On Tue, Apr 05, 2022 at 02:31:02PM -0700, Roman Gushchin wrote: > On Tue, Apr 05, 2022 at 01:58:59PM -0700, Yang Shi wrote: > > On Tue, Apr 5, 2022 at 9:36 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote: > > > On Tue, Apr 05, 2022 at 03:17:10PM +1000, Dave Chinner wrote: > > > > On Mon, Apr 04, 2022 at 12:08:25PM -0700, Roman Gushchin wrote: > > > > > On Mon, Apr 04, 2022 at 11:09:48AM +1000, Dave Chinner wrote: > > IMHO > > the number of really freed pages should be returned (I do understand > > it is not that easy for now), and returning 0 should be fine. > > It's doable, there is already a mechanism in place which hooks into > the slub/slab/slob release path and stops the slab reclaim as a whole > if enough memory was freed. The reclaim state that accounts for slab pages freed really needs to be first class shrinker state that is aggregated at the do_shrink_slab() level and passed back to the vmscan code. The shrinker infrastructure itself should be aware of the progress each shrinker is making - not just objects reclaimed but also pages reclaimed - so it can make better decisions about how much work should be done by each shrinker. e.g. lots of objects in cache, lots of objects reclaimed, no pages reclaimed is indicative of a fragmented slab cache. If this keeps happening, we should be trying to apply extra pressure to this specific cache because the only method we have for correcting a fragmented cache to return some memory is to reclaim lots more objects from it. > > The > > current logic (returning the number of objects) may feed up something > > over-optimistic. I, at least, experienced once or twice that a > > significant amount of slab caches were shrunk, but actually 0 pages > > were freed actually. TBH the new slab controller may make it worse > > since the page may be pinned by the objects from other memcgs. > > Of course, the more dense the placement of objects is, the harder is to get > the physical pages back. But usually it pays off by having a dramatically > lower total number of slab pages. Unless you have tens of millions of objects in the cache. The dentry cache is a prime example of this "lots of tiny cached objects" where we have tens of objects per slab page and so can suffer badly from internal fragmentation.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
