Re: [PATCH v2 7/7] Documentation: cgroup: add memory.swap.{current,max} description

Michal Hocko
2016-01-13T17:02:55Z

On Thu 17-12-15 15:30:00, Vladimir Davydov wrote:
> The rationale of separate swap counter is given by Johannes Weiner.
>
> Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> Changes in v2:
> - Add rationale of separate swap counter provided by Johannes.
>
> Documentation/cgroup.txt | 33 +++++++++++++++++++++++++++++++++

this went to Documentation/cgroup-v2.txt with the latest Tejun's pull
request.

> 1 file changed, 33 insertions(+)
>
> diff --git a/Documentation/cgroup.txt b/Documentation/cgroup.txt
> index 31d1f7bf12a1..f441564023e1 100644
> --- a/Documentation/cgroup.txt
> +++ b/Documentation/cgroup.txt
> @@ -819,6 +819,22 @@ PAGE_SIZE multiple when read back.
> the cgroup. This may not exactly match the number of
> processes killed but should generally be close.
>
> + memory.swap.current
> +
> + A read-only single value file which exists on non-root
> + cgroups.
> +
> + The total amount of swap currently being used by the cgroup
> + and its descendants.
> +
> + memory.swap.max
> +
> + A read-write single value file which exists on non-root
> + cgroups. The default is "max".
> +
> + Swap usage hard limit. If a cgroup's swap usage reaches this
> + limit, anonymous meomry of the cgroup will not be swapped out.
> +
>
> 5-2-2. General Usage
>
> @@ -1291,3 +1307,20 @@ allocation from the slack available in other groups or the rest of the
> system than killing the group. Otherwise, memory.max is there to
> limit this type of spillover and ultimately contain buggy or even
> malicious applications.
> +
> +The combined memory+swap accounting and limiting is replaced by real
> +control over swap space.
> +
> +The main argument for a combined memory+swap facility in the original
> +cgroup design was that global or parental pressure would always be
> +able to swap all anonymous memory of a child group, regardless of the
> +child's own (possibly untrusted) configuration. However, untrusted
> +groups can sabotage swapping by other means - such as referencing its
> +anonymous memory in a tight loop - and an admin can not assume full
> +swappability when overcommitting untrusted jobs.
> +
> +For trusted jobs, on the other hand, a combined counter is not an
> +intuitive userspace interface, and it flies in the face of the idea
> +that cgroup controllers should account and limit specific physical
> +resources. Swap space is a resource like all others in the system,
> +and that's why unified hierarchy allows distributing it separately.
> --
> 2.1.4
>
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--
Michal Hocko
SUSE Labs