HLTPrescaleProvider optimization: minimize repeated calls to retrieveL1Event

[slava77]

Sub-optimal behavior was found in analysis ntupling code.
The code loops over all triggers in the table (per event) and calls HLTPrescaleProvider::prescaleValuesInDetail, which turned out to be a significant user of CPU.

Profiler showed that more than 95% of CPU is taken by calls to l1t::L1TGlobalUtil::retrieveL1Event repeated twice for every call to HLTPrescaleProvider::prescaleValuesInDetail. One is called directly, and another via ::prescaleSet method call.
The retrieveL1Event call is the same per event, while the call to prescaleValuesInDetail is specific per trigger.

Calls to l1t::L1TGlobalUtil::retrieveL1Event can be done once per event with appropriate caching.

[slava77]

assign hlt

[cmsbuild]

New categories assigned: hlt

@Martin-Grunewald,@silviodonato,@fwyzard you have been requested to review this Pull request/Issue and eventually sign? Thanks

[cmsbuild]

A new Issue was created by @slava77 Slava Krutelyov.

@davidlange6, @Dr15Jones, @smuzaffar can you please review it and eventually sign/assign? Thanks.

cms-bot commands are listed here

[Martin-Grunewald]

It looks to me one could simply remove the calls to retrieveL1Event (case l1tType==2) here:
https://github.com/cms-sw/cmssw/blob/master/HLTrigger/HLTcore/src/HLTPrescaleProvider.cc#L163
(method prescaleValues) and here:
https://github.com/cms-sw/cmssw/blob/master/HLTrigger/HLTcore/src/HLTPrescaleProvider.cc#L285
(method prescaleValuesInDetail) because method prescaleSet is called beforehand within each method and that one calls retrieveL1Event already.

I'd just hope that MT would not screw this up!

@Dr15Jones ?

[Dr15Jones]

@Martin-Grunewald the HLTPrescaleProvider is already thread-friendly but not thread safe so as long as each module has its own copy of the object, there shouldn't be a threading problem.

[Martin-Grunewald]

PRs made - can close this issue!