doc: guidelines for library statistics

Signed-off-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>

diff --git a/doc/guides/guidelines/design.rst b/doc/guides/guidelines/design.rst
index b4ae1d1..04803ca 100644 (file)
--- a/doc/guides/guidelines/design.rst
+++ b/doc/guides/guidelines/design.rst
@@ -52,3 +52,108 @@ The following config options can be used:
 
 * CONFIG_RTE_EXEC_ENV is a string that contains the name of the executive environment.
 * CONFIG_RTE_EXEC_ENV_BSDAPP or CONFIG_RTE_EXEC_ENV_LINUXAPP are defined only if we are building for this execution environment.
+
+Library Statistics
+------------------
+
+Description
+~~~~~~~~~~~
+
+This document describes the guidelines for DPDK library-level statistics counter
+support. This includes guidelines for turning library statistics on and off and
+requirements for preventing ABI changes when implementing statistics.
+
+
+Mechanism to allow the application to turn library statistics on and off
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Each library that maintains statistics counters should provide a single build
+time flag that decides whether the statistics counter collection is enabled or
+not. This flag should be exposed as a variable within the DPDK configuration
+file. When this flag is set, all the counters supported by current library are
+collected for all the instances of every object type provided by the library.
+When this flag is cleared, none of the counters supported by the current library
+are collected for any instance of any object type provided by the library:
+
+.. code-block:: console
+
+       # DPDK file config/common_linuxapp, config/common_bsdapp, etc.
+       CONFIG_RTE_<LIBRARY_NAME>_STATS_COLLECT=y/n
+
+The default value for this DPDK configuration file variable (either "yes" or
+"no") is decided by each library.
+
+
+Prevention of ABI changes due to library statistics support
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The layout of data structures and prototype of functions that are part of the
+library API should not be affected by whether the collection of statistics
+counters is turned on or off for the current library. In practical terms, this
+means that space should always be allocated in the API data structures for
+statistics counters and the statistics related API functions are always built
+into the code, regardless of whether the statistics counter collection is turned
+on or off for the current library.
+
+When the collection of statistics counters for the current library is turned
+off, the counters retrieved through the statistics related API functions should
+have a default value of zero.
+
+
+Motivation to allow the application to turn library statistics on and off
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is highly recommended that each library provides statistics counters to allow
+an application to monitor the library-level run-time events. Typical counters
+are: number of packets received/dropped/transmitted, number of buffers
+allocated/freed, number of occurrences for specific events, etc.
+
+However, the resources consumed for library-level statistics counter collection
+have to be spent out of the application budget and the counters collected by
+some libraries might not be relevant to the current application. In order to
+avoid any unwanted waste of resources and/or performance impacts, the
+application should decide at build time whether the collection of library-level
+statistics counters should be turned on or off for each library individually.
+
+Library-level statistics counters can be relevant or not for specific
+applications:
+
+* For Application A, counters maintained by Library X are always relevant and
+  the application needs to use them to implement certain features, such as traffic
+  accounting, logging, application-level statistics, etc. In this case,
+  the application requires that collection of statistics counters for Library X is
+  always turned on.
+
+* For Application B, counters maintained by Library X are only useful during the
+  application debug stage and are not relevant once debug phase is over. In this
+  case, the application may decide to turn on the collection of Library X
+  statistics counters during the debug phase and at a later stage turn them off.
+
+* For Application C, counters maintained by Library X are not relevant at all.
+  It might be that the application maintains its own set of statistics counters
+  that monitor a different set of run-time events (e.g. number of connection
+  requests, number of active users, etc). It might also be that the application
+  uses multiple libraries (Library X, Library Y, etc) and it is interested in the
+  statistics counters of Library Y, but not in those of Library X. In this case,
+  the application may decide to turn the collection of statistics counters off for
+  Library X and on for Library Y.
+
+The statistics collection consumes a certain amount of CPU resources (cycles,
+cache bandwidth, memory bandwidth, etc) that depends on:
+
+* Number of libraries used by the current application that have statistics
+  counters collection turned on.
+
+* Number of statistics counters maintained by each library per object type
+  instance (e.g. per port, table, pipeline, thread, etc).
+
+* Number of instances created for each object type supported by each library.
+
+* Complexity of the statistics logic collection for each counter: when only
+  some occurrences of a specific event are valid, additional logic is typically
+  needed to decide whether the current occurrence of the event should be counted
+  or not. For example, in the event of packet reception, when only TCP packets
+  with destination port within a certain range should be recorded, conditional
+  branches are usually required. When processing a burst of packets that have been
+  validated for header integrity, counting the number of bits set in a bitmask
+  might be needed.