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Use CAS for safer counter collection #394

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Use CAS for safer counter collection #394

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15 changes: 15 additions & 0 deletions kamon-core/src/main/java/kamon/jsr166/LongAdder.java
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Expand Up @@ -151,6 +151,21 @@ public long sumThenReset() {
return sum;
}

public long sumAndReset() {
long sum = getAndSetBase(0L);
Cell[] as = cells;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null) {
sum += a.getAndSet(0L);
}
}
}
return sum;
}

/**
* Returns the String representation of the {@link #sum}.
* @return the String representation of the {@link #sum}
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11 changes: 11 additions & 0 deletions kamon-core/src/main/java/kamon/jsr166/Striped64.java
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Expand Up @@ -105,6 +105,10 @@ final boolean cas(long cmp, long val) {
return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
}

final long getAndSet(long val) {
return UNSAFE.getAndSetLong(this, valueOffset, val);
}

// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long valueOffset;
Expand Down Expand Up @@ -181,6 +185,13 @@ final boolean casBase(long cmp, long val) {
return UNSAFE.compareAndSwapLong(this, baseOffset, cmp, val);
}

/**
* CASes the base field.
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@dpsoft dpsoft Sep 28, 2016 •
edited

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@isaacl This code are using Java 8 Unsafe intrinsics (in this case lock xchg) and isn't supported in prior java versions.
Maybe we could check if support getAndSetObject in this way Unsafe.class.getMethod("getAndSetObject", Object.class, Long.TYPE,Object.class) and use this method, otherwise fallback to something like:

 final long getAndSetBase(long val) {
  long v;
     do {
          v = UNSAFE.getLongVolatile(this, baseOffset);
      } while (!UNSAFE.compareAndSwapLong(this, baseOffset, v, val));
  return v;
 }

or simply use the code above (CAS version, I do not know if the code compiles ;)) and the same for the final long getAndSet(long val)

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If getAndSetLong was added too recently I'd suggest adding the function as a static helper, as it's needed for both the base get and the cell get.

The code you wrote is identical to the actual impl.

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Yes, is a good idea!

*/
final long getAndSetBase(long val) {
return UNSAFE.getAndSetLong(this, baseOffset, val);
}

/**
* CASes the busy field from 0 to 1 to acquire lock.
*/
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2 changes: 1 addition & 1 deletion kamon-core/src/main/scala/kamon/metric/instrument/Counter.scala
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Expand Up @@ -48,7 +48,7 @@ class LongAdderCounter extends Counter {
counter.add(times)
}

def collect(context: CollectionContext): Counter.Snapshot = CounterSnapshot(counter.sumThenReset())
def collect(context: CollectionContext): Counter.Snapshot = CounterSnapshot(counter.sumAndReset())

def cleanup: Unit = {}
}
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