Add comments to clarify the easily misunderstood PG_readahead timing. PG_readahead is a trigger to say, when you get this far, it's time to think about kicking off the _next_ readahead. -- Hugh CC: Hugh Dickins <hughd@xxxxxxxxxx> CC: Matthew Wilcox <willy@xxxxxxxxxxxxxxx> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> --- mm/readahead.c | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) --- linux-next.orig/mm/readahead.c 2011-08-05 10:53:01.000000000 +0800 +++ linux-next/mm/readahead.c 2011-08-05 11:40:06.000000000 +0800 @@ -185,6 +185,14 @@ __do_page_cache_readahead(struct address break; page->index = page_offset; list_add(&page->lru, &page_pool); + /* + * set PG_readahead to trigger the _next_ ASYNC readahead. + * + * |----------------- nr_to_read ---------------->| + * |==================#===========================| + * |<---- lookahead_size ------| + * PG_readahead mark^ + */ if (page_idx == nr_to_read - lookahead_size) SetPageReadahead(page); ret++; @@ -321,6 +329,25 @@ static unsigned long get_next_ra_size(st * indicator. The flag won't be set on already cached pages, to avoid the * readahead-for-nothing fuss, saving pointless page cache lookups. * + * A typical readahead time chart for a sequential read stream. Note that when + * read(2) hits the PG_readahead mark, a new readahead will be started and the + * PG_readahead mark will be "pushed forward" by clearing the old PG_readahead + * and setting a new PG_readahead in the new readahead window. + * + * t0 + * t1 +#__ ==> SYNC readahead triggered by page miss + * t2 -+__#_______ ==> ASYNC readahead triggered by PG_readahead + * t3 --+_#_______ + * t4 ---+#_______ + * t5 ----+_______#_______________ ==> ASYNC readahead triggered by PG_readahead + * t6 -----+______#_______________ + * t7 ------+_____#_______________ + * + * [-] accessed page + * [+] the page read(2) is accessing + * [#] the PG_readahead mark + * [_] readahead page (newly brought into page cache but not yet accessed) + * * prev_pos tracks the last visited byte in the _previous_ read request. * It should be maintained by the caller, and will be used for detecting * small random reads. Note that the readahead algorithm checks loosely -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>