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src: register external references for source code #47055

Merged
merged 2 commits into from
May 10, 2023
Merged

src: register external references for source code #47055

merged 2 commits into from
May 10, 2023

Commits on Apr 9, 2023

  1. src: register external references for source code 

    Currently we use external strings for internalized builtin source code.
However when a snapshot is taken, any external string whose resource
is not registered is flattened into a SeqString (see ref). The
result is that module source code stored in the snapshot does not
use external strings after deserialization. This patch registers an
external string resource for each internalized builtin's source. The
savings are substantial: ~1.9 MB of heap memory per isolate, or ~44%
of an otherwise empty isolate's heap usage:

```console
$ node --expose-gc -p 'gc(),process.memoryUsage().heapUsed'
4190968
$ ./node --expose-gc -p 'gc(),process.memoryUsage().heapUsed'
2327536
```

The savings can be even higher for user snapshots which may include
more internal modules.

The existing UnionBytes implementation was ill-suited, because it only
created an external string resource when ToStringChecked was called,
but we need to register the external string resources before the
isolate even exists. We change UnionBytes to no longer own the data,
and shift ownership of the data to a new external resource class
called StaticExternalByteResource.  StaticExternalByteResource are
either statically allocated (for internalized builtin code) or owned
by the static `externalized_builtin_sources` map, so they will only be
destructed when static resources are destructed. We change JS2C to emit
statements to register a string resource for each internalized builtin.

Refs: https://github.com/v8/v8/blob/d2c8fbe9ccd1a6ce5591bb7dd319c3c00d6bf489/src/snapshot/serializer.cc#L633
    @kvakil
    kvakil committed Apr 9, 2023
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  2. fixup! src: register external references for source code

    @kvakil
    kvakil committed Apr 9, 2023
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    cd42560 View commit details
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