avoid inserting trampolines into the store at `Func`-insertion-time (aka instantiation time) and instead only lazily populate the map of trampolines when needed. The theory behind this is that almost all `Func` instances that are called indirectly from the host are actually wasm functions, not host-defined functions. This means that they already don't need to go through the map of host trampolines and can instead be looked up from the module they're defined in. With the assumed rarity of host functions making `lookup_trampoline` a bit slower seems ok. The `lookup_trampoline` function will now, on a miss from the wasm modules and `host_trampolines` map, lazily iterate over the functions within the store and insert trampolines into the `host_trampolines` map. This process will eventually reach something which matches the function provided because it should at least hit the same host function. The relevant `lookup_trampoline` now sports a new documentation block explaining all this as well for future readers. Concretely this commit speeds up instantiation of an empty module with 100 imports and ~80 unique signatures from 10.6us to 6.4us, a 40% improvement. * Review comments * Remove debug assert
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This commit is another installment of "how fast can we make
instantiation". Currently when instantiating a module with many function
imports each function, typically from the host, is inserted into the
store. This insertion process stores the
VMTrampolinefor the hostfunction in a side table so it can be looked up later if the host
function is called through the
Funcinterface. This insertion process,however, involves a hash map insertion which can be relatively expensive
at the scale of the rest of the instantiation process.
The optimization implemented in this commit is to avoid inserting
trampolines into the store at
Func-insertion-time (aka instantiationtime) and instead only lazily populate the map of trampolines when
needed. The theory behind this is that almost all
Funcinstances thatare called indirectly from the host are actually wasm functions, not
host-defined functions. This means that they already don't need to go
through the map of host trampolines and can instead be looked up from
the module they're defined in. With the assumed rarity of host functions
making
lookup_trampolinea bit slower seems ok.The
lookup_trampolinefunction will now, on a miss from the wasmmodules and
host_trampolinesmap, lazily iterate over the functionswithin the store and insert trampolines into the
host_trampolinesmap.This process will eventually reach something which matches the function
provided because it should at least hit the same host function. The
relevant
lookup_trampolinenow sports a new documentation blockexplaining all this as well for future readers.
Concretely this commit speeds up instantiation of an empty module with
100 imports and ~80 unique signatures from 10.6us to 6.4us, a 40%
improvement.