On loop execution speed

Question

In another thread the execution speed of loops was mentioned as a side issue. That warranted some testing.

It turns out that using if-construction inside the loop to deal with the first index instead of assigning that before the loop means that the loop takes almost twice as long ... if the loop just assigns a single value to a parameter. If the loop contents are even moderately complex, the effect of the if-construction will soon disappear into noise.

Another raised question was the speed of the two forks in the if-construction. From this simple test that is minimal. The random variation is much higher than the difference, but on the average the result below seems to be about right.

Another interesting thing, which doesn't show below, is the effect of multithreading. These examples are likely about as bad for multithreading as possible and that was also reflected in the results. The overhead from it was such that each execution time roughly quadrupled from the shown ones, which didn't use it.

JKT · Answer

I couldn't resist: Similar samples of matrix generation. Defining the matrix in dual loop starting from top left does take longer than doing it from bottom right, but the difference is relatively small. I expected more. On the other hand, using matrix instead of dual loop was faster as I expected, but it still took about half as long as the dual loop. The loops fared rather well in my opinion. If there is any real calculation for the matrix values, the difference would likely be insignificant.

In this case multithreading worked a bit differently. The first two clocked relatively consistently a bit over 2 seconds each. The second may have still been slightly faster. The last two were interesting, though. The time in the first of them varied from about 0.6 to somewhat over 1s. The second was much faster. In some cases it was roughly the same as without multithreading and the max was maybe 0.7s. That kind of difference I didn't expect. Another thing I didn't expect is that on average every third run the latter two produced an error when using multithreading. There are obviously some bugs in the code.

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