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

[Vladislav Bolkhovitin <vst@vlnb.net>]

Wu Fengguang, on 06/29/2009 01:34 PM wrote:
> 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.

No, SCST performance isn't an issue here. You simply can't get more than 
110 MB/s from iSCSI over 1GbE, hence 231 MB/s fundamentally isn't 
possible. There is only room for 20% improvement, which should be 
achieved with better client-side-driven pipelining (see our other 
discussions, e.g. http://lkml.org/lkml/2009/5/12/370)

> (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