Re: [-mm][PATCH 1/4] Add memrlimit controller documentation (v5)

[Andrew Morton]

On Wed, 21 May 2008 20:59:37 +0530 Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:

> 
> 
> Documentation patch - describes the goals and usage of the memrlimit
> controller.
> 
> 
> Signed-off-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
> ---
> 
>  Documentation/controllers/memrlimit.txt |   29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff -puN /dev/null Documentation/controllers/memrlimit.txt
> --- /dev/null	2008-05-16 21:23:36.290004010 +0530
> +++ linux-2.6.26-rc2-balbir/Documentation/controllers/memrlimit.txt	2008-05-21 20:53:33.000000000 +0530
> @@ -0,0 +1,29 @@
> +This controller is enabled by the CONFIG_CGROUP_MEMRLIMIT_CTLR option. Prior
> +to reading this documentation please read Documentation/cgroups.txt and
> +Documentation/controllers/memory.txt. Several of the principles of this
> +controller are similar to the memory resource controller.
> +
> +This controller framework is designed to be extensible to control any
> +memory resource limit with little effort.
> +
> +This new controller, controls the address space expansion of the tasks
> +belonging to a cgroup. Address space control is provided along the same lines as
> +RLIMIT_AS control, which is available via getrlimit(2)/setrlimit(2).

Still would like to see more details here.  RLIMIT_AS is simple because
it applies to a single mm.  But a control group may contain multiple
mm's, and those mm's can share stuff.

The sharing probably isn't important in this situation, where we're
only concerned with virtual address space size, rather than real
memory.  But still, readers will be wondering about this.

They also will be wondering about the handling of threads versus
heavyweight processes.  _we_ know that, but users and administrators
may not.


> +The interface for controlling address space is provided through
> +"rlimit.limit_in_bytes". The file is similar to "limit_in_bytes" w.r.t. the user
> +interface. Please see section 3 of the memory resource controller documentation
> +for more details on how to use the user interface to get and set values.
> +
> +The "memrlimit.usage_in_bytes" file provides information about the total address
> +space usage of the tasks in the cgroup, in bytes.
> +
> +Advantages of providing this feature
> +
> +1. Control over virtual address space allows for a cgroup to fail gracefully
> +   i.e., via a malloc or mmap failure as compared to OOM kill when no
> +   pages can be reclaimed.

Well it's more than "i.e.".  It would be better to precisely spell out
the behaviour of this feature.  It's mmap() and brk() only, is it not? 
malloc() isn't a system call ;)

Ideally, either the changelog, this document or code comments should be
sufficient for a manpage to be written.  And this level of description
will lead to better code review and quite likely a better feature.

> +2. It provides better control over how many pages can be swapped out when
> +   the cgroup goes over its limit. A badly setup cgroup can cause excessive
> +   swapping. Providing control over the address space allocations ensures
> +   that the system administrator has control over the total swapping that
> +   can take place.

IOW: what, exactly and completely, does this feature do?
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