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

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Date:   Fri, 18 Oct 2019 13:03:54 -0400
From:   Waiman Long <longman@...hat.com>
To:     Roman Gushchin <guro@...com>, linux-mm@...ck.org
Cc:     Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH 00/16] The new slab memory controller

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^
> 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.

Cheers,
Longman