* [PATCH] readahead: add comments on PG_readahead
@ 2011-08-05 3:50 Wu Fengguang
2011-08-07 14:27 ` Minchan Kim
0 siblings, 1 reply; 2+ messages in thread
From: Wu Fengguang @ 2011-08-05 3:50 UTC (permalink / raw)
To: Andrew Morton
Cc: Hugh Dickins, Matthew Wilcox, Linux Memory Management List, LKML
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@google.com>
CC: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] readahead: add comments on PG_readahead
2011-08-05 3:50 [PATCH] readahead: add comments on PG_readahead Wu Fengguang
@ 2011-08-07 14:27 ` Minchan Kim
0 siblings, 0 replies; 2+ messages in thread
From: Minchan Kim @ 2011-08-07 14:27 UTC (permalink / raw)
To: Wu Fengguang
Cc: Andrew Morton, Hugh Dickins, Matthew Wilcox,
Linux Memory Management List, LKML
On Fri, Aug 05, 2011 at 11:50:40AM +0800, Wu Fengguang wrote:
> 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@google.com>
> CC: Matthew Wilcox <willy@linux.intel.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
I expect it would be very helpful.
--
Kind regards,
Minchan Kim
^ permalink raw reply [flat|nested] 2+ messages in thread
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