It looks like the mmap call is not within the timed region in do_mmap_test. I'd call that a major flaw in the test methodology when combined with MAP_POPULATE and a cold cache. The read syscall version has to actually do IO in the timed region; the mmap(MAP_POPULATE) version just does TLB misses and memcpy.

edit: actually, sorry, I think the article points this out, mostly correctly:

> We will come back to __memmove_avx_unaligned_erms in a moment, but for the time being let’s try to figure out exactly how much time is spent mapping pages. I have a neat trick up my sleeve to do that. On Linux, the mmap system call can accept a MAP_POPULATE flag. What this flag does is it forces mmap to pre-populate all the page mappings during the actual system call, so none of the page-mapping work would be done when my test actually runs. So I changed my test to invoke mmap with MAP_POPULATE and learned that the experiment completes about 36% faster. (I only measure the timing of the main loop, and not that of the mmap system call). Therefore, I assume that in the above profile all those mapping functions take up about 36%.

They didn't point out that it doesn't actually do any IO in the main loop either then, but maybe they were only running the warm cache variant by here anyway. And it seems like they didn't use MAP_POPULATE when generating the comparison graphs earlier in the article.