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Twelve Ways to Build CMS Crossings from Root Files

Twelve Ways to Build CMS Crossings from Root Files. Benefits and deficiencies of Root trees and clones when : - NOT dealing with TObjects, - reading the trees entries NOT sequentially, - processing them NOT one by one. Outline. Goal & scope. Main use-case.

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Twelve Ways to Build CMS Crossings from Root Files

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  1. Twelve Ways to Build CMS Crossings from Root Files Benefits and deficiencies of Root trees and clones when :- NOT dealing with TObjects,- reading the trees entries NOT sequentially,- processing them NOT one by one. David Chamont (CMS - LLR)

  2. Outline • Goal & scope. • Main use-case. • 4 kinds of containers for the crossing data model. • 3 kinds of persistency managers. • Results. • Conclusions. David Chamont (CMS - LLR)

  3. Goal & Scope • Evaluate the benefits of TTree and TClonesArray for the persistency of CMSevent data (whose classes heavily rely on templates and external packages). • Focus on the generation of crossings (pile-up of about 160 simulated events chosen pseudo randomly). • Not covered yet : meta-data, associations between persistent objects, schema evolution . David Chamont (CMS - LLR)

  4. Main Use-Case Digitizer signal event (hits) minbias event (hits) digis Persistency Manager Persistency Manager Persistency Manager minbias events file digis file signal events file minbias events file David Chamont (CMS - LLR)

  5. Crossing Data Model • The folders //root/crossing/* represent the events composing the current crossing. • Each event folder contains a container for each kind of event objects :TrackHit,CaloHit,… • The kind of container is chosen among four : • std::vector<> (by value). • dynamic C array (each event object is wrapped inside a class instrumented with classdef). • TObjArray (each event object is wrapped inside a class derived from TObject). • TClonesArray (each event object is wrapped inside a class derived from TObject). David Chamont (CMS - LLR)

  6. Persistency Managers • The task of a persistency manager is to transfer an event from memory(TFolder) to disk(TFile) and vice-versa. • Three flavors have been implemented : • RtbPomKeys : directly write the TFolder in the TFile, each time with a different meaningful name. • RtbPomTreeMatrix : for each container in the folder, creates a TMatrixD and attach it to a branch of a TTree. • RtbPomTreeDirect : attach directly each container to a branch of a TTree. David Chamont (CMS - LLR)

  7. Implementation issues • Recent progress : • can now use -ansi -pedantic. • nice support of foreign classes. • better and better support of templates and std containers. • Recurrent problems with Root I/O : • must explicitly ask to parse namespaces, components types and template instances. • multiple containers sizes and misleading operator[], • tuning of chain branchs, • it is unclear which subset of C++ is supported by Root I/O, and which in TTree, and which in TClonesArray. David Chamont (CMS - LLR)

  8. Configuration • Pentium 4, 1.8 GHz. • ~ 512 Mo of RAM. • IDE disk. • RedHat Linux 7.3 • Gcc 3.2 • Root 3.05/03 David Chamont (CMS - LLR)

  9. Parameters of the Testbed • compression level. • size of buffers. • split level. • randomness : burst and jump. • size of the containers within the events. • number of crossings. • direct inheritance or not from TObject, direct instrumentation or not with ClassDef. • resetting or not the values in the empty constructors. David Chamont (CMS - LLR)

  10. Best Results David Chamont (CMS - LLR)

  11. Remove compression David Chamont (CMS - LLR)

  12. Then increase random David Chamont (CMS - LLR)

  13. Then reduce containers /10 David Chamont (CMS - LLR)

  14. Then remove random David Chamont (CMS - LLR)

  15. Conclusions • We succeeded to read pseudo-random entries from a chain and dispatch them to few hundred folders (despite tuning of TChain branches has not been straightforward). • Support for foreign classes, templates and C++ standard library has greatly improved. • The magic couple TTree/TClonesArray has proved very efficient, yet it requires top level TObjects and the benefits can become losses with less data or random access pattern. • One can simply use std vectors and store them directly into root files. Their integration in a TTree is not as worth as a TClonesArray. This could change in the next release of ROOT. • It would be interesting to retry with direct associations between objects, and to apply the testbed to POOL. David Chamont (CMS - LLR)

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