Every feature in ZEMAX is a separate thread, so it is independent of all other features. Not all features are internally multi-threaded however. There is an overhead in launching and managing threads, and in receiving the data from the threads on completion, and in stitching the results back together again.

Also, remember that each thread must include a full copy of all lens data. If every feature spawned multiple internal threads, the amount of memory would quickly rise.

Instead, the computationally demanding features are internally multi-threaded. This includes (but is not limited to) optimization, global optimization, tolerancing, Huygen's calculations, diffraction calculations, physical optics and non-sequential ray-tracing. Many individual Analysis features are also internally multi-threaded. ZEMAX manages the multi-threading to ensure optimal use of the resources in the machine for the task in hand.

For example, let's optimize the double Gauss sample file using a default wavefront merit function. Make all radii (except the surfaces with infinite radii) and thicknesses variable. Place an f/# solve on the last radius, and make it f/3. Then build a default merit function like so:



I then optimize it and tell ZEMAX to use all 8 CPUs



Now damped-least-squares optimization involves taking the derivative of the merit function with respect to each variable, and the calculation is multi-threaded at the variable level, so that each derivative can be computed as its own thread. However, when the optimizer is run, the result is surprising:



Only one CPU is being used! (13% CPU Usage represents only 1 of the 8 CPUs in the machine being used.) Why?

Well, in some respects this problem is too simple to benefit from multi-threading, or at least, it is too efficiently implemented in ZEMAX to need to be multi-threaded. This simple macro:

! How long does it take to compute the merit function?
FORMAT 4.3 EXP
cycles = 100
TIMER  # set the timer

FOR i = 1, cycles, 1
      dummy = MFCN() # update the merit function
NEXT i

PRINT "The average time to compute the merit function is: ", ETIM()/cycles, " seconds"

PRINT "Program End"
END

reveals:

Executing C:\Program Files (x86)\ZEMAX\MACROS\QUICKIE.ZPL.
The average time to compute the merit function is: 3.100E-004 seconds
Program End

The merit function takes only 310 microseconds to compute on one processor of this machine! Remember that the merit function in this case is the RMS wavefront error over three fields and three wavelengths. This shows the incredible efficiency of ZEMAX's Gaussian Quadrature and DLS algorithms. The overhead of copying all the data, launching threads, and receiving data back again is not justified when the merit function is computed this quickly. If a DENC (Diffraction Encircled Energy) operand is added to the merit function, the CPU utilization approaches 100%.



In general, a computation can be split up into "parallel-izable" tasks which can be split up over multiple CPUs, and serial tasks which can only take place in a single thread of execution. The CPU utilization shown in Task Manager is a rough representation of CPU usage, and during execution of a multi-threaded taks you will see the CPU usage vary from 13% (single CPU in this case) to 100%.