Difference between revisions of "Locking in JANA"

Revision as of 11:07, 11 April 2016

Plugin Processor Variables

• Do not place use any locks in JEventProcessor::init() or ::fini(): they are unnecessary, and a waste of time.
  • JANA is guaranteed to be single-threaded in these functions.

• Do not use or modify any global variables, unless you are in a lock.
  • Or even better, don't have any in the first place. It's terrible programming practice.

• Do NOT use or modify any class member-variables that you define for your JEventProcessor (i.e. in header file), unless you are in a lock (or in init(), fini()). These variables can be modified by other threads.
  • This ESPECIALLY includes data saved in brun(), that is read in evnt().
  • Or even better, don't have any in the first place: Reduce the amount of time spent in locks if at all possible. Use function-scope (local) variables instead.
    • Where possible of course. E.g. you can't avoid it with histograms, but you need to lock to fill those anyway.

Creating Histograms or TTrees

• If you are creating histograms or trees (outside of JEventProcessor::init()): Use the global ROOT JANA lock functions below.
  • This is because they depend on gDirectory being constant.

japp->RootWriteLock(); //ACQUIRE ROOT LOCK
japp->RootUnLock(); //RELEASE ROOT LOCK

Filling Histograms

• If you are filling histograms, you do NOT need to grab the global ROOT JANA lock functions. Try to instead grab a lock with a smaller scope, so that other threads can write to other histograms that don't interfere with yours.
  • In you are directly in the plugin processor, there's a special function to do this, that locks for the current plugin only. It is:

japp->RootFillLock(this); //ACQUIRE ROOT FILL LOCK //this: JEventProcessor pointer
japp->RootFillUnLock(this); //RELEASE ROOT FILL LOCK //this: JEventProcessor pointer

Filling TTrees

• If you are filling TTrees to the global ROOT file (i.e. hd_root.root), write to the TTrees with the global ROOT JANA lock functions below:
  • This is because TTree::Fill() periodically writes to the file, modifying it.

japp->RootWriteLock(); //ACQUIRE ROOT LOCK
japp->RootUnLock(); //RELEASE ROOT LOCK

• If you are filling TTrees to a local ROOT file (i.e. NOT hd_root.root), you do NOT need to grab the global ROOT JANA lock functions. Try to instead grab a lock with a smaller scope, so that other threads can write to other trees that don't interfere with yours.
  • Grab the same for every tree that is written to the same output file. This is because TTree::Fill() periodically writes to the file, modifying it.
  • This can be done with:

japp->WriteLock("MyFileName"); //ACQUIRE FILE LOCK
japp->Unlock("MyFileName"); //RELEASE FILE LOCK