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SubjectRe: [RFC] libcg: design and plans
Hi,

I was wonder if creating such library makes any sense at all,
considering the nature of cgroups, the way they work and their possible
application?
It seems to me that any attempt to create a 'simple' API would actualy
result in something that will be much harder to use that just making raw
mkdir/open/read/write/close operations. Another thing is suggested
config for this lib would be more appropriate for a daemon rather than a
library.
In general - cgroup is a very flexible subsystem that can be used in a
wide variety of ways and modes and trying to create a universal simple
API would more likely result in something hard to manage and work with.
The idea of having multiple container managers (applications that use
libcg) creates a lots of questions and possible issues. Containers are
supposed provide a flexible resource management and task grouping
ability, which somewhat implies that there cannot be two different
resource managers per node without one knowing well the
desires/goals/methods of the other and vice versa. And should there be
only one manager per node - probably it will be easier for it to use
cgroup subsystem directly rather than using a wrapper library?

Regards,
Peter Litov.

Dhaval Giani ??????:
> Hi,
> We have been working on a library for control groups which would provide
> simple APIs for programmers to utilize from userspace and make use of
> control groups.
>
> We are still designing the library and the APIs. I've attached the
> design (as of now) to get some feedback from the community whether we
> are heading in the correct direction and what else should be addressed.
>
> We have a project on sourceforge.net at
> https://sourceforge.net/projects/libcg and the mailing list (cc'd here)
> can be found at https://lists.sourceforge.net/lists/listinfo/libcg-devel
>
> Thanks,
> --
>
> libcg
> 1. Aims/Requirements
> 2. Design
> 3. APIs
> 4. Configuration Scheme
>
> 1. Aims/Requirements
>
> 1.1 What are Control Groups
>
> Control Groups provide a mechanism for aggregating/partitioning sets of
> tasks, and all their future children, into hierarchical groups with
> specialized behaviour [1]. It makes use of a filesystem interface.
>
> 1.2 Aims of libcg
>
> libcg aims to provide programmers easily usable APIs to use the control
> group file system. It should satisfy the following requirements
>
> 1.2.1. Provide a programmable interface for cgroups
>
> This should allow applications to create cgroups using something like
> create_cgroup() as opposed to having to go the whole filesystem route.
>
> 1.2.2. Provide persistent configuration across reboots
>
> Control Groups have a lifetime of only one boot cycle. The configuration
> is lost at reboot. Userspace needs to handle this issue. This is handled
> by libcg
>
> 1.2.3. Provide a programmable interface for manipulating configurations
>
> This should allow libcg to handle changing application requirements. For
> example, while gaming, you might want to reduce the cpu power of other
> groups whereas othertimes you would want greater CPU power for those
> groups.
>
> 2. Design
>
> 2.1 Architecture
>
> 2.1.1 Global overview
>
> libcg will be consumed in the following fashion
>
> ---------------------------
> | applications |
> ---------------------------
> | libcg |
> ---------------------------
> | kernel |
> ---------------------------
>
> A more detailed example would be as follows. Consider various applications
> running at the same time on a system. A typical system would be running
> a web browser, a mail client, a media player and office software. libcg
> could be used to group these applications into various groups and give
> them various resources. A possible example would be three groups, Internet,
> Entertainment and Office. A daemon could attach tasks to these groups according
> to some rules and the adminstrator can control the resources attached to each
> group via the configuration manager.i
>
> Internet Office Entertainment
> ---------------------- -------------- -----------
> | firefox mutt | | openoffice | | mplayer |
> ---------------------- -------------- -----------
> \ | /
> \ | /
> ------------------------------------------------------
> | Some daemon |
> ------------------------------------------------------
> | libcg |
> ------------------------------------------------------
> | kernel |
> ------------------------------------------------------
>
> 2.1.2
>
> libcg will consist of two main parts. The configuration manager and the
> library.
>
> The configuration manager will used to maintain the configurations, to load
> and unload the configurations, to set the bootup configurations and so on.
> This is similar to the network configuration. A configuration file is used
> to setup the networking at bootup. Similarly a configuration file will be
> used to setup the default control groups (and maybe the top level control
