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

[Wu Fengguang]

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@xxxxxxxxx> 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.

Thanks,
Fengguang
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