TTreeProcessorMP.cxx

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/* @(#)root/multiproc:$Id$ */
// Author: Enrico Guiraud July 2015
// Modified: G Ganis Jan 2017
 
/*************************************************************************
 * Copyright (C) 1995-2000, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
#include "TEnv.h"
#include "ROOT/TTreeProcessorMP.hxx"
#include "TMPWorkerTree.h"
 
#include <thread>
static const std::thread::id MAIN_THREAD_ID = std::this_thread::get_id();
 
//////////////////////////////////////////////////////////////////////////
///
/// \class ROOT::TTreeProcessorMP
/// \ingroup Parallelism
/// \brief This class provides an interface to process a TTree dataset
///        in parallel with multi-process technology
///
/// ###ROOT::TTreeProcessorMP::Process
/// The possible usages of the Process method are the following:\n
/// * `Process(<dataset>, F func, const std::string& treeName, ULong64_t nToProcess)`:
///     func is executed nToProcess times with argument a TTreeReader&, initialized for
///     the TTree with name treeName, from the dataset `<dataset>`. The dataset can be
///     expressed as:
/// \code{.cpp}
///                     const std::string& fileName  -> single file name
///                     const std::vector<std::string>& fileNames -> vector of file names
///                     TFileCollection& files       -> collection of TFileInfo objects
///                     TChain& files                -> TChain with the file paths
///                     TTree& tree                  -> Reference to an existing TTree object
/// \endcode
/// For legacy, the following signature is also supported:
/// * `Process(<dataset>, TSelector& selector, const std::string& treeName, ULong64_t nToProcess)`:
///   where selector is a TSelector derived class describing the analysis and the other arguments
///   have the same meaning as above.
///
/// For either set of signatures, the processing function is executed as many times as
/// needed by a pool of fNWorkers workers; the number of workers can be passed to the constructor
/// or set via SetNWorkers. It defaults to the number of cores.\n
/// A collection containing the result of each execution is returned.\n
/// **Note:** the user is responsible for the deletion of any object that might
/// be created upon execution of func, returned objects included: ROOT::TTreeProcessorMP never
/// deletes what it returns, it simply forgets it.\n
/// **Note:** that the usage of ROOT::TTreeProcessorMP::Process is indicated only when the task to be
/// executed takes more than a few seconds, otherwise the overhead introduced
/// by Process will outrun the benefits of parallel execution on most machines.
///
/// \param func
/// \parblock
/// a lambda expression, an std::function, a loaded macro, a
/// functor class or a function that takes zero arguments (for the first signature)
/// or one (for the second signature).
/// \endparblock
/// \param args
/// \parblock
/// a standard container (vector, list, deque), an initializer list
/// or a pointer to a TCollection (TList*, TObjArray*, ...).
/// \endparblock
/// **Note:** the version of ROOT::TTreeProcessorMP::Process that takes a TFileCollection* as argument incurs
/// in the overhead of copying data from the TCollection to an STL container. Only
/// use it when absolutely necessary.\n
/// **Note:** in cases where the function to be executed takes more than
/// zero/one argument but all are fixed except zero/one, the function can be wrapped
/// in a lambda or via std::bind to give it the right signature.\n
/// **Note:** the user should take care of initializing random seeds differently in each
/// process (e.g. using the process id in the seed). Otherwise several parallel executions
/// might generate the same sequence of pseudo-random numbers.
///
/// #### Return value:
/// Methods taking 'F func' return the return type of F.
/// Methods taking a TSelector return a 'TList *' with the selector output list; the output list
/// content is owned by the caller.
///
/// #### Examples:
///
/// See tutorials/multicore/mp102_readNtuplesFillHistosAndFit.C and tutorials/multicore/mp103__processSelector.C .
///
//////////////////////////////////////////////////////////////////////////
 
namespace ROOT {
//////////////////////////////////////////////////////////////////////////
/// Class constructor.
/// nWorkers is the number of times this ROOT session will be forked, i.e.
/// the number of workers that will be spawned.
TTreeProcessorMP::TTreeProcessorMP(UInt_t nWorkers) : TMPClient(nWorkers)
{
   Reset();
}
 
