Re: [PATCH v2] mm: fix ununiform page status when writing new file with small buffer

[Robin Dong <hao.bigrat@gmail.com>]

2012/6/11 KOSAKI Motohiro <kosaki.motohiro@gmail.com>:
> (6/11/12 6:42 AM), Robin Dong wrote:
>> From: Robin Dong<sanbai@taobao.com>
>>
>> When writing a new file with 2048 bytes buffer, such as write(fd, buffer, 2048), it will
>> call generic_perform_write() twice for every page:
>>
>>       write_begin
>>       mark_page_accessed(page)
>>       write_end
>>
>>       write_begin
>>       mark_page_accessed(page)
>>       write_end
>>
>> The page 1~13th will be added to lru-pvecs in write_begin() and will *NOT* be added to
>> active_list even they have be accessed twice because they are not PageLRU(page).
>> But when page 14th comes, all pages in lru-pvecs will be moved to inactive_list
>> (by __lru_cache_add() ) in first write_begin(), now page 14th *is* PageLRU(page).
>> And after second write_end() only page 14th  will be in active_list.
>>
>> In Hadoop environment, we do comes to this situation: after writing a file, we find
>> out that only 14th, 28th, 42th... page are in active_list and others in inactive_list. Now
>> kswapd works, shrinks the inactive_list, the file only have 14th, 28th...pages in memory,
>> the readahead request size will be broken to only 52k (13*4k), system's performance falls
>> dramatically.
>>
>> This problem can also replay by below steps (the machine has 8G memory):
>>
>>       1. dd if=/dev/zero of=/test/file.out bs=1024 count=1048576
>>       2. cat another 7.5G file to /dev/null
>>       3. vmtouch -m 1G -v /test/file.out, it will show:
>>
>>       /test/file.out
>>       [oooooooooooooooooooOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO] 187847/262144
>>
>>       the 'o' means same pages are in memory but same are not.
>>
>>
>> The solution for this problem is simple: the 14th page should be added to lru_add_pvecs
>> before mark_page_accessed() just as other pages.
>>
>> Signed-off-by: Robin Dong<sanbai@taobao.com>
>> Reviewed-by: Minchan Kim<minchan@kernel.org>
>> ---
>>   mm/swap.c |    8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/swap.c b/mm/swap.c
>> index 4e7e2ec..08e83ad 100644
>> --- a/mm/swap.c
>> +++ b/mm/swap.c
>> @@ -394,13 +394,19 @@ void mark_page_accessed(struct page *page)
>>   }
>>   EXPORT_SYMBOL(mark_page_accessed);
>>
>> +/*
>> + * Check pagevec space before adding new page into as
>> + * it will prevent ununiform page status in
>> + * mark_page_accessed() after __lru_cache_add()
>> + */
>>   void __lru_cache_add(struct page *page, enum lru_list lru)
>>   {
>>       struct pagevec *pvec =&get_cpu_var(lru_add_pvecs)[lru];
>>
>>       page_cache_get(page);
>> -     if (!pagevec_add(pvec, page))
>> +     if (!pagevec_space(pvec))
>>               __pagevec_lru_add(pvec, lru);
>> +     pagevec_add(pvec, page);
>>       put_cpu_var(lru_add_pvecs);
>>   }
>>   EXPORT_SYMBOL(__lru_cache_add);
>
> No change from v1?
>

Adding function comment from Minchan Kim's suggestion.

I know that the best solution may be removing all pagevecs completely,
 as you say,
but removing pagevecs would be a very very long-term subject (I guess)  because
many developers will argue it again and again before coming to compromise.
I don't think I have the power to make a so big change,
so...."hacking" the __lur_cache_add
would be a good solution recently, at least in many Hadoop Clusters  :)

-- 
--
Best Regard
Robin Dong

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