When ScavengeObject is Called in libv8

When is ScavangeObject called in libv8

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The user is asking about when ScavangeObject is called in libv8. This seems to be about the V8 JavaScript engine’s garbage collection mechanism. Let me search for this in a V8 repository.

First, I should search for “ScavangeObject” (though I notice it might be a typo - it’s likely “ScavengeObject” with an ‘e’). Let me search for both spellings to be thorough.

The main V8 repository is at v8/v8 on GitHub.

I’ll search for ScavangeObject (and also ScavengeObject in case of spelling variation) in the V8 repository.

Thinking...

GitHub code search
Searched for ‘ScavangeObject’ in v8/v8

GitHub code search
Searched for ‘ScavengeObject’ in v8/v8

GitHub file content
Retrieved content of src/heap/scavenger.cc#L2220-L2260, src/heap/scavenger-inl.h#L440-L460 from v8/v8@main

GitHub code search
Searched for ‘ScavengeObject call’ in v8/v8

GitHub file content
Retrieved content of src/heap/scavenger-inl.h#L440-L460, src/heap/scavenger-inl.h#L540-L565 from v8/v8@main

GitHub search files
Searched for ‘scavenger-inl.h’ in v8/v8/main

GitHub search files
Searched for ‘scavenger, inl’ in v8/v8/main

GitHub search files
Searched for ‘heap, scavenger.cc’ in v8/v8/main

Based on my search of the V8 repository, here’s when ScavengeObject is called in libv8:

Overview

ScavengeObject is called during V8’s Scavenge garbage collection (a type of generational GC for young generation objects). It’s located in src/heap/scavenger.cc.

When It’s Called

Based on the code and stack traces, ScavengeObject is called by:

1. VisitHeapObjectImpl - When visiting heap object slots during scavenging

   • This processes individual object references

2. ScavengeVisitor::VisitPointersImpl - When iterating through pointer fields in objects

   • This traverses all the pointers in an object that need to be scavenged

3. During object graph traversal - As part of copying live objects from the “from-space” to “to-space” in the young generation

What It Does

The function:

• Takes a slot p and a HeapObject that needs to be scavenged
• Checks if the object has already been forwarded (moved)
• If already forwarded, updates the reference
• If not forwarded, calls EvacuateObject to copy it to the new location
• Returns a SlotCallbackResult indicating whether to keep or remove the slot

Call Chain Example

From the test data trace:

EvacuateObjectDefault → EvacuateShortcutCandidate → EvacuateObject → ScavengeObject → VisitHeapObjectImpl → VisitPointersImpl

This is a core part of V8’s generational garbage collector for managing young generation memory.

Can this be issued programatically?

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The user is asking if ScavengeObject can be issued/triggered programmatically. Let me search for information about how scavenging is triggered, and what public APIs exist for triggering garbage collection in V8.

I should look for:

1. How scavenging is triggered in V8
2. Public APIs for triggering GC
3. How to programmatically trigger a scavenge operation

Let me search for information about triggering scavenge and GC APIs.

GitHub code search
Searched for ‘RequestGarbageCollectionForTesting’ in v8/v8

GitHub code search
Searched for ‘PerformScavenge’ in v8/v8

GitHub code search
Searched for ‘CollectGarbage SCAVENGE’ in v8/v8

GitHub file content
Retrieved content of include/v8-isolate.h#L461-L475, include/v8-isolate.h#L1320-L1330 from v8/v8@main

GitHub code search
Searched for ‘LowMemoryNotification RequestInterrupt’ in v8/v8

Yes! ScavengeObject can be triggered programmatically through V8’s public APIs, though you cannot call it directly. Here are the ways:

1. For Testing - RequestGarbageCollectionForTesting()

The most direct way to trigger a scavenge (which calls ScavengeObject internally):

v8::Isolate* isolate = ...;
isolate->RequestGarbageCollectionForTesting(
    v8::Isolate::kMinorGarbageCollection  // This triggers scavenge
);

From include/v8-isolate.h#L1320-L1327:

• kMinorGarbageCollection - Triggers a scavenge of the young generation
• kFullGarbageCollection - Triggers a full mark-compact collection

Warning: This API is only for testing and requires --expose_gc flag. It has a strong negative performance impact.

2. Production Use - Memory Pressure APIs

For production code, use these instead:

// Signal low memory condition - may trigger GC including scavenge
isolate->LowMemoryNotification();

// Or trigger GC via interrupt
isolate->RequestInterrupt([](v8::Isolate* isolate, void*) {
    isolate->RequestGarbageCollectionForTesting(
        v8::Isolate::kMinorGarbageCollection);
}, nullptr);

3. JavaScript APIs

If --expose-gc flag is enabled:

// Triggers minor GC (scavenge)
gc({type: 'minor'});

// or just
gc();  // Can trigger scavenge when appropriate

Call Chain

When you trigger a minor GC, the chain is:

RequestGarbageCollectionForTesting() 
→ Heap::CollectGarbage(NEW_SPACE) 
→ Heap::Scavenge() 
→ ScavengerCollector::CollectGarbage() 
→ ScavengeObject()

So yes, you can trigger it, but ScavengeObject itself is an internal implementation detail that happens during young generation garbage collection.