[PATCH] speed up readahead for seeky loads

[PATCH] speed up readahead for seeky loads

From: Ram Pai <linuxram@us.ibm.com>

Currently the readahead code tends to read one more page than it should with
seeky database-style loads.  This was to prevent bogus readahead triggering
when we step into the last page of the current window.

The patch removes that workaround and fixes up the suboptimal logic instead.

wrt the "rounding errors" mentioned in this patch, Ram provided the following
description:

  Say the i/o size is 20 pages.

  Our algorithm starts by a initial average i/o size of 'ra_pages/2' which
  is mostly say 16.

  Now every time we take a average, the 'average' progresses as follows
  (16+20)/2=18
  (18+20)/2=19
  (19+20)/2=19
  (19+20)/2=19.....
  and the rounding error makes it never touch 20

Benchmarking sitrep:

			IOZONE

	run on a nfs mounted filesystem:
	client machine 2proc, 733MHz, 2GB memory
	server machine 8proc, 700Mhz, 8GB memory

./iozone -c -t1 -s 4096m -r 128k