DirectIO will bypass OS buffer, where buffering is unnecessary in our sampling process. Further, it seems that DirectIO will be faster on some FS such as APFS. E.g., using 32 threads sampling on APFS, DirectIO can yield 1GB/s randread (actual disk IO is 1GB/s), while buffer IO will yield 500GB/s (actual disk IO is 2GB/s, no sure where the overhead comes from).
DirectIO will bypass OS buffer, where buffering is unnecessary in our sampling process. Further, it seems that DirectIO will be faster on some FS such as APFS. E.g., using 32 threads sampling on APFS, DirectIO can yield 1GB/s randread (actual disk IO is 1GB/s), while buffer IO will yield 500GB/s (actual disk IO is 2GB/s, no sure where the overhead comes from).