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Make ThreadSafeMapBenchmark lookup index per-thread (remove shared-counter contention) #11988

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Make ThreadSafeMapBenchmark lookup index per-thread (remove shared-counter contention) #11988

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17 changes: 12 additions & 5 deletions internal-api/src/jmh/java/datadog/trace/util/ThreadSafeMapBenchmark.java
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,8 @@
import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.Fork;
import org.openjdk.jmh.annotations.Measurement;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.State;
import org.openjdk.jmh.annotations.Threads;
import org.openjdk.jmh.annotations.Warmup;

Expand Down Expand Up @@ -65,6 +67,7 @@
@Warmup(iterations = 2)
@Measurement(iterations = 3)
@Threads(8)
@State(Scope.Thread)
public class ThreadSafeMapBenchmark {
static final String[] INSERTION_KEYS = {
"foo", "bar", "baz", "quux", "foobar", "foobaz", "key0", "key1", "key2", "key3"
Expand All @@ -84,16 +87,20 @@ static <T> T init(Supplier<T> supplier) {
return supplier.get();
}

static int sharedLookupIndex = 0;
// Per-thread (@State(Scope.Thread)) so cycling the lookup key doesn't contend a shared counter.
// The maps below stay static/shared (the point — concurrent reads of one map); only the index is
// per-thread. A shared counter's cache-line ping-pong would otherwise floor the fastest reads
// (e.g. FlatHashtable's lock-free probe), hiding exactly the differences this benchmark compares.
int lookupIndex = 0;

static String nextLookupKey() {
String nextLookupKey() {
return nextLookupKey(EQUAL_KEYS);
}

static String nextLookupKey(String[] keys) {
int localIndex = ++sharedLookupIndex;
String nextLookupKey(String[] keys) {
int localIndex = ++lookupIndex;
if (localIndex >= keys.length) {
sharedLookupIndex = localIndex = 0;
lookupIndex = localIndex = 0;
}
return keys[localIndex];
}
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