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

[Sean Dobbs]

Richard,

Some anecdotal numbers:  Running with a few plugins for calibration &
compton analysis gives a reconstruction rate ~400 Hz with one thread.
Both 15 and 30 threads give ~2.8kHz.  However, when we are running a
monitoring launch, the limiting factor is filling all of the
histograms, so the scaling is better (but the rate lower).  But
pressure should still be applied to remove this Fortran code from
halld_recon.

---Sean

On Thu, Mar 7, 2019 at 8:38 AM Richard Jones <richard.t.jones at uconn.edu> wrote:
>
> 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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