More comment, release notes

README/ReleaseNotes/v608/index.md

@@ -32,9 +32,11 @@ The following people have contributed to this new version:
  Paul Russo, Fermilab,\
  Enric Tejedor Saavedra, CERN/SFT,\
  Liza Sakellari, CERN/SFT,\
+ Alex Saperstein, ANL,\
  Manuel Tobias Schiller, CERN/LHCb,\
  David Smith, CERN/IT,\
  Matevz Tadel, UCSD/CMS, Eve,\
+ Peter van Gemmeren, ANL, ATLAS,\
  Vassil Vassilev, Fermilab/CMS,\
  Wouter Verkerke, NIKHEF/Atlas, RooFit
 
@@ -128,6 +130,7 @@ Add a new mode for `TClass::SetCanSplit` (2) which indicates that this class and
 * The with `goff` option one can use as many variables as needed. There no more
   limitation, like with the options `para`and `candle`.
 * Fix detection of errors that appears in nested TTreeFormula [ROOT-8218]
+* Better basket size optimization by taking into account meta date and rounding up to next 512 bytes, ensuring a complete cluster fits into a single basket.
 
 ### Fast Cloning
 

tree/tree/src/TTree.cxx

@@ -6585,6 +6585,12 @@ void TTree::OptimizeBaskets(ULong64_t maxMemory, Float_t minComp, Option_t *opti
                newBsize = newBsize + (branch->GetBasket(0)->GetNevBuf() * sizeof(Int_t) * 2); // make room for meta data
             }
             // rounds up, increases basket size to ensure all entries fit into single basket as intended
+            // We used ATLAS fully-split xAOD for testing, which is a rather unbalanced TTree, 10K branches,
+            // with 8K having baskets smaller than 512 bytes. To achieve good I/O performance ATLAS uses auto-flush 100,
+            // resulting in the smallest baskets being ~300-400 bytes, so this change increases their memory by about 8k*150B =~ 1MB,
+            // at the same time it significantly reduces the number of total baskets because it ensures that all 100 entries can be
+            // stored in a single basket (the old optimization tended to make baskets too small). In a toy example with fixed sized
+            // structures we found a factor of 2 fewer baskets needed in the new scheme.
             newBsize = newBsize - newBsize%512 + 512;
          }
          if (newBsize < sizeOfOneEntry) newBsize = sizeOfOneEntry;