> >Yes. The kernel readahead code by design will outperform simple > >fadvise in the case of clustered random reads. Imagine the access > >pattern 1, 3, 2, 6, 4, 9. fadvise will trigger 6 IOs literally. While > > You mean it will trigger 6 IOs in the POSIX_FADV_RANDOM case or > POSIX_FADV_WILLNEED case? Yes. However note that I'm assuming 1-page sized and prefetch depth fadvise(POSIX_FADV_WILLNEED) calls in this example. Given more prefetch depth or good timing, there will be possibility for IO requests (eg. 3 and 2) be merged at block layer. > >kernel readahead will likely trigger 3 IOs for 1, 3, 2-9. Because on > >the page miss for 2, it will detect the existence of history page 1 > >and do readahead properly. For hard disks, it's mainly the number of > > If the first IO read 1, it will call page_cache_sync_read() since > cache miss, > if (offset - (ra->prev_pos) >> PAGE_CACHE_SHIFT) <= 1UL) > goto initial_readahead; > If the initial_readahead will be called? Because offset is equal to > 1 and ra->prev_pos is equal to 0. If my assume is true, 2 also will > be readahead. ra->prev_pos is initialized to -1 in file_ra_state_init(), so that if the very first read is on page 0, it will trigger readahead. Sorry I gave a confusing example. We may as well use 1001, 1003, 1002, 1006, 1004, 1009 as the example numbers. Thanks, Fengguang -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>