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

Balbir Singh
2008-05-22T10:14:56Z

Andrew Morton wrote:
> On Wed, 21 May 2008 20:59:37 +0530 Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
>>
>> Documentation patch - describes the goals and usage of the memrlimit
>> controller.
>>
>>
>> Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
>> ---
>>
>> 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 ;)
>

Yes, true. I thought about wording it that way, but end users understand mmap()
and malloc() better that mmap with MAP_ANONYMOUS or brk(), sbrk().

> 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.
>

That makes a lot of sense. I'll work on enhancing this documentation.

>> +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?

I'll send an update with more details with an example if possible.

--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL