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

[Vladislav Bolkhovitin <vst@vlnb.net>]

Wu Fengguang, on 06/29/2009 04:54 PM wrote:
> 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@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.
>>> 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? :)

We started with 2.6.29, so why not complete with it (to save additional 
Ronald's effort to move on 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.

OK. Ronald, can you 4 more test cases, please:

7. Default vanilla 2.6.29 kernel, 2MB read-ahead, the rest is default

8. Default vanilla 2.6.29 kernel, 2MB read-ahead, 64 KB
max_sectors_kb, the rest is default

9. Patched by the Fengguang's patch vanilla 2.6.29 kernel, 2MB
read-ahead, the rest is default

10. Patched by the Fengguang's patch vanilla 2.6.29 kernel, 2MB
read-ahead, 64 KB max_sectors_kb, the rest is default

>> 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
> 
>