> groups) at bootup.
>
> The administrator can directly access the configuration files, and
> applications can access it through the library. The configuration manager
> is used to provide the persistence.
>
> Application Administrator
> | |
> V |
> library APIs |
> \ /
> \ /
> \ /
> V V
> libcg configuration files
> |
> |
> V
> libcg configuration manager
>
> The configuration manager has to provide isolation between various users of
> libcg. That is, if two different users A and B are making use of libcg, then
> the configuration manager has to ensure that user A does not affect user B's
> settings/configurations.
>
> The top level limits and permissions for A and B are to be provided by the
> administrator. The permissions are filesystem permissions as cgroup is
> filesystem based.
>
> With this architecure in mind, we expect two levels of configuration files.
> One would be the global configuration which the administrator would control
> and setup the groups, and a local configuration which the group owner will
> control.
>
> A simple example could be that the administrator could split the top level
> according to uid, and then each user could control the resources available
> to him and group those applications accordingly.
>
> root
> |
> -------------------------------------------------
> | |
> A B
> |- browsers |- compilers
> |- games |- internet
> |- office |- dev-environment
> |- entertainment |- others
>
> In this example, we have an example cgroup filesystem configuration.
>
> The administrator decides the resources available to "A" and "B". Both "A" and
> "B" have followed grouping according to their usage. They decide the resources
> availble to their groups (which is dependent of the resources alloted to them
> by the adminstrator).
>
> libcg will be written mainly in C with lex and yacc for parsing the configuration
> files.
>
> 3. APIs
>
> The APIs are envisaged to be of two main types
>
> 3.1. Manipulating Control Groups
>
> 3.1.1. Create Control Group: This API is proposed to create control groups.
> It should take care of the following scenarios
>
> 3.1.1.1 Create non persistent control groups: These groups should exist
> for just duration of this run. They should not stick across different sessions.
>
> 3.1.1.2 Create persistent control groups: These groups should stick across
> different sessions.
>
> 3.1.2 Delete Control Group: This API is proposed to delete control groups.
> It would have the same scenarios as expected for Create Control Group.
>
> 3.1.2 Modify Control Group: This API proposed to modify an already
> existing group's control files. It too should handle the persistence issue
> as like Create Control Group does.
>
> More details about configuration are available in sections 2 and 4.
>
> 3.2. Manipulating Configurations
>
> 3.2.1. Generate Configuration File: If a cgroup filesystem hierarchy already exists,
> it should be possible to generate a configuration file which can create it. This
> is proposed to be provided by this API.
>
> 3.2.2. Change Configuration File: If one configuration is currently loaded in
> memory, it is possible for it to be replaced with the new file. This API proposes
> to implement that.
>
> 3.2.3. Manipulate Configuration File: This API proposes to allow the configuration
> file to be modified.
>
> We should also plan on taking care of statistics once its available in mainline.
>
> 4. Configuration Scheme
>
> There are multiple configuration levels. The basic wlm.conf file will provide
> the mount points and the controller details. This can only be manipulated by
> the adminstrator. No APIs will be provided to modify this file.
>
> There will be group specific configuration files as well. The exact details
> of the same still need to be worked out.
>
> 4.1. Sample configuration files
>
> 4.1.1. Sample wlm.conf
>
> #
> # controller file
> #
>
> group ca1 {
> perm {
> task {
> uid = balbir;
> gid = cgroup;
> }
> admin {
> uid = root;
> gid = cgroup;
> }
> }
>
> cpu {
> cpu.shares = 500;
> }
> }
>
> mount {
> cpu = /container;
> }
>
> This is an example of a top level group. The mount{} block is used to provide
> the mount point of the various controllers. For eg, the cpu controller is
> mounted at /container. Next we have the group ca1. This is the top level group
> and its permissions are given by the uid and gid fields for tasks and admin. The
> next is the individual controller block. For the mount point of cpu, the cpu.shares
> value is provided. Thus the above file can be represented as the following script
>
> mkdir /container
> mount -t cgroup -o cpu none /container
> mkdir /container/ca1
> /bin/echo 500 > /container/ca1/cpu.shares
> chown -R root /container/ca1
> chgrp -R cgroup /container/ca1
> chown balbir /container/ca1/tasks
> chgrp cgroup /container/ca1/tasks
>
> 5. References
>
> 1. Documentation/cgroups.txt in kernel sources.
>


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