Set ApproxEventsPerBasket to 1 for FSRs

In gaudi/Gaudi!1381 (merged) the TTree basket sizes are adapted to improve the compression of output files by using larger baskets in some cases. For FSRs there is only ever one event per branch so making baskets large enough for multiple entries is a waste of vmem.

Reqires a new Gaudi release that includes gaudi/Gaudi!1381 (merged).

GaudiConf/python/GaudiConf/IOHelper.py

@@ -421,9 +421,9 @@ class IOHelper(object):
             ApplicationMgr().ExtSvc += [rootSvc]
 
             fileSvc = Gaudi__RootCnvSvc("FileRecordCnvSvc")
-            #optimization, see Core Soft meeting, indico.cern.ch/conferenceDisplay?confId=207776
-            if hasattr(fileSvc, "BufferSize"):
-                fileSvc.BufferSize = 512
+            # The FSRs are trees with only one entry per branch
+            fileSvc.MinBufferSize = 512
+            fileSvc.ApproxEventsPerBasket = 1
 
             self._doConfFileRecords(fileSvc)