fio sync read 4k block size 35% regression

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

Comapraing with 2.6.30, fio sync read (block size 4k) has about 35% regression
with kernel 2.6.31-rc1 on my stoakley machine with a JBOD (13 SCSI disks).

Every disk has 1 partition and 4 1-GB files. Start 10 processes per disk to
do sync read sequentinally.

Bisected down to below patch.

51daa88ebd8e0d437289f589af29d4b39379ea76 is first bad commit
commit 51daa88ebd8e0d437289f589af29d4b39379ea76
Author: Wu Fengguang <fengguang.wu@intel.com>
Date:   Tue Jun 16 15:31:24 2009 -0700

    readahead: remove sync/async readahead call dependency

    The readahead call scheme is error-prone in that it expects the call sites
    to check for async readahead after doing a sync one.  I.e.

                        if (!page)
                                page_cache_sync_readahead();
                        page = find_get_page();
                        if (page && PageReadahead(page))
                                page_cache_async_readahead();


I also test block size 64k and 128k, but they don't have regression. Perhaps
the default read_ahead_kb is equal to 128?

Other 2 machines have no such regression. The JBODS of the 2 machines consists
of 12 and 7 SATA/SAS disks while every disk has 2 partitions.