The existing slab memory controller is based on the idea of replicating slab allocator internals for each memory cgroup. This approach promises a low memory overhead (one pointer per page), and isn't adding too much code on hot allocation and release paths. But is has a very serious flaw: it leads to a low slab utilization. Using a drgn* script I've got an estimation of slab utilization on a number of machines running different production workloads. In most cases it was between 45% and 65%, and the best number I've seen was around 85%. Turning kmem accounting off brings it to high 90s. Also it brings back 30-50% of slab memory. It means that the real price of the existing slab memory controller is way bigger than a pointer per page. The real reason why the existing design leads to a low slab utilization is simple: slab pages are used exclusively by one memory cgroup. If there are only few allocations of certain size made by a cgroup, or if some active objects (e.g. dentries) are left after the cgroup is deleted, or the cgroup contains a single-threaded application which is barely allocating any kernel objects, but does it every time on a new CPU: in all these cases the resulting slab utilization is very low. If kmem accounting is off, the kernel is able to use free space on slab pages for other allocations. Arguably it wasn't an issue back to days when the kmem controller was introduced and was an opt-in feature, which had to be turned on individually for each memory cgroup. But now it's turned on by default on both cgroup v1 and v2. And modern systemd-based systems tend to create a large number of cgroups. This patchset provides a new implementation of the slab memory controller, which aims to reach a much better slab utilization by sharing slab pages between multiple memory cgroups. Below is the short description of the new design (more details in commit messages). Accounting is performed per-object instead of per-page. Slab-related vmstat counters are converted to bytes. Charging is performed on page-basis, with rounding up and remembering leftovers. Memcg ownership data is stored in a per-slab-page vector: for each slab page a vector of corresponding size is allocated. To keep slab memory reparenting working, instead of saving a pointer to the memory cgroup directly an intermediate object is used. It's simply a pointer to a memcg (which can be easily changed to the parent) with a built-in reference counter. This scheme allows to reparent all allocated objects without walking them over and changing memcg pointer to the parent. Instead of creating an individual set of kmem_caches for each memory cgroup, two global sets are used: the root set for non-accounted and root-cgroup allocations and the second set for all other allocations. This allows to simplify the lifetime management of individual kmem_caches: they are destroyed with root counterparts. It allows to remove a good amount of code and make things generally simpler. The patchset contains a couple of semi-independent parts, which can find their usage outside of the slab memory controller too: 1) subpage charging API, which can be used in the future for accounting of other non-page-sized objects, e.g. percpu allocations. 2) mem_cgroup_ptr API (refcounted pointers to a memcg, can be reused for the efficient reparenting of other objects, e.g. pagecache. The patchset has been tested on a number of different workloads in our production. In all cases, it saved hefty amounts of memory: 1) web frontend, 650-700 Mb, ~42% of slab memory 2) database cache, 750-800 Mb, ~35% of slab memory 3) dns server, 700 Mb, ~36% of slab memory So far I haven't found any regression on all tested workloads, but potential CPU regression caused by more precise accounting is a concern. Obviously the amount of saved memory depend on the number of memory cgroups, uptime and specific workloads, but overall it feels like the new controller saves 30-40% of slab memory, sometimes more. Additionally, it should lead to a lower memory fragmentation, just because of a smaller number of non-movable pages and also because there is no more need to move all slab objects to a new set of pages when a workload is restarted in a new memory cgroup. * https://github.com/osandov/drgn Roman Gushchin (14): mm: memcg: subpage charging API mm: memcg: introduce mem_cgroup_ptr mm: vmstat: use s32 for vm_node_stat_diff in struct per_cpu_nodestat mm: vmstat: convert slab vmstat counter to bytes mm: memcg/slab: allocate space for memcg ownership data for non-root slabs mm: slub: implement SLUB version of obj_to_index() mm: memcg/slab: save memcg ownership data for non-root slab objects mm: memcg: move memcg_kmem_bypass() to memcontrol.h mm: memcg: introduce __mod_lruvec_memcg_state() mm: memcg/slab: charge individual slab objects instead of pages mm: memcg: move get_mem_cgroup_from_current() to memcontrol.h mm: memcg/slab: replace memcg_from_slab_page() with memcg_from_slab_obj() mm: memcg/slab: use one set of kmem_caches for all memory cgroups mm: slab: remove redundant check in memcg_accumulate_slabinfo() drivers/base/node.c | 11 +- fs/proc/meminfo.c | 4 +- include/linux/memcontrol.h | 102 ++++++++- include/linux/mm_types.h | 5 +- include/linux/mmzone.h | 12 +- include/linux/slab.h | 3 +- include/linux/slub_def.h | 9 + include/linux/vmstat.h | 8 + kernel/power/snapshot.c | 2 +- mm/list_lru.c | 12 +- mm/memcontrol.c | 431 +++++++++++++++++++++-------------- mm/oom_kill.c | 2 +- mm/page_alloc.c | 8 +- mm/slab.c | 37 ++- mm/slab.h | 300 +++++++++++++------------ mm/slab_common.c | 449 ++++--------------------------------- mm/slob.c | 12 +- mm/slub.c | 63 ++---- mm/vmscan.c | 3 +- mm/vmstat.c | 38 +++- mm/workingset.c | 6 +- 21 files changed, 683 insertions(+), 834 deletions(-) -- 2.21.0
