Stream: git-wasmtime

Topic: wasmtime / PR #4051 Reduce clones of `Arc<HostFunc>` duri...

Wasmtime GitHub notifications bot (Apr 19 2022 at 14:31):

alexcrichton opened PR #4051 from less-arc-clone to main:

This commit implements an optimization to help improve concurrently
creating instances of a module on many threads simultaneously. One
bottleneck to this measured has been the reference count modification on
Arc<HostFunc>. Each host function stored within a Linker<T> is
wrapped in an Arc<HostFunc> structure, and when any of those host
functions are inserted into a store the reference count is incremented.
When the store is dropped the reference count is then decremented.

This ends up meaning that when a module imports N functions it ends up
doing 2N atomic modifications over the lifetime of the instance. For
embeddings where the Linker<T> is rarely modified but instances are
frequently created this can be a surprising bottleneck to creating many
instances.

A change implemented here is to optimize the instantiation process when
using an InstancePre<T>. An InstancePre serves as an opportunity to
take the list of items used to instantiate a module and wrap them all up
in an Arc<[T]>. Everything is going to get cloned into a Store<T>
anyway so to optimize this the Arc<[T]> is cloned at the top-level and
then nothing else is cloned internally. This continues to, however,
preserve a strong reference count for all contained items to prevent
them from being deallocated.

A new variant of FuncKind was added for host functions which is
effectively stored via *mut HostFunc. This variant is unsafe to create
and manage and has been documented internally.

Performance-wise the overall impact of this change is somewhat minor.
It's already a bit esoteric if this atomic increment and decrement are a
bottleneck due to the number of concurrent instances being created. In
my measurements I've seen that this can reduce instantiation time by up
to 10% for a module that imports two dozen functions. For larger modules
with more imports this is expected to have a larger win.

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Wasmtime GitHub notifications bot (Apr 19 2022 at 17:13):

cfallin submitted PR review.

Wasmtime GitHub notifications bot (Apr 19 2022 at 19:23):

alexcrichton merged PR #4051.

Last updated: Dec 23 2024 at 12:05 UTC