Re: [PATCH 0/8] mm: add page cache limit and reclaim feature

[Xishi Qiu]

On 2014/6/23 19:29, Michal Hocko wrote:

> On Mon 23-06-14 10:05:48, Xishi Qiu wrote:
>> On 2014/6/20 23:32, Michal Hocko wrote:
>>
>>> On Fri 20-06-14 15:56:56, Xishi Qiu wrote:
>>>> On 2014/6/17 9:35, Xishi Qiu wrote:
>>>>
>>>>> On 2014/6/16 20:50, Rafael Aquini wrote:
>>>>>
>>>>>> On Mon, Jun 16, 2014 at 01:14:22PM +0200, Michal Hocko wrote:
>>>>>>> On Mon 16-06-14 17:24:38, Xishi Qiu wrote:
>>>>>>>> When system(e.g. smart phone) running for a long time, the cache often takes
>>>>>>>> a large memory, maybe the free memory is less than 50M, then OOM will happen
>>>>>>>> if APP allocate a large order pages suddenly and memory reclaim too slowly. 
>>>>>>>
>>>>>>> Have you ever seen this to happen? Page cache should be easy to reclaim and
>>>>>>> if there is too mach dirty memory then you should be able to tune the
>>>>>>> amount by dirty_bytes/ratio knob. If the page allocator falls back to
>>>>>>> OOM and there is a lot of page cache then I would call it a bug. I do
>>>>>>> not think that limiting the amount of the page cache globally makes
>>>>>>> sense. There are Unix systems which offer this feature but I think it is
>>>>>>> a bad interface which only papers over the reclaim inefficiency or lack
>>>>>>> of other isolations between loads.
>>>>>>>
>>>>>> +1
>>>>>>
>>>>>> It would be good if you could show some numbers that serve as evidence
>>>>>> of your theory on "excessive" pagecache acting as a trigger to your
>>>>>> observed OOMs. I'm assuming, by your 'e.g', you're running a swapless
>>>>>> system, so I would think your system OOMs are due to inability to
>>>>>> reclaim anon memory, instead of pagecache.
>>>>>>
>>>>
>>>> I asked some colleagues, when the cache takes a large memory, it will not
>>>> trigger OOM, but performance regression. 
>>>>
>>>> It is because that business process do IO high frequency, and this will 
>>>> increase page cache. When there is not enough memory, page cache will
>>>> be reclaimed first, then alloc a new page, and add it to page cache. This
>>>> often takes too much time, and causes performance regression.
>>>
>>> I cannot say I would understand the problem you are describing. So the
>>> page cache eats the most of the memory and that increases allocation
>>> latency for new page cache? Is it because of the direct reclaim?
>>
>> Yes, allocation latency causes performance regression.
> 
> This doesn't make much sense to me. So you have a problem with latency
> caused by direct reclaim so you add a new way of direct page cache
> reclaim.
> 
>> A user process produces page cache frequently, so free memory is not
>> enough after running a long time. Slow path takes much more time because 
>> direct reclaim. And kswapd will reclaim memory too, but not much. Thus it
>> always triggers slow path. this will cause performance regression.
> 
> If I were you I would focus on why the reclaim doesn't catch up with the
> page cache users. The mechanism you are proposing in unacceptable.

Hi Michal,

Do you mean why the reclaim is slower than page cache increase?

I think there are two reasons:
1. kswapd and direct_reclaim will be triggered only when there is not 
enough memory(e.g. __alloc_pages_slowpath()). That means it will not 
reclaim when memory is enough(e.g. get_page_from_freelist()).
2. __alloc_pages_direct_reclaim
	try_to_free_pages
		nr_to_reclaim = SWAP_CLUSTER_MAX
And "#define SWAP_CLUSTER_MAX 32UL", that means it expect to reclaim 32
pages. It is too few, if we alloc 2^10 pages in one time.

Thanks,
Xishi Qiu

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