Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev

[Wu Fengguang <fengguang.wu@intel.com>]

On Sat, Jun 20, 2009 at 08:29:31PM +0800, Vladislav Bolkhovitin wrote:
> 
> Wu Fengguang, on 06/20/2009 07:55 AM wrote:
> > On Fri, Jun 19, 2009 at 03:04:36AM +0800, Andrew Morton wrote:
> >> On Sun, 7 Jun 2009 06:45:38 +0800
> >> Wu Fengguang <fengguang.wu@intel.com> wrote:
> >>
> >>>>> Do you have a place where the raw blktrace data can be retrieved for
> >>>>> more in-depth analysis?
> >>>> I think your comment is really adequate. In another thread, Wu Fengguang pointed
> >>>> out the same issue.
> >>>> I and Wu also wait his analysis.
> >>> And do it with a large readahead size :)
> >>>
> >>> Alan, this was my analysis:
> >>>
> >>> : Hifumi, can you help retest with some large readahead size?
> >>> :
> >>> : Your readahead size (128K) is smaller than your max_sectors_kb (256K),
> >>> : so two readahead IO requests get merged into one real IO, that means
> >>> : half of the readahead requests are delayed.
> >>>
> >>> ie. two readahead requests get merged and complete together, thus the effective
> >>> IO size is doubled but at the same time it becomes completely synchronous IO.
> >>>
> >>> :
> >>> : The IO completion size goes down from 512 to 256 sectors:
> >>> :
> >>> : before patch:
> >>> :   8,0    3   177955    50.050313976     0  C   R 8724991 + 512 [0]
> >>> :   8,0    3   177966    50.053380250     0  C   R 8725503 + 512 [0]
> >>> :   8,0    3   177977    50.056970395     0  C   R 8726015 + 512 [0]
> >>> :   8,0    3   177988    50.060326743     0  C   R 8726527 + 512 [0]
> >>> :   8,0    3   177999    50.063922341     0  C   R 8727039 + 512 [0]
> >>> :
> >>> : after patch:
> >>> :   8,0    3   257297    50.000760847     0  C   R 9480703 + 256 [0]
> >>> :   8,0    3   257306    50.003034240     0  C   R 9480959 + 256 [0]
> >>> :   8,0    3   257307    50.003076338     0  C   R 9481215 + 256 [0]
> >>> :   8,0    3   257323    50.004774693     0  C   R 9481471 + 256 [0]
> >>> :   8,0    3   257332    50.006865854     0  C   R 9481727 + 256 [0]
> >>>
> >> I haven't sent readahead-add-blk_run_backing_dev.patch in to Linus yet
> >> and it's looking like 2.6.32 material, if ever.
> >>
> >> If it turns out to be wonderful, we could always ask the -stable
> >> maintainers to put it in 2.6.x.y I guess.
> > 
> > Agreed. The expected (and interesting) test on a properly configured
> > HW RAID has not happened yet, hence the theory remains unsupported.
> 
> Hmm, do you see anything improper in the Ronald's setup (see
> http://sourceforge.net/mailarchive/forum.php?thread_name=a0272b440906030714g67eabc5k8f847fb1e538cc62%40mail.gmail.com&forum_name=scst-devel)?
> It is HW RAID based.

No. Ronald's HW RAID performance is reasonably good.  I meant Hifumi's
RAID performance is too bad and may be improved by increasing the
readahead size, hehe.

> As I already wrote, we can ask Ronald to perform any needed tests.

Thanks!  Ronald's test results are:

231   MB/s   HW RAID                        
 69.6 MB/s   HW RAID + SCST                 
 89.7 MB/s   HW RAID + SCST + this patch

So this patch seem to help SCST, but again it would be better to
improve the SCST throughput first - it is now quite sub-optimal.
(Sorry for the long delay: currently I have not got an idea on
 how to measure such timing issues.)

And if Ronald could provide the HW RAID performance with this patch,
then we can confirm if this patch really makes a difference for RAID.

Thanks,
Fengguang