Re: [PATCH] md: Enhancements and clean to linear RAID

[SandeepKsinha <sandeepksinha@gmail.com>]

Hi Neil,

I have made all the required changes and have tested the changes.
It works fine.

Thanks for enlightening me, the changes really optimized the code further.

The patches will be followed by this mail.

On Tue, May 19, 2009 at 1:01 PM, SandeepKsinha <sandeepksinha@gmail.com> wrote:
> Thanks Neil,
>
>
> On Tue, May 19, 2009 at 12:55 PM, NeilBrown <neilb@suse.de> wrote:
>> On Tue, May 19, 2009 4:23 pm, SandeepKsinha wrote:
>>> Hi Neil,
>>>
>>> I have made the desired changes and have tested the changes as well.
>>> As expected the performance gain with number of devices ( > 20 ) is
>>> quite visible.
>>
>> How did you measure the performance gain and what were your
>> results?
>>
>
> I created arrays with 20 devices and performed random I/O it.
> The results with linear search were slower as compared to binary.
>
>>> The code also looks simpler as compared to what it earlier was.
>>>
>>> I have created the following patches for the changes:
>>>
>>> PATCH [01/03] md: Removal of num_sectors from dev_info in linear raid
>>>
>>> This patch removes the num_sectors from dev_info and makes use of the
>>> rdev->sectors for
>>> all further usage.
>>
>> Could I as you to redo this one a little differently?
>> I want to keep the linear search very tight, and you have just added
>> a pointer de-reference to it.
>>
>> If you could replace start_sector by end_sector, we wouldn't need that
>> de-reference or the addition.
>>
>>
> I would make the required changes.
>
>>>
>>> PATCH[02/03] md: Getting rid of sector_div and hash table in linear raid
>>>
>>> Removal of all the code pertaining hast table and other related data
>>> structures.
>>> This also makes the code really really simple.
>>> The searching being made linear for the time being to test the patch.
>>>
>>> PATCH[03/03] md: Replacing linear with a binary search
>>>
>>> This patch replaces the linear search in which_dev with binary search.
>>
>> Two things wrong with you binary search.
>> 1/ the while() condition is "hi >= lo".  That will always be true, so it
>>  would be better to make that explicit.  i.e. "while(1)".
>>  However the while loop of a binary search should be
>>   while (hi > lo)
>>
>> 2/ You have 3 comparisons against 'sector' inside the loop.  There should
>>   only be one.  We are aiming for speed remember :-)
>>
>>   while (hi > lo) {
>>       mid = (hi + lo + 1) / 2;
>>       dev = conf->disks + mid;
>>
>>       if (sector >= dev->start_sector)
>>            lo = mid;
>>       else
>>            hi = mid - 1;
>>    }
>>    return dev;
>>
>>    With that formulation, you don't even need to replace
>>    sector_start by sector_end.. However doing so would make other
>>    code simpler, so please do proceed with replacing sector_start by
>>    sector_end.
>>    Then the binary search (with a fix because dev can be uninitialised),
>>    becomes:
>>
>>    while (hi > lo) {
>>       mid = (hi + lo) / 2;
>>
>>       if (sector < conf->disks[mid].end_sector)
>>            hi = mid;
>>       else
>>            lo = mid + 1;
>>    }
>>    return conf->disks + lo;
>>
>
> I will make this change too.
>
>
> Thanks.
>
>> NeilBrown
>>
>>
>>
>>
>
>
>
> --
> Regards,
> Sandeep.
>
>
>
>
>
>
> “To learn is to change. Education is a process that changes the learner.”
>



-- 
Regards,
Sandeep.





 	
“To learn is to change. Education is a process that changes the learner.”
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