Re: [PATCH 00/16] The new slab memory controller

From: Waiman Long
Date: Fri Oct 18 2019 - 13:03:58 EST

On 10/17/19 8:28 PM, Roman Gushchin wrote:
> 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.

In the case of slub memory allocator, it is not just unused space within
a slab. It is also the use of per-cpu slabs that can hold up a lot of
memory, especially if the tasks jump around to different cpus. The
problem is compounded if a lot of memcgs are being used. Memory
utilization can improve quite significantly if per-cpu slabs are
disabled. Of course, it comes with a performance cost.