On Fri, Jun 28, 2019 at 10:16:13AM -0700, Yang Shi wrote: > On Fri, Jun 28, 2019 at 8:32 AM Christopher Lameter <cl@xxxxxxxxx> wrote: > > > > On Thu, 27 Jun 2019, Roman Gushchin wrote: > > > > > so that objects belonging to different memory cgroups can share the same page > > > and kmem_caches. > > > > > > It's a fairly big change though. > > > > Could this be done at another level? Put a cgoup pointer into the > > corresponding structures and then go back to just a single kmen_cache for > > the system as a whole? You can still account them per cgroup and there > > will be no cleanup problem anymore. You could scan through a slab cache > > to remove the objects of a certain cgroup and then the fragmentation > > problem that cgroups create here will be handled by the slab allocators in > > the traditional way. The duplication of the kmem_cache was not designed > > into the allocators but bolted on later. > > I'm afraid this may bring in another problem for memcg page reclaim. > When shrinking the slabs, the shrinker may end up scanning a very long > list to find out the slabs for a specific memcg. Particularly for the > count operation, it may have to scan the list from the beginning all > the way down to the end. It may take unbounded time. > > When I worked on THP deferred split shrinker problem, I used to do > like this, but it turns out it may take milliseconds to count the > objects on the list, but it may just need reclaim a few of them. I don't think the shrinker mechanism should be altered. Shrinker lists already contain individual objects, and I don't see any reasons, why these objects can't reside on a shared set of pages. What we're discussing is that it's way too costly (under some conditions) to have many sets of kmem_caches, if each of them is containing only few objects. Thanks!
