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tarabyte

Why in the left graph does the red line go down as we move from left to right?

Answer from class: Since f = 0.9999, this program is almost entirely parallelizable. This program gets a lot of use out of running on a machine with 16 cores, even if each of them is not as fast within a single-thread. So, we get a 16x speedup. As we move from 16 cores to 1 big, fancy core (running at 4 times the performance on a single-thread) we must do everything sequentially. So, we can't exploit the fact that the program is very parallelizable, and we can only run 4 times as fast.

jmc

It's important to bear in mind that perf(r) = \sqrt(r) is a rough model. We know that perf(r) = r would be too optimistic -- you cannot just keep building a linearly faster core with more transistors, as we saw when we looked at the history of processor design at the beginning of this class. Does anyone know if there is a more principled explanation for modelling perf(r) as \sqrt(r)?

bochet

In what sense is the plot symmetric?

Edit: Oh, looking at next slide, I think symmetric means each processor will have same performance.

kayvonf

@bochet. Yes, "symmetric" refers to the fact that these graphs are modeling the performance of symmetric (homogeneous) multi-core processors.