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Simplify moving average calculator #1350

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Merged
merged 3 commits into from
Apr 8, 2021
Merged

Simplify moving average calculator #1350

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ac487ee
Simplify moving average calculator
andrewlock Apr 7, 2021
0024775
Update src/Datadog.Trace/Agent/MovingAverageKeepRateCalculator.cs
andrewlock Apr 7, 2021
af358a6
Apply Kevin-level optimisations
andrewlock Apr 7, 2021
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53 changes: 18 additions & 35 deletions src/Datadog.Trace/Agent/MovingAverageKeepRateCalculator.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,9 @@ internal class MovingAverageKeepRateCalculator : IKeepRateCalculator
private readonly TaskCompletionSource<bool> _processExit = new TaskCompletionSource<bool>();

private int _index = 0;
private long _totals = 0;
private ulong _sumDrops = 0;
private ulong _sumCreated = 0;
private double _keepRate = 0;

private long _latestDrops = 0;
private long _latestKeeps = 0;
Expand Down Expand Up @@ -84,14 +86,8 @@ public void IncrementDrops(int count)
/// </summary>
public double GetKeepRate()
{
var totals = Interlocked.Read(ref _totals);
UnpackBits(totals, out var totalDropped, out var totalCreated);
if (totalCreated == 0)
{
return 0;
}

return 1 - ((double)totalDropped / totalCreated);
// Essentially Interlock.Read(double)
return Volatile.Read(ref _keepRate);
}

/// <summary>
Expand All @@ -111,21 +107,23 @@ internal void UpdateBucket()
var previousDropped = _dropped[index];
var previousCreated = _created[index];

var latestDropped = Math.Min(Interlocked.Exchange(ref _latestDrops, 0), uint.MaxValue);
var latestCreated = Math.Min(Interlocked.Exchange(ref _latestKeeps, 0) + latestDropped, uint.MaxValue);
// Cap at uint.MaxValue events. Very unlikely to reach this value!
var latestDropped = (uint)Math.Min(Interlocked.Exchange(ref _latestDrops, 0), uint.MaxValue);
var latestCreated = (uint)Math.Min(Interlocked.Exchange(ref _latestKeeps, 0) + latestDropped, uint.MaxValue);

var totals = _totals;
UnpackBits(totals, out var previousTotalDropped, out var previousTotalCreated);
var newSumDropped = _sumDrops - previousDropped + latestDropped;
var newSumCreated = _sumCreated - previousCreated + latestCreated;

var newTotalDropped = (uint)Math.Min(((long)previousTotalDropped) - previousDropped + latestDropped, uint.MaxValue);
var newTotalCreated = (uint)Math.Min(((long)previousTotalCreated) - previousCreated + latestCreated, uint.MaxValue);
var keepRate = newSumCreated == 0
? 0
: 1 - ((double)newSumDropped / newSumCreated);

// Pack the totals into a single value we can read and write atomically
var packedTotal = PackBits(newTotalDropped, newTotalCreated);
Interlocked.Exchange(ref _totals, packedTotal);
Volatile.Write(ref _keepRate, keepRate);

_dropped[index] = (uint)latestDropped;
_created[index] = (uint)latestCreated;
_sumDrops = newSumDropped;
_sumCreated = newSumCreated;
_dropped[index] = latestDropped;
_created[index] = latestCreated;
_index = (index + 1) % _windowSize;
}

Expand Down Expand Up @@ -156,20 +154,5 @@ await Task.WhenAny(
#endif
}
}

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private long PackBits(uint hits, uint total)
{
long result = hits;
result = result << 32;
return result | (long)total;
}

[MethodImpl(MethodImplOptions.AggressiveInlining)]
private void UnpackBits(long bits, out uint hits, out uint total)
{
hits = (uint)(bits >> 32);
total = (uint)(bits & 0xffffffffL);
}
}
}