Re: [RFC] [Patch 4/6] statistics infrastructure - documentation
Date: Thu May 18 2006 - 02:33:19 EST
On Wed, 17 May 2006 20:54:45 +0200 Martin Peschke wrote:
> documentation for developers and users
> Signed-off-by: Martin Peschke <mp3@xxxxxxxxxx>
> 00-INDEX | 2
> statistics.txt | 760 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 762 insertions(+)
> diff -Nurp a/Documentation/statistics.txt b/Documentation/statistics.txt
> --- a/Documentation/statistics.txt 1970-01-01 01:00:00.000000000 +0100
> +++ b/Documentation/statistics.txt 2006-05-17 19:11:32.000000000 +0200
> @@ -0,0 +1,760 @@
> + 0. Which problems is it meant to solve?
> +This common code layer implements statistics in a device driver independent
> +and architecture independent way. It minimizes the effort by providing a simple
> +programming interface. Added benefit is a generic user interfaces.
... generic user interface.
> + 1. Concepts
> + Overview
> + The form of reported data
> +Exploiters report data in the form of (X, Y) value pairs with X being
> +a quantity for the main characteristic of the statistic, like a request size
> +or request latency, and with Y being a qualifier for that characteristic,
> +i.e. the occurrence of a particular X-value.
> +Thus, the Y-part can be seen an optimisation that allows to report a bunch
that allows <something> to report a bunch
e.g., allows a function or a module or a driver or a filesystem
> +of similar measurements in one go ( see statistic_add() ).
Drop extra spaces: (see statistic_add()).
Is "go" a technical term? Can we get something better, like
in one call?
> +For the programmer's convenience, Y can be ommited when it would be always 1
> +( see statistic_inc() ).
drop extra spaces
> + How data is reported
> +There are two methods how such data can be provided to the statistics
> +infrastructure, a push interface and a pull interface. Each statistic
> +is either a pull-type or push-type statistic as determined by the exploiter.
> +The push-interface is suitable for data feeds that report incremental updates
> +to statistics, and where actual accumulation can be left to the statistics
> +infrastructure. New measurements usually trigger pushing data.
> +( see statistics_add() and statistic_inc() )
drop extra spaces
> +The pull-interface is suitable for data that already comes in an aggregated
> +form, like hardware measurement data or counters already maintained and
> +used by exploiters for other purposes. Reading statistics data from files
> +triggers an optional callback of the exploiter, which can update pull-type
> +stattistics then ( see statistic_set() ).
drop extra spaces
> + How data is processed
> +(X, Y) pairs can be processed in different ways by the statistics
> +infrastructure, according to the current settings applicable to a
> +particular statistic.
> +For example, X might be used to distinguish certain buckets (see histogram).
> +Minimum, average and maximum X values might be determined (see utilisation).
> +Y might be summed up (see counter, for example).
> +See below for a reference of processing options.
> +Please note that the statistics infrastructure does not care about the
> +actual semantics of (X, Y), an that it just adheres to abstract rules
> +describing what to do with (X, Y) pairs for certain settings.
> +It is up to the user to interpret processed data, to add semantics
> +back to it, and to choose settings and, therewith, data processing modes
> +according to their needs.
> + Why debugfs for now
> +While sysfs comes with a refined structure reflecting almost everything
> +in the system, it is (by design) not good at representing large and variable
> +amounts of data, that is, more than one value per file. As for statistics,
> +we could make good use of the former, but not of the latter.
Why is debugfs not suitable for large & variable amounts of data?
> + 2. Performance
> + Path length of statistic_add() & friends
> +Some flexibility and functionality are achieved on the expense of slightly
> +increased path length.
> +A function pointer is used to implement different data processing modes
> +users can chose from.
> + 3. Modes of data processing
> +So far, available are:
> + type=counter_inc
> +A counter sums up all Y-values of (X, Y) data pairs reported, regardless of the
> + type=sparse
> +This one is similar to other histograms, with the exception that it provides
> +buckets for discrete X-values instead of ranges of X-values. Since it
> +utilises a list instead of an array, it is suited for compiling histogram-like
> +results for rather few, sparse X-values which users want to measure
> +Additional required parameters include:
> +- entries (list is capped at this number of entries)
> +For example, a (request size, occurrence)-statistic would yield the
> +occurrences of all request sizes. Since records precise resize numbers,
"Since this records... " (?)
> +it can also show the odd one out, which might be problematic; who knows...
> + Other
> +The statistic infrastructure has been designed to make the addition
> +of more ways of data processing easy (see struct statistic_discipline).
> +For example, two more types had been implemented which are not included
> +in the source code:
> +- A "raw" type statistic which provides a record of (X, Y)-pairs.
> + Nice for verification and debugging purposes.
> +- An enhancement of other basic types, like "counter" or "utilisation"
> + by the dimension time, which provides a time-tagged history of their
> + results for successive periods of time.
> + For example, a (request size, occurrence)-statistic could yield the
> + transfer rate over time, like bytes per seconds.
> + 4. User interface
> + Locating statistics
> +The statistics infrastructure's user interface is in the
> +/sys/kernel/debug/statistics directory, assuming debugfs has been mounted at
> +/sys/kernel/debug. The "statistics" directory holds interface subdirectories
> +created on the behalf of exploiters, for example:
Is /sys/kernel/debug a common mount point for debugfs ?
> + Changing statistics
> +One can either write the entire line describing a statistic, including
> +read-only attributes (which are ignored by the statistics infrastructure, as
as is any junk).
> +any is junk). Or one may modify single attributes, assuming this results
> +in a valid configuration.
> +This simplifies the procedures (copy, paste to command line,
> +modify command line, echo attributes from command line into definition file)
> +and (cat to file, modify file content, cat file back into definition file).
> +Some operations can be done in an atomic fashion for all statistics grouped
> +within the scope of an interface. Simply, omit the name= attribute:
> + 5. Programming interface
> + Creating statistics
> + Reporting statistics data
> +The above examples show statistics that feed on incremental updates that
> +get accumulated by the statistics infrastructure on top of data already
> +gathered by the statistics infrastructure.
> +That is why, statistic_add() or statistic_inc() respectively are used.
> +There might be statistics that come as total numbers, e.g. because they feed
> +on counters alredy maintained by the exploiter or some hardware feature.
> +These numbers can be exported through the statistics infrastructure along
> +with any other statistic. In this case, use statistic_set() to report data.
> +Usually it is sufficient to do so when the user opens the corresponding
> +file to read statistic data. This will trigger the optional callback function
> +to be executed. Place statistic_set() calls there. In the same context calling
> +statistic_add() or statistic_inc() for incremental data feeds works as well,
> +in case that's needed:
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