On Tue 24-06-14 10:25:32, Xishi Qiu wrote: > On 2014/6/23 19:29, Michal Hocko wrote: [...] > > 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()). Yeah and that is the whole point. If you want to start to reclaim earlier because you need a bigger pillow for the free memory for sudden memory pressure then increase min_free_kbytes. > 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. Maybe _userspace_ allocates that much of memory but it is not faulted in/allocated by kernel in one shot. Besides that at the time you enter direct reclaim kswapd should be reclaiming memory to balance zones. So reclaiming SWAP_CLUSTER_MAX from the direct reclaim shouldn't matter that much. If it does then show us some numbers to prove it. SWAP_CLUSTER_MAX is kind of arbitrary number but I haven't seen any reclaim regression becuse of this value being too small AFAIR. -- 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>