On Wed, Jul 01, 2009 at 01:03:55PM +0800, Zhang, Yanmin wrote:
> On Wed, 2009-07-01 at 12:10 +0800, Wu Fengguang wrote:
> > On Wed, Jul 01, 2009 at 11:25:33AM +0800, Zhang, Yanmin wrote:
> > > 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.
> > 
> > Yanmin, thanks for the tests!
> > 
> > Maybe the patch posted here can restore the performance:
> > 
> > http://lkml.org/lkml/2009/5/21/319
> I tried it and it doesn't help.

Then let's check what's happening behind the scene :)

Please apply the attached patch and run

        echo 1 > /debug/readahead/trace_enable
        # do benchmark
        echo 0 > /debug/readahead/trace_enable

and send the dmesg which will contain lots of lines like

        [   54.738105] readahead-initial0(pid=3290(zsh), dev=00:10(0:10), ino=105910(dmesg), req=0+1, ra=0+4-3, async=0) = 2
        [   54.751801] readahead-subsequent(pid=3290(dmesg), dev=00:10(0:10), ino=105910(dmesg), req=1+60, ra=4+8-8, async=1, miss=0) = 0


Thank you very much,
Fengguang