//////////////////////////////////////////////////////////////////////////
/// TSelector-based tree processing: memory resident tree
TList *TTreeProcessorMP::Process(TTree &tree, TSelector &selector, TEntryList &entries, ULong64_t nToProcess,
                                 ULong64_t jFirst)
{
 
   // Warn for yet unimplemented functionality
   if (jFirst > 0) {
      Warning("Process", "support for generic 'first entry' (jFirst > 0) not implemented yet - ignoring");
      jFirst = 0;
   }
 
   // Warn about forking of already threaded program
   if (std::this_thread::get_id() != MAIN_THREAD_ID) {
      Warning("Process",
              "multiprocessing from an already multi-threaded program "
              "(first thread '%zu' vs current thread id '%zu') might lead to inefficient forking",
              std::hash<std::thread::id>{}(MAIN_THREAD_ID), std::hash<std::thread::id>{}(std::this_thread::get_id()));
   }
 
   //prepare environment
   Reset();
   UInt_t nWorkers = GetNWorkers();
   selector.Begin(nullptr);
 
   // Check the entry list
   TEntryList *elist = (entries.IsValid()) ? &entries : nullptr;
   //fork
   TMPWorkerTreeSel worker(selector, &tree, elist, nWorkers, nToProcess / nWorkers, jFirst);
   bool ok = Fork(worker);
   if(!ok) {
      Error("TTreeProcessorMP::Process", "[E][C] Could not fork. Aborting operation");
      return nullptr;
   }
 
   //divide entries equally between workers
   fTaskType = ETask::kProcByRange;
 
   //tell workers to start processing entries
   fNToProcess = nWorkers; //this is the total number of ranges that will be processed by all workers cumulatively
   std::vector<UInt_t> args(nWorkers);
   std::iota(args.begin(), args.end(), 0);
   fNProcessed = Broadcast(MPCode::kProcTree, args);
   if (fNProcessed < nWorkers)
      Error("TTreeProcessorMP::Process", "[E][C] There was an error while sending tasks to workers."
                                   " Some entries might not be processed.");
 
   //collect results, distribute new tasks
   std::vector<TObject*> outLists;
   Collect(outLists);
 
   // The first element must be a TList instead of a TSelector List, to avoid duplicate problems with merging
   FixLists(outLists);
 
   PoolUtils::ReduceObjects<TObject *> redfunc;
   auto outList = static_cast<TList*>(redfunc(outLists));
 
   // Import the resulting list in the selector
   selector.ImportOutput(outList);
   // outList is empty after this: just delete it
   delete outList;
 
   // Finalize the selector tasks
   selector.Terminate();
 
   //clean-up and return
   ReapWorkers();
   fTaskType = ETask::kNoTask;
   return selector.GetOutputList();
}
 
//////////////////////////////////////////////////////////////////////////
/// TSelector-based tree processing: dataset as a vector of files
TList *TTreeProcessorMP::Process(const std::vector<std::string> &fileNames, TSelector &selector, TEntryList &entries,
                                 const std::string &treeName, ULong64_t nToProcess, ULong64_t jFirst)
{
 
   // Warn for yet unimplemented functionality
   if (jFirst > 0) {
      Warning("Process", "support for generic 'first entry' (jFirst > 0) not implemented yet - ignoring");
      jFirst = 0;
   }
 
   //prepare environment
   Reset();
   UInt_t nWorkers = GetNWorkers();
   selector.Begin(nullptr);
 
