Actually, you are looking at the
perfect situation when the software was made thinking on multi-tasking on the same task.
I'm looking based on this - The graphic shows no performance loss (its theoretical after all), but it can actually decrease performance if not perfectly programmed
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For a better explanation, if the software (in this case, builders) was made to be half multi-tasked the increase of processors show no advance when the number of processors is above 16.
The example I did before was based on this graphic and it is used to measure performance gained with high performance computers - gaming ones, where there are always a big number of processors and there are not big differences between a 8-core and 16-core on 90% of the games
