[PATCH 02/14] readahead: state based method: decouple readahead_ratio from growth_limit

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

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Remove readahead_ratio from the growth_limit computing in state based method.

It simplifies the size ramp up rules to:

        ahead_window_size = request_size +
                            current_window_size * 2 +
                            readahead_max / 16;

The first two sizes are apparent. The last one has two effects:

1) Ensure that we get acceptable readahead size in early start up phase.
   It is an analog to the old 'first x4 then x2' trick.

2) Ensure that we reach readahead_max within 8 steps.
   It is good for the laptop mode. A user may set a huge readahead_max, and
   expect the kernel to do large I/Os right away.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
 mm/readahead.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- linux-2.6.21-rc3-mm2.orig/mm/readahead.c
+++ linux-2.6.21-rc3-mm2/mm/readahead.c
@@ -1099,7 +1099,7 @@ state_based_readahead(struct address_spa
 
 	growth_limit = req_size;
 	growth_limit += ra_max / 16;
-	growth_limit += (2 + readahead_ratio / 64) * ra_old;
+	growth_limit += 2 * ra_old;
 	if (growth_limit > ra_max)
 	    growth_limit = ra_max;
 

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