Re: [PATCH] mm: consider all swapped back pages in used-once logic

[Minchan Kim]

On 06/25/2012 05:08 PM, Johannes Weiner wrote:

> On Mon, Jun 25, 2012 at 08:53:11AM +0900, Minchan Kim wrote:
>> Hi Hannes,
>>
>> On 06/23/2012 08:04 PM, Johannes Weiner wrote:
>>
>>> On Mon, May 21, 2012 at 07:07:00PM +0800, Zheng Liu wrote:
>>>> On Mon, May 21, 2012 at 11:37:05AM +0200, Johannes Weiner wrote:
>>>> [snip]
>>>>>>> Is it because the read()/write() IO is high throughput and pushes
>>>>>>> pages through the LRU lists faster than the mmap pages are referenced?
>>>>>>
>>>>>> Yes, in this application, one query needs to access mapped file page
>>>>>> twice and file page cache twice.  Namely, one query needs to do 4 disk
>>>>>> I/Os.  We have used fadvise(2) to reduce file page cache accessing to
>>>>>> only once.  For mapped file page, in fact them are accessed only once
>>>>>> because in one query the same data is accessed twice.  Thus, one query
>>>>>> causes 2 disk I/Os now.  The size of read/write is quite larger than
>>>>>> mmap/munmap.  So, as you see, if we can keep mmap/munmap file in memory
>>>>>> as much as possible, we will gain the better performance.
>>>>>
>>>>> You access the same unmapped cache twice, i.e. repeated reads or
>>>>> writes against the same file offset?
>>>>
>>>> No.  We access the same mapped file twice.
>>>>
>>>>>
>>>>> How do you use fadvise?
>>>>
>>>> We access the header and content of the file respectively using read/write.
>>>> The header and content are sequentially.  So we use fadivse(2) with
>>>> FADV_WILLNEED flag to do a readahead.
>>>>
>>>>>> In addition, another factor also has some impacts for this application.
>>>>>> In inactive_file_is_low_global(), it is different between 2.6.18 and
>>>>>> upstream kernel.  IMHO, it causes that mapped file pages in active list
>>>>>> are moved into inactive list frequently.
>>>>>>
>>>>>> Currently, we add a parameter in inactive_file_is_low_global() to adjust
>>>>>> this ratio.  Meanwhile we activate every mapped file pages for the first
>>>>>> time.  Then the performance gets better, but it still doesn't reach the
>>>>>> performance of 2.6.18.
>>>>>
>>>>> 2.6.18 didn't have the active list protection at all and always
>>>>> forcibly deactivated pages during reclaim.  Have you tried fully
>>>>> reverting to this by making inactive_file_is_low_global() return true
>>>>> unconditionally?
>>>>
>>>> No, I don't try it.  AFAIK, 2.6.18 didn't protect the active list.  But
>>>> it doesn't always forcibly deactivate the pages.  I remember that in
>>>> 2.6.18 kernel we calculate 'mapped_ratio' in shrink_active_list(), and
>>>> then we get 'swap_tendency' according to 'mapped_ratio', 'distress', and
>>>> 'sc->swappiness'.  If 'swap_tendency' is not greater than 100.  It
>>>> doesn't reclaim mapped file pages.  By this equation, if the sum of the
>>>> anonymous pages and mapped file pages is not greater than the 50% of
>>>> total pages, we don't deactivate these pages.  Am I missing something?
>>>
>>> I think we need to go back to protecting mapped pages based on how
>>> much of reclaimable memory they make up, one way or another.
>>
>>
>> I partly agreed it with POV regression.
>> But I would like to understand rationale of "Why we should handle specially mmapped page".
>> In case of code pages(VM_EXEC), we already have handled it specially and
>> I understand why we did. At least, my opinion was that our LRU algorithm doesn't consider
>> _frequency_ fully while it does _recency_ well. I thought code page would be high frequency of access
>> compared to other pages.
>> But in case of mapped data pages, why we should handle specially?
>> I guess mapped data pages would have higher access chance than unmapped page because
>> unmapped page doesn't have any owner(it's just for caching for reducing I/O) while mapped page
>> has a owner above.
>>
>> Doesn't it make sense?
> 
> I agree that the reason behind protecting VM_EXEC pages was that our
> frequency information for mapped pages is at LRU cycle granularity.
> 
> But I don't see why you think this problem wouldn't apply to all
> mapped pages in general.


Code page is very likely to share by other processes so I think it's very special
than normal mmaped page. So I would like to raise bonus on code page than normal mmaped pages.
So I would like to make following as if we can.

Reclaim preference :
unmapped page >> mapped page > VM_EXEC mapped page

But at least, we can't solve Zheng's regression with current VM_EXEC protection logic
because it seems he already used VM_EXEC tric :(
I hope Erecalaimbe LRU list can solve it.

-- 
Kind regards,
Minchan Kim
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