llvm/tools/llvm-exegesis/lib/LatencyBenchmarkRunner.cpp

   1 //===-- LatencyBenchmarkRunner.cpp ------------------------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "LatencyBenchmarkRunner.h"
  10
  11 #include "BenchmarkRunner.h"
  12 #include "Target.h"
  13 #include "llvm/ADT/Twine.h"
  14 #include "llvm/Support/Error.h"
  15 #include <algorithm>
  16 #include <cmath>
  17
  18 namespace llvm {
  19 namespace exegesis {
  20
  21 LatencyBenchmarkRunner::LatencyBenchmarkRunner(
  22     const LLVMState &State, Benchmark::ModeE Mode,
  23     BenchmarkPhaseSelectorE BenchmarkPhaseSelector,
  24     Benchmark::ResultAggregationModeE ResultAgg, ExecutionModeE ExecutionMode)
  25     : BenchmarkRunner(State, Mode, BenchmarkPhaseSelector, ExecutionMode) {
  26   assert((Mode == Benchmark::Latency || Mode == Benchmark::InverseThroughput) &&
  27          "invalid mode");
  28   ResultAggMode = ResultAgg;
  29 }
  30
  31 LatencyBenchmarkRunner::~LatencyBenchmarkRunner() = default;
  32
  33 static double computeVariance(const llvm::SmallVector<int64_t, 4> &Values) {
  34   if (Values.empty())
  35     return 0.0;
  36   double Sum = std::accumulate(Values.begin(), Values.end(), 0.0);
  37
  38   const double Mean = Sum / Values.size();
  39   double Ret = 0;
  40   for (const auto &V : Values) {
  41     double Delta = V - Mean;
  42     Ret += Delta * Delta;
  43   }
  44   return Ret / Values.size();
  45 }
  46
  47 static int64_t findMin(const llvm::SmallVector<int64_t, 4> &Values) {
  48   if (Values.empty())
  49     return 0;
  50   return *std::min_element(Values.begin(), Values.end());
  51 }
  52
  53 static int64_t findMax(const llvm::SmallVector<int64_t, 4> &Values) {
  54   if (Values.empty())
  55     return 0;
  56   return *std::max_element(Values.begin(), Values.end());
  57 }
  58
  59 static int64_t findMean(const llvm::SmallVector<int64_t, 4> &Values) {
  60   if (Values.empty())
  61     return 0;
  62   return std::accumulate(Values.begin(), Values.end(), 0.0) /
  63          static_cast<double>(Values.size());
  64 }
  65
  66 Expected<std::vector<BenchmarkMeasure>> LatencyBenchmarkRunner::runMeasurements(
  67     const FunctionExecutor &Executor) const {
  68   // Cycle measurements include some overhead from the kernel. Repeat the
  69   // measure several times and return the aggregated value, as specified by
  70   // ResultAggMode.
  71   constexpr const int NumMeasurements = 30;
  72   llvm::SmallVector<int64_t, 4> AccumulatedValues;
  73   double MinVariance = std::numeric_limits<double>::infinity();
  74   const char *CounterName = State.getPfmCounters().CycleCounter;
  75   // Values count for each run.
  76   int ValuesCount = 0;
  77   for (size_t I = 0; I < NumMeasurements; ++I) {
  78     auto ExpectedCounterValues = Executor.runAndSample(CounterName);
  79     if (!ExpectedCounterValues)
  80       return ExpectedCounterValues.takeError();
  81     ValuesCount = ExpectedCounterValues.get().size();
  82     if (ValuesCount == 1)
  83       AccumulatedValues.push_back(ExpectedCounterValues.get()[0]);
  84     else {
  85       // We'll keep the reading with lowest variance (ie., most stable)
  86       double Variance = computeVariance(*ExpectedCounterValues);
  87       if (MinVariance > Variance) {
  88         AccumulatedValues = std::move(ExpectedCounterValues.get());
  89         MinVariance = Variance;
  90       }
  91     }
  92   }
  93
  94   std::string ModeName;
  95   switch (Mode) {
  96   case Benchmark::Latency:
  97     ModeName = "latency";
  98     break;
  99   case Benchmark::InverseThroughput:
 100     ModeName = "inverse_throughput";
 101     break;
 102   default:
 103     break;
 104   }
 105
 106   switch (ResultAggMode) {
 107   case Benchmark::MinVariance: {
 108     if (ValuesCount == 1)
 109       llvm::errs() << "Each sample only has one value. result-aggregation-mode "
 110                       "of min-variance is probably non-sensical\n";
 111     std::vector<BenchmarkMeasure> Result;
 112     Result.reserve(AccumulatedValues.size());
 113     for (const int64_t Value : AccumulatedValues)
 114       Result.push_back(BenchmarkMeasure::Create(ModeName, Value));
 115     return std::move(Result);
 116   }
 117   case Benchmark::Min: {
 118     std::vector<BenchmarkMeasure> Result;
 119     Result.push_back(
 120         BenchmarkMeasure::Create(ModeName, findMin(AccumulatedValues)));
 121     return std::move(Result);
 122   }
 123   case Benchmark::Max: {
 124     std::vector<BenchmarkMeasure> Result;
 125     Result.push_back(
 126         BenchmarkMeasure::Create(ModeName, findMax(AccumulatedValues)));
 127     return std::move(Result);
 128   }
 129   case Benchmark::Mean: {
 130     std::vector<BenchmarkMeasure> Result;
 131     Result.push_back(
 132         BenchmarkMeasure::Create(ModeName, findMean(AccumulatedValues)));
 133     return std::move(Result);
 134   }
 135   }
 136   return llvm::make_error<Failure>(llvm::Twine("Unexpected benchmark mode(")
 137                                        .concat(std::to_string(Mode))
 138                                        .concat(" and unexpected ResultAggMode ")
 139                                        .concat(std::to_string(ResultAggMode)));
 140 }
 141
 142 } // namespace exegesis
 143 } // namespace llvm