[Halld-offline] effects of CCal mutex on scaling in jana

[Richard Jones]

Hello David,

Do you have any measurements of the multi-thread scaling of our
reconstruction code with this CCal mutex in place? Prior to this, you
showed this very nice scaling up to >100 cores in this plot that was shown
at the November review and elsewhere. We need to keep Amdahl's Law in mind
here, which says that at some point it is the single-threaded piece of the
code that limits performance, even if it is a relatively minor fraction of
the total event processing. If there is a substantial change in the scaling
behavior of our reconstruction code coming from this CCal mutex, it will
give an idea how urgent the need is to multithread this portion of our
codebase, where "multithreading" does not mean merely mutexing it.

Knowing how everyone has more than they can handle on their plate, this
tedious job of rewriting existing code (I know, I have done it plenty of
times) may never get done. It would be helpful to know how much that
matters, if at all.

-Richard Jones
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