With Ram Pai <linuxram@us.ibm.com>

- request size is now passed into page_cache_readahead.  This allows the
  removal of the size averaging code in the current readahead logic.

- readahead rampup is now faster  (especially for larger request sizes)

- No longer "slow read path".  Readahead is turn off at first random access,
  turned back on at first sequential access.

- Code now handles thrashing, slowly reducing readahead window until
  thrashing stops, or min size reached.

- Returned to old behavior where first access is assumed sequential only if
  at offset 0.

- designed to handle larger (1M or above) window sizes efficiently


Benchmark results:

machine 1: 8 way pentiumIV 1GB memory, tests run to 36GB SCSI disk
(Similar results were seen on a 1 way 866Mhz box with IDE disk.)

TioBench:

tiobench.pl --dir /mnt/tmp --block 4096 --size 4000 --numruns 2 --threads 1(4,16,64)

4k request size sequential read results in MB/sec

  Threads         2.6.9    w/patches    %diff         diff