   // Check the entry list
   TEntryList *elist = (entries.IsValid()) ? &entries : nullptr;
   //fork
   TMPWorkerTreeSel worker(selector, fileNames, elist, treeName, nWorkers, nToProcess, jFirst);
   bool ok = Fork(worker);
   if (!ok) {
      Error("TTreeProcessorMP::Process", "[E][C] Could not fork. Aborting operation");
      return nullptr;
   }
 
   Int_t procByFile = gEnv->GetValue("MultiProc.TestProcByFile", 0);
 
   if (procByFile) {
      if (fileNames.size() < nWorkers) {
         // TTree entry granularity: for each file, we divide entries equally between workers
         fTaskType = ETask::kProcByRange;
         // Tell workers to start processing entries
         fNToProcess = nWorkers*fileNames.size(); //this is the total number of ranges that will be processed by all workers cumulatively
         std::vector<UInt_t> args(nWorkers);
         std::iota(args.begin(), args.end(), 0);
         fNProcessed = Broadcast(MPCode::kProcRange, args);
         if (fNProcessed < nWorkers)
            Error("TTreeProcessorMP::Process", "[E][C] There was an error while sending tasks to workers."
                                         " Some entries might not be processed");
      } else {
         // File granularity: each worker processes one whole file as a single task
         fTaskType = ETask::kProcByFile;
         fNToProcess = fileNames.size();
         std::vector<UInt_t> args(nWorkers);
         std::iota(args.begin(), args.end(), 0);
         fNProcessed = Broadcast(MPCode::kProcFile, args);
         if (fNProcessed < nWorkers)
            Error("TTreeProcessorMP::Process", "[E][C] There was an error while sending tasks to workers."
                                         " Some entries might not be processed.");
      }
   } else {
      // TTree entry granularity: for each file, we divide entries equally between workers
      fTaskType = ETask::kProcByRange;
      // Tell workers to start processing entries
      fNToProcess = nWorkers*fileNames.size(); //this is the total number of ranges that will be processed by all workers cumulatively
      std::vector<UInt_t> args(nWorkers);
      std::iota(args.begin(), args.end(), 0);
      fNProcessed = Broadcast(MPCode::kProcRange, args);
      if (fNProcessed < nWorkers)
         Error("TTreeProcessorMP::Process", "[E][C] There was an error while sending tasks to workers."
                                      " Some entries might not be processed.");
   }
 
   // collect results, distribute new tasks
   std::vector<TObject*> outLists;
   Collect(outLists);
 
   // The first element must be a TList instead of a TSelector List, to avoid duplicate problems with merging
   FixLists(outLists);
 
   PoolUtils::ReduceObjects<TObject *> redfunc;
   auto outList = static_cast<TList*>(redfunc(outLists));
 
   // Import the resulting list in the selector
   selector.ImportOutput(outList);
   // outList is empty after this: just delete it
   delete outList;
 
   // Finalize the selector tasks
   selector.Terminate();
 
   //clean-up and return
   ReapWorkers();
   fTaskType = ETask::kNoTask;
 
   return selector.GetOutputList();
}
 
//////////////////////////////////////////////////////////////////////////
/// TSelector-based tree processing: dataset as a TFileCollection
TList *TTreeProcessorMP::Process(TFileCollection &files, TSelector &selector, TEntryList &entries,
                                 const std::string &treeName, ULong64_t nToProcess, ULong64_t firstEntry)
{
   std::vector<std::string> fileNames(files.GetNFiles());
   UInt_t count = 0;
   for(auto f : *static_cast<THashList*>(files.GetList()))
      fileNames[count++] = static_cast<TFileInfo*>(f)->GetCurrentUrl()->GetUrl();
 
   TList *rl = Process(fileNames, selector, entries, treeName, nToProcess, firstEntry);
   return rl;
}
 
