Fixups from code review.

test/MainEvent.cxx

@@ -131,9 +131,12 @@ int main(int argc, char **argv)
    Int_t branchStyle = 1; //new style by default
    if (split < 0) {branchStyle = 0; split = -1-split;}
 
+   ROOT::EnableImplicitMT(4);
+
    TFile *hfile;
    TTree *tree;
    Event *event = 0;
+   Event *event2 = 0;
 
    // Fill event, header and tracks with some random numbers
    //   Create a timer object to benchmark this loop
@@ -195,6 +198,7 @@ int main(int argc, char **argv)
       } else
          hfile = new TFile("Event.root","RECREATE","TTree benchmark ROOT file");
       hfile->SetCompressionLevel(comp);
+      //hfile->SetCompressionAlgorithm(2);
 
      // Create histogram to show write_time in function of time
      Float_t curtime = -0.5;
@@ -213,9 +217,16 @@ int main(int argc, char **argv)
       bufsize = 64000;
       if (split)  bufsize /= 4;
       event = new Event();           // By setting the value, we own the pointer and must delete it.
+      event2 = new Event();           // By setting the value, we own the pointer and must delete it.
       TTree::SetBranchStyle(branchStyle);
       TBranch *branch = tree->Branch("event", &event, bufsize,split);
+      TBranch *branch2 = tree->Branch("event2", &event, bufsize,split);
+      TBranch *branch3 = tree->Branch("event3", &event2, bufsize,split);
+      TBranch *branch4 = tree->Branch("event4", &event2, bufsize,split);
       branch->SetAutoDelete(kFALSE);
+      branch2->SetAutoDelete(kFALSE);
+      branch3->SetAutoDelete(kFALSE);
+      branch4->SetAutoDelete(kFALSE);
       if(split >= 0 && branchStyle) tree->BranchRef();
       Float_t ptmin = 1;
 
@@ -230,6 +241,7 @@ int main(int argc, char **argv)
          }
 
          event->Build(ev, arg5, ptmin);
+         event2->Build(ev, arg5, ptmin);
 
          if (write) nb += tree->Fill();  //fill the tree
 
@@ -242,7 +254,8 @@ int main(int argc, char **argv)
       }
    }
    // We own the event (since we set the branch address explicitly), we need to delete it.
-   delete event;  event = 0;
+   // delete event;  event = 0;
+   // delete event2;  event2 = 0;
 
    //  Stop timer and print results
    timer.Stop();

tree/tree/inc/TTree.h

@@ -163,11 +163,11 @@ class TTree : public TNamed, public TAttLine, public TAttFill, public TAttMarker
    TTree(const TTree& tt);              // not implemented
    TTree& operator=(const TTree& tt);   // not implemented
 
-#ifdef R__USE_IMT
+   // For simplicity, although fIMTFlush is always disabled in non-IMT builds, we don't #ifdef it out.
    mutable Bool_t fIMTFlush{false};               ///<! True if we are doing a multithreaded flush.
    mutable std::atomic<Long64_t> fIMTTotBytes;    ///<! Total bytes for the IMT flush baskets
    mutable std::atomic<Long64_t> fIMTZipBytes;    ///<! Zip bytes for the IMT flush baskets.
-#endif
+
    void             InitializeSortedBranches();
    void             SortBranchesByTime();
 

tree/tree/src/TBasket.cxx

@@ -932,6 +932,12 @@ Int_t TBasket::WriteBuffer()
       return -1;
    }
    fMotherDir = file; // fBranch->GetDirectory();
+
+   // This mutex prevents multiple TBasket::WriteBuffer invocations from interacting
+   // with the underlying TFile at once - TFile is assumed to *not* be thread-safe.
+   //
+   // The only parallelism we'd like to exploit (right now!) is the compression
+   // step - everything else should be serialized at the TFile level.
 #ifdef R__USE_IMT
    std::unique_lock<std::mutex> sentry(file->fWriteMutex);
 #endif  // R__USE_IMT
@@ -1000,7 +1006,9 @@ Int_t TBasket::WriteBuffer()
       for (Int_t i = 0; i < nbuffers; ++i) {
          if (i == nbuffers - 1) bufmax = fObjlen - nzip;
          else bufmax = kMAXZIPBUF;
-         //compress the buffer
+         // Compress the buffer.  Note that we allow multiple TBasket compressions to occur at once
+         // for a given TFile: that's because the compression buffer when we use IMT is no longer
+         // shared amongst several threads.
 #ifdef R__USE_IMT
          sentry.unlock();
 #endif  // R__USE_IMT

tree/tree/src/TTree.cxx

@@ -4792,6 +4792,8 @@ Int_t TTree::Fit(const char* funcname, const char* varexp, const char* selection
    return -1;
 }
 
+namespace {
+
 struct BoolRAIIToggle {
 
 Bool_t &m_val;
@@ -4801,6 +4803,7 @@ BoolRAIIToggle(Bool_t &val) : m_val(val) { m_val = true; }
 ~BoolRAIIToggle() { m_val = false; }
 };
 
+}
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Write to disk all the basket that have not yet been individually written.