Memory-Mapped File is Faster on Huge Sequential Read? Why?

Question

I used the code below to measure the performance difference between reading large, sequential reads of a memory-mapped file, as compared to just calling ReadF

Answer 1

FILE_FLAG_NO_BUFFERING cripples ReadFile. The memory-mapped file is free to use whatever read-ahead algorithm it wants, and you've forbidden ReadFile to do the same. You've turned off caching only in the ReadFile version. Memory-mapped files can't work without file cache.