[RF] Avoid using static std::vector<double> buffer in RooBatchCompute

In the RooBatchCompute CPU library, all scalar inputs have to be copied
n times into a buffer that is as long as the SIMD registers, to allow
for vectorization in all cases.

To avoid frequent memory allocations, this buffer was made a `static`
variable in the original implementation of the batchcompute library,
which of course made it non-threadsafe.

This is now hitting us, because RooFit needs to be able to do multiple
fits concurrently. This is a requirement for CMSSW, and a blocker for
ROOT master adoption in CMSSW since the new CPU backend is the default:
cms-sw/cmsdist#9034

This commit fixes the concurrency problem by doing the buffering in the
DataMaps that are used in the `RooFit::Evaluator`. Like this, multiple
computation graphs can be concurrently evaluated.

It was tested with the ATLAS benchmarks in `rootbench` that the fitting
performance remains the same.

roofit/batchcompute/res/RooBatchCompute.h

@@ -46,6 +46,8 @@ typedef std::span<double> ArgSpan;
 typedef double *__restrict RestrictArr;
 typedef const double *__restrict InputArr;
 
+constexpr std::size_t bufferSize = 64;
+
 void init();
 
 /// Minimal configuration struct to steer the evaluation of a single node with

roofit/batchcompute/src/Batches.h

@@ -29,8 +29,6 @@ so that they can contain data for every kind of compute function.
 
 namespace RooBatchCompute {
 
-constexpr std::size_t bufferSize = 64;
-
 namespace RF_ARCH {
 
 class Batch {

roofit/batchcompute/src/RooBatchCompute.cxx

@@ -50,20 +50,10 @@ void fillBatches(Batches &batches, RestrictArr output, size_t nEvents, std::size
    batches._output = output;
 }
 
-void fillArrays(std::vector<Batch> &arrays, VarSpan vars, double *buffer, std::size_t nEvents)
+void fillArrays(std::span<Batch> arrays, VarSpan vars, std::size_t nEvents)
 {
-
-   arrays.resize(vars.size());
-   for (size_t i = 0; i < vars.size(); i++) {
-      const std::span<const double> &span = vars[i];
-      if (!span.empty() && span.size() < nEvents) {
-         // In the scalar case, copy the value to each element of vector input
-         // buffer.
-         std::fill_n(&buffer[i * bufferSize], bufferSize, span.data()[0]);
-         arrays[i].set(&buffer[i * bufferSize], false);
-      } else {
-         arrays[i].set(span.data(), true);
-      }
+   for (std::size_t i = 0; i < vars.size(); i++) {
+      arrays[i].set(vars[i].data(), vars[i].empty() || vars[i].size() >= nEvents);
    }
 }
 
@@ -111,9 +101,6 @@ class RooBatchComputeClass : public RooBatchComputeInterface {
    void compute(Config const &, Computer computer, RestrictArr output, size_t nEvents, VarSpan vars,
                 ArgSpan extraArgs) override
    {
-      static std::vector<double> buffer;
-      buffer.resize(vars.size() * bufferSize);
-
       if (ROOT::IsImplicitMTEnabled()) {
          ROOT::Internal::TExecutor ex;
          std::size_t nThreads = ex.GetPoolSize();
@@ -127,9 +114,9 @@ class RooBatchComputeClass : public RooBatchComputeInterface {
             // Fill a std::vector<Batches> with the same object and with ~nEvents/nThreads
             // Then advance every object but the first to split the work between threads
             Batches batches;
-            std::vector<Batch> arrays;
+            std::vector<Batch> arrays(vars.size());
             fillBatches(batches, output, nEventsPerThread, vars.size(), extraArgs);
-            fillArrays(arrays, vars, buffer.data(), nEvents);
+            fillArrays(arrays, vars, nEvents);
             batches._arrays = arrays.data();
             batches.advance(batches.getNEvents() * idx);
 
