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

[Xishi Qiu <qiuxishi@huawei.com>]

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.
> 

Thank you for your reply.
I'll try to find some examples in my company. 

>  
>>> Use "echo 3 > /proc/sys/vm/drop_caches" will drop the whole cache, this will
>>> affect the performance, so it is used for debugging only. 
>>>
> 
> If you are able to drop the whole pagecache by issuing the command
> above, than it means the majority of it is just unmapped cache pages, 
> and those would be normally reclaimed upon demand by the PFRA. One more 
> thing that makes me wonder you're just seeing the effect of a leaky app 
> making the system unable to swap out anon pages.
> 

I find the page cache will only be reclaimed when there is not enough
memory. And in some smart phones, there is no swap disk.
So I add a parameter to reclaim in circles.

Thanks,
Xishi Qiu

> 
>>> suse has this feature, I tested it before, but it can not limit the page cache
>>> actually. So I rewrite the feature and add some parameters.
>>
>> The feature is there for historic reasons and I _really_ think the
>> interface is not appropriate. If there is a big pagecache usage which
>> affects other loads then Memory cgroup controller can be used to help
>> from interference.
>>
>>> Christoph Lameter has written a patch "Limit the size of the pagecache"
>>> http://marc.info/?l=linux-mm&m=116959990228182&w=2
>>> It changes in zone fallback, this is not a good way.
>>>
>>> The patchset is based on v3.15, it introduces two features, page cache limit
>>> and page cache reclaim in circles.
>>>
>>> Add four parameters in /proc/sys/vm
>>>
>>> 1) cache_limit_mbytes
>>> This is used to limit page cache amount.
>>> The input unit is MB, value range is from 0 to totalram_pages.
>>> If this is set to 0, it will not limit page cache.
>>> When written to the file, cache_limit_ratio will be updated too.
>>> The default value is 0.
>>>
>>> 2) cache_limit_ratio
>>> This is used to limit page cache amount.
>>> The input unit is percent, value range is from 0 to 100.
>>> If this is set to 0, it will not limit page cache.
>>> When written to the file, cache_limit_mbytes will be updated too.
>>> The default value is 0.
>>>
>>> 3) cache_reclaim_s
>>> This is used to reclaim page cache in circles.
>>> The input unit is second, the minimum value is 0.
>>> If this is set to 0, it will disable the feature.
>>> The default value is 0.
>>>
>>> 4) cache_reclaim_weight
>>> This is used to speed up page cache reclaim.
>>> It depend on enabling cache_limit_mbytes/cache_limit_ratio or cache_reclaim_s.
>>> Value range is from 1(slow) to 100(fast).
>>> The default value is 1.
>>>
>>> I tested the two features on my system(x86_64), it seems to work right.
>>> However, as it changes the hot path "add_to_page_cache_lru()", I don't know
>>> how much it will the affect the performance, maybe there are some errors
>>> in the patches too, RFC.
>>
>> I haven't looked at patches yet but you would need to explain why the
>> feature is needed much better and why the existing features are not
>> sufficient.
>> -- 
>> Michal Hocko
>> SUSE Labs
> 
> .
> 



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