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

[Michal Hocko]

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.
-- 
Michal Hocko
SUSE Labs
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