@@ -159,9 +146,9 @@ class RooBatchComputeClass : public RooBatchComputeInterface {
          // Fill a std::vector<Batches> with the same object and with ~nEvents/nThreads
          // Then advance every object but the first to split the work between threads
          Batches batches;
-         std::vector<Batch> arrays;
+         std::vector<Batch> arrays(vars.size());
          fillBatches(batches, output, nEvents, vars.size(), extraArgs);
-         fillArrays(arrays, vars, buffer.data(), nEvents);
+         fillArrays(arrays, vars, nEvents);
          batches._arrays = arrays.data();
 
          std::size_t events = batches.getNEvents();

roofit/roofitcore/inc/RooFit/Detail/DataMap.h

@@ -82,10 +82,7 @@ namespace Detail {
 
 class DataMap {
 public:
-   auto size() const
-   {
-      return _dataMap.size();
-   }
+   auto size() const { return _dataMap.size(); }
    void resize(std::size_t n);
 
    inline void set(RooAbsArg const *arg, std::span<const double> const &span)
@@ -119,8 +116,14 @@ class DataMap {
 
    RooBatchCompute::Config config(RooAbsArg const *arg) const;
 
+   void enableVectorBuffers(bool enable) { _enableVectorBuffers = enable; }
+   void resetVectorBuffers() { _bufferIdx = 0; }
+
 private:
    std::vector<std::span<const double>> _dataMap;
+   bool _enableVectorBuffers = false;
+   std::vector<std::vector<double>> _buffers;
+   std::size_t _bufferIdx = 0;
    std::vector<RooBatchCompute::Config> _cfgs;
 };
 

roofit/roofitcore/src/RooFit/Detail/DataMap.cxx

@@ -16,17 +16,39 @@
 #include <RooBatchCompute.h>
 #include <RooRealVar.h>
 
+#include <algorithm>
+
 namespace RooFit {
 namespace Detail {
 
 std::span<const double> DataMap::at(RooAbsArg const *arg, RooAbsArg const * /*caller*/)
 {
+   std::span<const double> out;
+
    if (!arg->hasDataToken()) {
       auto var = static_cast<RooRealVar const *>(arg);
-      return {&var->_value, 1};
+      out = std::span<const double>{&var->_value, 1};
+   } else {
+      std::size_t idx = arg->dataToken();
+      out = _dataMap[idx];
    }
-   std::size_t idx = arg->dataToken();
-   return _dataMap[idx];
+
+   if (!_enableVectorBuffers || out.size() != 1) {
+      return out;
+   }
+
+   if (_bufferIdx == _buffers.size()) {
+      _buffers.emplace_back(RooBatchCompute::bufferSize);
+   }
+
+   double *buffer = _buffers[_bufferIdx].data();
+
+   std::fill_n(buffer, RooBatchCompute::bufferSize, out[0]);
+   out = std::span<const double>{buffer, 1};
+
+   ++_bufferIdx;
+
+   return out;
 }
 
 void DataMap::setConfig(RooAbsArg const *arg, RooBatchCompute::Config const &config)

roofit/roofitcore/src/RooFit/Evaluator.cxx

@@ -384,7 +384,12 @@ void Evaluator::computeCPUNode(const RooAbsArg *node, NodeInfo &info)
       buffer = info.buffer->cpuWritePtr();
    }
    _dataMapCPU.set(node, {buffer, nOut});
+   if (nOut > 1) {
+      _dataMapCPU.enableVectorBuffers(true);
+   }
    nodeAbsReal->computeBatch(buffer, nOut, _dataMapCPU);
+   _dataMapCPU.resetVectorBuffers();
+   _dataMapCPU.enableVectorBuffers(false);
 #ifdef ROOFIT_CUDA
    if (info.copyAfterEvaluation) {
       _dataMapCUDA.set(node, {info.buffer->gpuReadPtr(), nOut});