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Possibly use a fast path for accessing scope stacks #10071

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Possibly use a fast path for accessing scope stacks #10071

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37 changes: 37 additions & 0 deletions dd-trace-core/src/main/java/datadog/trace/core/scopemanager/ContinuableScopeManager.java
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Original file line number Diff line number Diff line change
Expand Up @@ -391,6 +391,8 @@ public Context swap(Context context) {

static final class ScopeStackThreadLocal extends ThreadLocal<ScopeStack> {

private final ScopeStack[] fastStacks = new ScopeStack[2048]; // This should be tuned
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@mcculls mcculls Dec 3, 2025

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Something to consider from https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html#getId--:

When a thread is terminated, this thread ID may be reused

What if an instrumentation left one or more unclosed scopes on a scope stack and the owning thread died and its id was reused? Or what if a thread dies during traced work before the scope stack could be cleaned up?


private final ProfilingContextIntegration profilingContextIntegration;

ScopeStackThreadLocal(ProfilingContextIntegration profilingContextIntegration) {
Expand All @@ -401,6 +403,41 @@ static final class ScopeStackThreadLocal extends ThreadLocal<ScopeStack> {
protected ScopeStack initialValue() {
return new ScopeStack(profilingContextIntegration);
}

@Override
public ScopeStack get() {
final long id = Thread.currentThread().getId();
if (id >= 0 && id < this.fastStacks.length) {
ScopeStack ret = fastStacks[(int) id];
if (ret == null) {
ret = initialValue();
fastStacks[(int) id] = ret;
}
return ret;
} else {
return super.get();
}
}

@Override
public void set(ScopeStack value) {
final long id = Thread.currentThread().getId();
if (id >= 0 && id < this.fastStacks.length) {
fastStacks[(int) id] = value;
} else {
super.set(value);
}
}

@Override
public void remove() {
final long id = Thread.currentThread().getId();
if (id >= 0 && id < this.fastStacks.length) {
fastStacks[(int) id] = null;
} else {
super.remove();
}
}
}

private void scheduleRootIterationScopeCleanup(ScopeStack scopeStack, ContinuableScope scope) {
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