cgroup information proc file format

Daniel Lezcano daniel.lezcano at free.fr
Thu Aug 11 14:45:58 PDT 2011


On 08/11/2011 11:30 PM, Glauber Costa wrote:
> On 08/11/2011 05:55 PM, Daniel Lezcano wrote:
>> Hi all,
>>
>> the cgroup cpuset and memory reduce access to a part of the resources on
>> the system. Some applications use the /proc/cpuinfo and /proc/meminfo to
>> allocate the resources. For instance, HPC jobs look at /proc/cpuinfo to
>> fork the number of cpu found in this file either look at /proc/meminfo
>> to allocate a big chunk of memory. Each process set the affinity on each
>> cpu, which in case a subset of cpus is used, some affinity will fail.
>>
>> In the case of the container, the cgroup is used to reduce the memory or
>> to assign a cpu to the container. Unfortunately, as this partitioning is
>> not reflected in /proc, the different system tools (ps, top, free, ...)
>> show a wrong information.
>>
>> I was wondering if that would make sense to create for the different
>> cgroup subsystem, when it is relevant, a proc formatted file we can bind
>> mount /proc.
>>
>> For example: /cgroup/memory.proc and /cgroup/cpuset.proc
>
> Not only that. user/sys/nice,etc statistics also are expected to be
> different than the main system one, among other things.
>
> One way I was thinking of doing it, was to always show per-cgroup
> data in /proc files when relevant, using the cgroup of the current
> process as a base.

That was proposed initially but refused. I tried to do that from
userspace with a fuse filesystem and by translating the cgroup
information into proc information. I was proud of the result but I
noticed fuse is not really friendly with us for the containers: adds a
lot of processes, does not support some file operations and adds an
important overhead, so I gave up because it leads to a deadend.

http://lxc.sourceforge.net/download/procfs/

>
> bind mounting proc files from their cgroup is a nice alternative,
> though. But it leaves the possibility of any user of it not setting it
> up. 

AFAIK, an user can set up an cgroup, so I guess it is up to the cgroup
creator to handle that.

> Although it is certainly more flexible, it makes me wonder if a
> constrained process should ever know about resources it can't access...
>
> If bind mounts are used, I'd suggest we represent them as directories,
> like cpuset.proc/cpuinfo. (It is not clear for me what exactly you meant
> in your proposal, sorry if it was just that). 

Well this can not be organized in directory because a directory is a
cgroup :)
The naming was an example, that would make more sense to name them
cpuset.cpuinfo and memory.meminfo.

> This might make things easier to setup an isolated /proc, since we can
> convention that anything
> in this format is available to be bind mounted, instead of having to
> convention that /cgroup/cpuset.proc means /proc/cpuinfo (May matter
> for future features)


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