### TTreeProcessorMT API * on jira: [TTreeProcMT breaks without EnableImplicitMT on](https://sft.its.cern.ch/jira/browse/ROOT-10561) * on github: [EnableThreadSafety is not enough to run TTreeProcMT without problems](https://github.com/root-project/root/pull/4947) Need to pick the correct pattern and document it.

TThreadExecutor

explicit parallelism

EnableThreadSafety();
TThreadExecutor p(4); // creates a thread-pool
p.Map(func, data);
	    

EnableImplicitMT(4);
TThreadExecutor p; // uses the existing thread-pool
p.Map(func, data);
	    

RDataFrame

implicit parallelism

EnableThreadSafety();
RDataFrame df(data);
df.Func().GetValue(); // runs sequentially
	    

EnableImplicitMT(4);
RDataFrame df(data);
df.Func().GetValue(); // uses the existing thread-pool
	    

TTreeProcessorMT

??? parallelism

EnableThreadSafety();
TTreeProcessorMT tp(data); // creates a thread-pool, can't specify size
tp.Process(func);
	    

EnableImplicitMT(4);
TTreeProcessorMT tp(data);
tp.Process(func); // uses the existing thread-pool
	    

I _think_ we need to pick one: * explicit MT: require `EnableThreadSafety` and let user specify number of cores at construction time * implicit MT: require `EnableImplicitMT`, run sequentially(?) otherwise ...and document it