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Subject[PATCH] fix readahead breakage for sequential after random reads
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Hi Andrew,

Current readahead logic is broken when a random read pattern is
followed by a long sequential read. The cause is that on a window
miss ra->next_size is set to ra->average, but ra->average is only
updated at the end of a sequence, so window size will remain 1 until
the end of the sequential read.

This patch fixes this by taking the current sequence length into
account (code taken from towards end of page_cache_readahead()), and
also setting ra->average to a decent value in handle_ra_miss() when
sequential access is detected.

Please apply!

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>

==============================================================================
--- linux-2.6.8-rc2/mm/readahead.c.orig 2004-06-16 07:18:57.000000000 +0200
+++ linux-2.6.8-rc2/mm/readahead.c 2004-07-18 23:52:31.000000000 +0200
@@ -470,7 +470,11 @@ do_io:
* pages shall be accessed in the next
* current window.
*/
- ra->next_size = min(ra->average , (unsigned long)max);
+ average = ra->average;
+ if (ra->serial_cnt > average)
+ average = (ra->serial_cnt + ra->average + 1) / 2;
+
+ ra->next_size = min(average , (unsigned long)max);
}
ra->start = offset;
ra->size = ra->next_size;
@@ -552,6 +556,7 @@ void handle_ra_miss(struct address_space
ra->size = max;
ra->ahead_start = 0;
ra->ahead_size = 0;
+ ra->average = max / 2;
}
}
ra->prev_page = offset;


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