Re: [PATCH] Add a document describing the resource counterabstraction

[Randy Dunlap]

On Wed, 12 Mar 2008 13:25:20 +0300 Pavel Emelyanov wrote:

> The resource counter is supposed to facilitate the resource accounting
> of arbitrary resource (and it already does this for memory controller).
> 
> However, it is about to be used in other resources controllers (swap,
> kernel memory, networking, etc), so provide a doc describing how to
> work with it.  This will eliminate all the possible future duplications
> in the appropriate controllers' docs.
> 
> Signed-off-by: Pavel Emelyanov <xemul@xxxxxxxxxx>
> 
> ---
> 
> diff --git a/Documentation/controllers/resource_counter.txt b/Documentation/controllers/resource_counter.txt
> new file mode 100644
> index 0000000..99f16c6
> --- /dev/null
> +++ b/Documentation/controllers/resource_counter.txt
> @@ -0,0 +1,173 @@
> +
> +		The Resource Counter
> +
> +The resource counter, declared at include/linux/res_counter.h

                                     include/linux/res_counter.h,

> +is supposed to facilitate the resource management by controllers
> +by providing common stuff for accounting.
> +
> +This "stuff" includes the res_counter structure and routines
> +to work with it.
> +
> +
> +
> +1. Crucial parts of the res_counter structure
> +
> + a. unsigned long long usage
> +
> + 	The usage value shows the amount of a resource that is consumed
> +	by a group at a given time. The units of measurement should be
> +	determined by the controller, that uses this counter. E.g. it can

                          controller that

> +	be bytes, items or any other unit the controller operates on.
> +
> + b. unsigned long long max_usage
> +
> + 	The maximal value of the usage over the time.

                                       over time.

> +
> + 	This value is useful when gathering a statistical information about

                                  gathering statistical information

> +	the particular group, as it shows the actual resource requirements
> +	for a particular group, not just some usage snapshot.
> +
> + c. unsigned long long limit
> +
> + 	The maximal allowed amount of resource to consume by the group. In
> +	case the group requests for more resources, so that the usage value
> +	would exceed the limit, the resource allocation is rejected (see
> +	the next section)

                 section).

> +
> + d. unsigned long long failcnt
> +
> + 	The failcnt stands for "failures counter". This is the number of
> +	resource allocation attempts, that failed.

                            attempts that failed.

> +
> + c. spinlock_t lock
> +
> + 	Protects changes of the above values.
> +
> +
> +
> +2. Basic accounting routines
> +
> + a. void res_counter_init(struct res_counter *rc)
> +
> + 	Initializes the resource counter. As usual, should be the first
> +	routine called for a new counter.
> +
> + b. int res_counter_charge[_locked]
> +			(struct res_counter *rc, unsigned long val)
> +
> +	When a resource is about to be allocated it has to be accounted
> +	with the appropriate resource counter (controller should determine
> +	which one to use by its own). This operation is called "charging".

                         on its own).

	This description implies (at least to me) that
	res_counter_charge[_locked]() should be called for accounting
	before the actual allocation is performed.  Right?
	So if the allocation fails, then the uncharge function below
	should also be called...

> +
> + c. void res_counter_uncharge[_locked]
> +			(struct res_counter *rc, unsigned long val)
> +
> +	When a resource is released (freed) is should be de-accounted
> +	from the resource counter it was accounted to.  This is called
> +	"uncharging".
> +
> +    The _locked routines imply that the res_counter->lock is taken.
> +
> +
> + 2.1 Other accounting routines
> +
> +    There are more routines that may help you with the common need, like

                                                 with common needs, like

> +    checking whether the limit is reached or re-setting the max_usage

                                                resetting

> +    value. They are all declared in include/linux/res_counter.h

end with period.

> +
> +
> +
> +3. Analyzing the resource counter registrations
> +
> + a. If the failcnt value constantly grows this means, that the counter's

       If the failcnt value constantly grows, this means that the counter's

> +    limit is too tight. Either the group is misbehaving and consumes too
> +    many resources, or the configuration is not suitable for the group
> +    and the limit should be increased.
> +
> + b. The max_usage value can be used to quickly tune the group. One may
> +    set the limits to maximal values and either load the container with
> +    a common patterns or leave one for a while. After this the max_usage

                pattern

> +    value shows the amount of memory the container would require during
> +    its common activity.
> +
> +    Setting the limit a bit above this value gives a pretty good
> +    configuration, that works in most of the cases.

No comma.

> +
> + c. If the max_usage is much less than the limit, but the failcnt value
> +    is growing, then the group tries to allocate a big chunk of resource
> +    at once.
> +
> + d. If the max_usage is much less than the limit, but the failcnt value
> +    is 0, then this group is given too high limit, that it does not
> +    require. It is better to lower the limit a bit leaving more resource
> +    for other groups.
> +
> +
> +
> +4. Communication with the control groups subsystem (cgroups)
> +
> +All the resource controllers, that are using cgroups and resource

No comma.

> +counters should provide files to work with the resource counter fields.

Is this in some cgroup filesystem or in sysfs?


> +They are recommended to adhere to the following rules:
> +
> + a. File names
> +
> + 	Field name	File name
> +	---------------------------------------------------
> +	usage		usage_in_<unit_of_measurement>
> +	max_usage	max_usage_in_<unit_of_measurement>
> +	limit		limit_in_<unit_of_measurement>
> +	failcnt		failcnt
> +	lock		no file :)
> +
> + b. Reading from file should show the according field value in the 

s/according/corresponding/ ?

Line ends with a space.  :(
Please check the entire file for that.

> +    appropriate format.
> +
> + c. Writing to file
> +
> + 	Field		Expected behavior
> +	----------------------------------
> +	usage		prohibited
> +	max_usage	reset to usage
> +	limit		set the limit
> +	failcnt		reset to zero

---
~Randy
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