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

[Ronald Moesbergen]

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.
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