One point is outside of the area of Roofline model

Question

I used roofline model for analysis of code optimization.

But I found the point with green color is out of the area of boundary of bandwidth.The program can run without problem.

I don't understand why the green point is not in the area of red region.

The machine is a virtual linux machine with 2 cores,my local physical machine has 4 cores.The analysis tool is intel advisor.

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Answer 1

This is possible if the application is not bound by the DRAM but by the CPU caches which have a significantly higher throughput. For example, the application can be bound by the LLC cache of your processor (LLC means Last Level Cache, which is typically a level 3 cache).

Note that a 4.26 peak flop performance is pretty low for a relatively-recent processor (AFAIK my i5-9600KF reaches ~200 GFlops in double-precision and ~400 GFlops for simple-precision). 36.31 is more realistic but still quite low unless your target processor is pretty old or is an embedded/mobile one. Also please note that virtual machine running on shared machine are generally not great for profiling as users can possibly simultaneously run other application using the DRAM resulting in misleading profiling results.