dotnet/BenchmarkDotNet · error · InvalidOperationException

One overhead iteration stop event is missing, unable to…

Error message

One overhead iteration stop event is missing, unable to calculate stats

What it means

CalculateMetrics requires overhead (jitting/allocations floor) iterations to come in start/stop pairs. It throws when overheadTimestamps.Count is odd, meaning an OverheadActualStart was seen without its matching OverheadActualStop, so an iteration boundary cannot be reconstructed.

Solutions

Increase ETW session buffer sizes / use a real-time session with larger buffers to avoid dropped events.
Ensure the benchmark run completes without exceptions so all Start/Stop pairs are emitted.
Re-run the benchmark; transient event loss often does not recur.

Defensive patterns

Strategy: retry

Try / catch

try { /* run + parse trace */ }
catch (InvalidOperationException ex) when (ex.Message.Contains("overhead iteration stop event is missing"))
{
    log.Warning("ETW dropped an overhead stop event; increasing buffers and rerunning.");
    // increase ETW buffer sizes and rerun
}

Prevention

Use sufficiently large ETW session buffers to avoid dropped events.
Ensure the benchmark run completes without exceptions so all Start/Stop pairs fire.
Capture the .etl over the full run window.

When it happens

Trigger: The ETW trace is missing an OverheadActualStop (or Start) event. Caused by event loss in the ETW session, the benchmarked process crashing/exiting mid-iteration, or the trace being captured over only part of the run.

Common situations: Capturing an .etl with insufficient buffer size (events dropped); the process under test was killed; trace started after the first overhead iteration or stopped before the last; running the PMC diagnoser on a benchmark that threw an exception partway through.

Related errors

AI-assisted analysis of dotnet/BenchmarkDotNet@b515068b61 (2026-08-13). Data as JSON: /api/errors/76c6c5db5b2a9046. Report an issue: GitHub.

Appendix: source

Thrown at src/BenchmarkDotNet.Diagnostics.Windows/Tracing/TraceLogParser.cs:124

            }
            else if (iterationMode == IterationMode.Workload)
            {
                if (!totalOperationsPerIteration.HasValue)
                    totalOperationsPerIteration = totalOperations;
                else if (totalOperationsPerIteration.Value != totalOperations)
                    throw new InvalidOperationException($"TotalOperations count can't change during the benchmark run! Invalid trace!");

                workloadTimestamps.Add(timeStamp);
            }
        }

        public void HandleNewSample(double timeStamp, ulong instructionPointer, int profileSourceId)
            => samples.Add((timeStamp, instructionPointer, profileSourceId));

        public IEnumerable<Metric> CalculateMetrics(Dictionary<int, int> profileSourceIdToInterval, PreciseMachineCounter[] counters)
        {
            if (overheadTimestamps.Count % 2 != 0)
                throw new InvalidOperationException("One overhead iteration stop event is missing, unable to calculate stats");
            if (workloadTimestamps.Count % 2 != 0)
                throw new InvalidOperationException("One workload iteration stop event is missing, unable to calculate stats");
            if (!totalOperationsPerIteration.HasValue)
                throw new InvalidOperationException("TotalOperations is missing, unable to calculate stats");

            var overheadIterations = CreateIterationData(overheadTimestamps);
            var workloadIterations = CreateIterationData(workloadTimestamps);

            SumCountersPerIterations(profileSourceIdToInterval, workloadIterations, overheadIterations, counters);

            var workloadTotalPerCounter = Sum(workloadIterations);
            var overheadTotalPerCounter = Sum(overheadIterations);

            return workloadTotalPerCounter.Select(perCounter =>
            {
                var pmc = counters.Single(counter => counter.ProfileSourceId == perCounter.Key);

                overheadTotalPerCounter.TryGetValue(perCounter.Key, out var overhead);

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