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

From: Wu Fengguang
Date: Mon Jun 29 2009 - 08:55:21 EST

On Mon, Jun 29, 2009 at 06:55:40PM +0800, Vladislav Bolkhovitin wrote:
> Ronald Moesbergen, on 06/29/2009 02:26 PM wrote:
> > 2009/6/29 Wu Fengguang <fengguang.wu@xxxxxxxxx>:
> >> 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@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.
> >>> 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.
> >
> > I just tested raw HW RAID throughput with the patch applied, same
> > readahead setting (512KB), and it doesn't look promising:
> >
> > ./blockdev-perftest -d -r /dev/cciss/c0d0
> > blocksize W W W R R R
> > 67108864 -1 -1 -1 5.59686 5.4098 5.45396
> > 33554432 -1 -1 -1 6.18616 6.13232 5.96124
> > 16777216 -1 -1 -1 7.6757 7.32139 7.4966
> > 8388608 -1 -1 -1 8.82793 9.02057 9.01055
> > 4194304 -1 -1 -1 12.2289 12.6804 12.19
> > 2097152 -1 -1 -1 13.3012 13.706 14.7542
> > 1048576 -1 -1 -1 11.7577 12.3609 11.9507
> > 524288 -1 -1 -1 12.4112 12.2383 11.9105
> > 262144 -1 -1 -1 7.30687 7.4417 7.38246
> > 131072 -1 -1 -1 7.95752 7.95053 8.60796
> > 65536 -1 -1 -1 10.1282 10.1286 10.1956
> > 32768 -1 -1 -1 9.91857 9.98597 10.8421
> > 16384 -1 -1 -1 10.8267 10.8899 10.8718
> > 8192 -1 -1 -1 12.0345 12.5275 12.005
> > 4096 -1 -1 -1 15.1537 15.0771 15.1753
> > 2048 -1 -1 -1 25.432 24.8985 25.4303
> > 1024 -1 -1 -1 45.2674 45.2707 45.3504
> > 512 -1 -1 -1 87.9405 88.5047 87.4726
> >
> > It dropped down to 189 MB/s. :(
>
> Ronald,
>
> Can you, please, rerun this test locally on the target with the latest
> version of blockdev-perftest, which produces much more readable results,

Is blockdev-perftest public available? It's not obvious from google search.

> for the following 6 cases:
>
> 1. Default vanilla 2.6.29 kernel, default parameters, including read-ahead

Why not 2.6.30? :)

> 2. Default vanilla 2.6.29 kernel, 512 KB read-ahead, the rest is default

How about 2MB RAID readahead size? That transforms into about 512KB
per-disk readahead size.

> 3. Default vanilla 2.6.29 kernel, 512 KB read-ahead, 64 KB
> max_sectors_kb, the rest is default
>
> 4. Patched by the Fengguang's patch http://lkml.org/lkml/2009/5/21/319
> vanilla 2.6.29 kernel, default parameters, including read-ahead
>
> 5. Patched by the Fengguang's patch vanilla 2.6.29 kernel, 512 KB
> read-ahead, the rest is default
>
> 6. Patched by the Fengguang's patch vanilla 2.6.29 kernel, 512 KB
> read-ahead, 64 KB max_sectors_kb, the rest is default

Thanks,
Fengguang

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