Multithreading timings quandry

AmiBroker
#1

The timing information shown below (at the end of this post) for a back test doesn't seem to add up in the same way as shown in the "Limits of multithreading" article. Specifically, using a PC with a processor with six cores, I would expect the following to be true:

total time = UI thread-time + worker-thread-time/12

However, I find instead that

UI thread-time + worker-thread-time/12 = 173.00 + 2478.87/12 = 379.57 seconds

But we also have: total time = 283.35 seconds, with a difference of 96.21 seconds!

Can anyone tell me what might be causing such a great discrepancy?

All suggestions are welcome...Thanks,
Ken Metz

AB "info" tab:
Completed in 283.35 seconds. Number of rows: 4708
( Timings: data: 163.99, setup: 0.02, afl: 2469.35, job: 9.53, lock: 0.08, pbt: 8.99, UI thread: 173.00, worker threads: 2478.87/2478.80 )

#2

They don’t add up because they are not supposed to add up. Time spent in multiple threads is “parallel” it is like you were living multiple parallel lives. Also six core CPU can’t really execute 12 threads at full speed, limiting factors are Shared resources like memory and shared L3 cache. This all makes such calculation fundamentally flawed. Also Ui thread time goes in parallel to worker threads, except pbt and setup, so actual math is a whole lot more complex than you assumed.