On Mon, Jun 20, 2016 at 04:42:08PM +0900, Minchan Kim wrote: > On Fri, Jun 17, 2016 at 01:01:29PM -0400, Johannes Weiner wrote: > > On Fri, Jun 17, 2016 at 04:49:45PM +0900, Minchan Kim wrote: > > > On Thu, Jun 16, 2016 at 11:12:07AM -0400, Johannes Weiner wrote: > > > > On Wed, Jun 15, 2016 at 11:23:41AM +0900, Minchan Kim wrote: > > > > > Do we want to retain [1]? > > > > > > > > > > This patch motivates from swap IO could be much faster than file IO > > > > > so that it would be natural if we rely on refaulting feedback rather > > > > > than forcing evicting file cache? > > > > > > > > > > [1] e9868505987a, mm,vmscan: only evict file pages when we have plenty? > > > > > > > > Yes! We don't want to go after the workingset, whether it be cache or > > > > anonymous, while there is single-use page cache lying around that we > > > > can reclaim for free, with no IO and little risk of future IO. Anon > > > > memory doesn't have this equivalent. Only cache is lazy-reclaimed. > > > > > > > > Once the cache refaults, we activate it to reflect the fact that it's > > > > workingset. Only when we run out of single-use cache do we want to > > > > reclaim multi-use pages, and *then* we balance workingsets based on > > > > cost of refetching each side from secondary storage. > > > > > > If pages in inactive file LRU are really single-use page cache, I agree. > > > > > > However, how does the logic can work like that? > > > If reclaimed file pages were part of workingset(i.e., refault happens), > > > we give the pressure to anonymous LRU but get_scan_count still force to > > > reclaim file lru until inactive file LRU list size is enough low. > > > > > > With that, too many file workingset could be evicted although anon swap > > > is cheaper on fast swap storage? > > > > > > IOW, refault mechanisme works once inactive file LRU list size is enough > > > small but small inactive file LRU doesn't guarantee it has only multiple > > > -use pages. Hm, Isn't it a problem? > > > > It's a trade-off between the cost of detecting a new workingset from a > > stream of use-once pages, and the cost of use-once pages impose on the > > established workingset. > > > > That's a pretty easy choice, if you ask me. I'd rather ask cache pages > > to prove they are multi-use than have use-once pages put pressure on > > the workingset. > > Make sense. > > > > > Sure, a spike like you describe is certainly possible, where a good > > portion of the inactive file pages will be re-used in the near future, > > yet we evict all of them in a burst of memory pressure when we should > > have swapped. That's a worst case scenario for the use-once policy in > > a workingset transition. > > So, the point is how such case it happens frequently. A scenario I can > think of is that if we use one-cgroup-per-app, many file pages would be > inactive LRU while active LRU is almost empty until reclaim kicks in. > Because normally, parallel reclaim work during launching new app makes > app's startup time really slow. That's why mobile platform uses notifiers > to get free memory in advance via kiling/reclaiming. Anyway, once we get > amount of free memory and lauching new app in a new cgroup, pages would > live his born LRU list(ie, anon: active file: inactive) without aging. > > Then, activity manager can set memory.high of less important app-cgroup > to reclaim it with high value swappiness because swap device is much > faster on that system and much bigger anonymous pages compared to file- > backed pages. Surely, activity manager will expect lots of anonymous > pages be able to swap out but unlike expectation, he will see such spike > easily with reclaiming file-backed pages a lot and refault until inactive > file LRU is enough small. > > I think it's enough possible scenario in small system one-cgroup-per- > app. That's the workingset transition I was talking about. The algorithm is designed to settle towards stable memory patterns. We can't possibly remove one of the key components of this - the use-once policy - to speed up a few seconds of workingset transition when it comes at the risk of potentially thrashing the workingset for *hours*. The fact that swap IO can be faster than filesystem IO doesn't change this at all. The point is that the reclaim and refetch IO cost of use-once cache is ZERO. Causing swap IO to make room for more and more unused cache pages doesn't make any sense, no matter the swap speed. I really don't see the relevance of this discussion to this patch set. -- 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>