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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>