Re: [PATCH] f2fs: optimize gc for better performance

[Jin Xu <jinuxstyle@gmail.com>]

Yes, I will submit the patch later.

On 05/09/2013 07:50, Jaegeuk Kim wrote:
> Hi Jin,
>
> 2013-09-04 (수), 21:17 +0800, Jin Xu:
>> Hi Jaegeuk,
>>
>> On 04/09/2013 17:40, Jaegeuk Kim wrote:
>>> Hi Jin,
>>>
>>> 2013-09-04 (수), 07:59 +0800, Jin Xu:
>>>> Hi Jaegeuk,
>>>>
>>>> On 03/09/2013 08:45, Jaegeuk Kim wrote:
>>>>> Hi Jin,
>>>>>
>>>>>> [...]
>>>>>>>
>>>>>>> It seems that we can obtain the performance gain just by setting the
>>>>>>> MAX_VICTIM_SEARCH to 4096, for example.
>>>>>>> So, how about just adding an ending criteria like below?
>>>>>>>
>>>>>>
>>>>>> I agree that we could get the performance improvement by simply
>>>>>> enlarging the MAX_VICTIM_SEARCH to 4096, but I am concerning the
>>>>>> scalability a little bit. Because it might always searching the whole
>>>>>> bitmap in some cases, for example, when dirty segments is 4000 and
>>>>>> total segments is 409600.
>>>>>>> [snip]
>>>>>> [...]
>>>>>>>
>>>>>>> 	if (p->max_search > MAX_VICTIM_SEARCH)
>>>>>>> 		p->max_search = MAX_VICTIM_SEARCH;
>>>>>>>
>>>>>>
>>>>>> The optimization does not apply to SSR mode. There has a reason.
>>>>>> As noticed in the test, when SSR selected the segments that have most
>>>>>> garbage blocks, then when gc is needed, all the dirty segments might
>>>>>> have very less garbage blocks, thus the gc overhead is high. This might
>>>>>> lead to performance degradation. So the patch does not change the
>>>>>> victim selection policy for SSR.
>>>>>
>>>>> I think it doesn't care.
>>>>> GC is only triggered during the direct node block allocation.
>>>>> What it means that we need to consider the number of GC triggers where
>>>>> the GC triggers more frequently during the normal data allocation than
>>>>> the node block allocation.
>>>>> So, I think it would not degrade performance significatly.
>>>>>
>>>>> BTW, could you show some numbers for this?
>>>>> Or could you test what I suggested?
>>>>>
>>>>> Thanks,
>>>>>
>>>>
>>>> I re-ran the test and got the following result:
>>>>
>>>> ---------------------------------------
>>>> 2GB SDHC
>>>> create 52023 files of size 32768 bytes
>>>> random re-write 100000 records of 4KB
>>>> ---------------------------------------
>>>>
>>>> | file creation (s) | rewrite time (s) | gc count | gc garbage blocks |
>>>>
>>>> no patch       341         4227             1174          174840
>>>> patched        296         2995             634           109314
>>>> patched (KIM)  324         2958             645           106682
>>>>
>>>> In this test, it does not show the minor performance degradation caused
>>>> by applying the patch to SSR mode. Instead, the performance is a little
>>>> better with what you suggested.
>>>>
>>>> I agree that the performance degradation would not be significant even
>>>> it does degrade. I ever saw the minor degradation in some workloads, but
>>>> I didn't save the data.
>>>>
>>>> So, I agree that we can apply the patch to SSR mode as well.
>>>>
>>>> And do you still have concerns about the formula for calculating the #
>>>> of search?
>>>
>>> Thank you for the test. :)
>>> What I've concerned is that, if it is really important to get a victim
>>> more accurately for the performance as you described, it doesn't need to
>>> calculate the number of searches IMO. Just let's select nr_dirty. Why
>>> not?
>>> Only the thing that we should consider is to handle the case where the
>>> nr_dirty is too large.
>>> For this, we can just limit the # of searches to avoid performance
>>> degradation.
>>>
>>> Still actually, I'm not convincing the effectiveness of your formula.
>>> If possible, could you show it with numbers?
>>
>> It's not easy to prove the effectiveness of the formula. It's just for
>> eliminating my concern on the scalability of searching. Since it does
>> not matter much for the performance improvement, we can put it aside
>> and choose the simpler method as you suggested.
>>
>> So, should I revise the patch based on what you suggested or will
>> you take care of it?
>
> Could you make a patch with your performance description and sumbit it
> again?
> Thanks a lot,
>