//////////////////////////////////////////////////////////////////////////
/// TSelector-based tree processing: dataset as a TChain
TList *TTreeProcessorMP::Process(TChain &files, TSelector &selector, TEntryList &entries, const std::string &treeName,
                                 ULong64_t nToProcess, ULong64_t firstEntry)
{
   TObjArray* filelist = files.GetListOfFiles();
   std::vector<std::string> fileNames(filelist->GetEntries());
   UInt_t count = 0;
   for(auto f : *filelist)
      fileNames[count++] = f->GetTitle();
 
   return Process(fileNames, selector, entries, treeName, nToProcess, firstEntry);
}
 
//////////////////////////////////////////////////////////////////////////
/// TSelector-based tree processing: dataset as a single file
TList *TTreeProcessorMP::Process(const std::string &fileName, TSelector &selector, TEntryList &entries,
                                 const std::string &treeName, ULong64_t nToProcess, ULong64_t firstEntry)
{
   std::vector<std::string> singleFileName(1, fileName);
   return Process(singleFileName, selector, entries, treeName, nToProcess, firstEntry);
}
 
///
/// No TEntryList versions of selector processor
///
 
TList *TTreeProcessorMP::Process(const std::vector<std::string> &fileNames, TSelector &selector,
                                 const std::string &treeName, ULong64_t nToProcess, ULong64_t jFirst)
{
   TEntryList noelist;
   return Process(fileNames, selector, noelist, treeName, nToProcess, jFirst);
}
 
TList *TTreeProcessorMP::Process(const std::string &fileName, TSelector &selector, const std::string &treeName,
                                 ULong64_t nToProcess, ULong64_t jFirst)
{
   TEntryList noelist;
   return Process(fileName, selector, noelist, treeName, nToProcess, jFirst);
}
 
TList *TTreeProcessorMP::Process(TFileCollection &files, TSelector &selector, const std::string &treeName,
                                 ULong64_t nToProcess, ULong64_t jFirst)
{
   TEntryList noelist;
   return Process(files, selector, noelist, treeName, nToProcess, jFirst);
}
 
TList *TTreeProcessorMP::Process(TChain &files, TSelector &selector, const std::string &treeName, ULong64_t nToProcess,
                                 ULong64_t jFirst)
{
   TEntryList noelist;
   return Process(files, selector, noelist, treeName, nToProcess, jFirst);
}
 
TList *TTreeProcessorMP::Process(TTree &tree, TSelector &selector, ULong64_t nToProcess, ULong64_t jFirst)
{
   TEntryList noelist;
   return Process(tree, selector, noelist, nToProcess, jFirst);
}
 
/// Fix list of lists before merging (to avoid errors about duplicated objects)
void TTreeProcessorMP::FixLists(std::vector<TObject*> &lists) {
 
   // The first element must be a TList instead of a TSelector List, to avoid duplicate problems with merging
   TList *firstlist = new TList;
   TList *oldlist = (TList *) lists[0];
   TIter nxo(oldlist);
   TObject *o = nullptr;
   while ((o = nxo())) { firstlist->Add(o); }
   oldlist->SetOwner(false);
   lists.erase(lists.begin());
   lists.insert(lists.begin(), firstlist);
   delete oldlist;
}
 
//////////////////////////////////////////////////////////////////////////
/// Reset TTreeProcessorMP's state.
void TTreeProcessorMP::Reset()
{
   fNProcessed = 0;
   fNToProcess = 0;
   fTaskType = ETask::kNoTask;
}
 
//////////////////////////////////////////////////////////////////////////
/// Reply to a worker who is idle.
/// If still events to process, tell the worker. Otherwise
/// ask for a result
void TTreeProcessorMP::ReplyToIdle(TSocket *s)
{
   if (fNProcessed < fNToProcess) {
      //we are executing a "greedy worker" task
      if (fTaskType == ETask::kProcByRange)
         MPSend(s, MPCode::kProcRange, fNProcessed);
      else if (fTaskType == ETask::kProcByFile)
         MPSend(s, MPCode::kProcFile, fNProcessed);
      ++fNProcessed;
   } else
      MPSend(s, MPCode::kSendResult);
}
 
} // namespace ROOT