.. highlight:: none .. index:: pair: diagnostic feedback; design .. _design-diag: Diagnostic feedback =================== .. mps:prefix:: design.mps.diag Introduction ------------ :mps:tag:`intro` This document describes how to use the diagnostic feedback mechanism in the Memory Pool System. :mps:tag:`sources` Initially abased on [RHSK_2007-04-13]_ and [RHSK_2007-04-18]_. Overview -------- Diagnostic feedback is information created by the MPS diagnostic system for the purpose of helping MPS programmers and client programmers. Such a piece of information is called "a diagnostic". (See also :mps:ref:`.parts`.) A diagnostic is not intended to be visible to end users, or readable by them. A diagnostic is not intended to be stable from one release to the next: it may be modified or removed at any time. Requirements ------------ MPS diagnostic feedback code must do these things: - calculate, store, and propagate data; - collate, synthesise, and format it into a human-useful diagnostic; - control (for example, filter) output of diagnostics; - use a channel to get the diagnostic out. Usage ----- To get diagnostic output from the MPS, you must use a variety with diagnostics compiled-in. Currently, that means variety.cool. See ``config.h``. There are two mechanism for getting diagnostic output: #. Automatically via the telemetry system. See design.mps.telemetry_, and the "Telemetry" chapter in the manual. .. _design.mps.telemetry: telemetry.html #. Manually via the debugger. In the debugger, set break points at the places where you want to inspect data structures (or wait for the debugger to be entered via an :c:func:`abort()` call or unhandled segmentation fault). Then at the debugger command prompt, run :c:func:`Describe()` commands of your choice. For example:: (gdb) run Starting program: mv2test Reading symbols for shared libraries +............................. done cbs.c:94: MPS ASSERTION FAILED: !cbs->inCBS Program received signal SIGABRT, Aborted. 0x00007fff83e42d46 in __kill () (gdb) frame 12 #12 0x000000010000b1fc in MVTFree (pool=0x103ffe160, base=0x101dfd000, size=5024) at poolmv2.c:711 711 Res res = CBSInsert(MVTCBS(mvt), base, limit); (gdb) p MVTDescribe(mvt, mps_lib_get_stdout(), 0) MVT 0000000103FFE160 { minSize: 8 meanSize: 42 maxSize: 8192 fragLimit: 30 reuseSize: 16384 fillSize: 8192 availLimit: 90931 abqOverflow: FALSE splinter: TRUE splinterBase: 0000000106192FF0 splinterLimit: 0000000106193000 size: 303104 allocated: 262928 available: 40176 unavailable: 0 # ... etc ... } How to write a diagnostic ------------------------- Compile away in non-diag varieties; no side effects ................................................... Wrap code with the :c:macro:`STATISTIC` and :c:macro:`METER` macros, to make sure that non-diagnostic varieties do not execute diagnostic-generating code. Diagnostic-generating code must have no side effects. Writing good paragraph text ........................... Make your diagnostics easy to understand! Other people will read your diagnostics! Make them clear and helpful. Do not make them terse and cryptic. If you use symbols, print a key in the diagnostic. How the MPS diagnostic system works ----------------------------------- Parts of the MPS diagnostic system .................................. :mps:tag:`parts` The following facilities are considered part of the MPS diagnostic system: - the :c:func:`Describe()` methods. - the :c:macro:`STATISTIC` macros (see ``mpm.h``); - the :c:macro:`METER` macros and meter subsystem. Statistics .......... :mps:tag:`stat` The statistic system collects information about the behaviour and performance of the MPS that may be useful for MPS developers and customers, but which is not needed by the MPS itself for internal decision-making. :mps:tag:`stat.remove` The space needed for these statistics, and the code for maintaining them, can therefore be removed (compiled out) in some varieties. :mps:tag:`stat.config` Statistics are compiled in if :c:macro:`CONFIG_STATS` is defined (in the cool variety) and compiled out if :c:macro:`CONFIG_STATS_NONE` is defined (in the hot and rash varieties). .. c:macro:: STATISTIC_DECL(decl) :mps:tag:`stat.decl` The :c:macro:`STATISTIC_DECL` macro is used to wrap the declaration of storage for a statistic. Note that the expansion supplies a terminating semi-colon and so it must not be followed by a semi-colon in use. This is so that it can be used in structure declarations. .. c:macro:: STATISTIC(gather) :mps:tag:`stat.gather` The :c:macro:`STATISTIC` macro is used to gather statistics. The argument is a statement and the expansion followed by a semicolon is syntactically a statement. The macro expends to :c:macro:`NOOP` in non-statistical varieties. (Note that it can't use :c:macro:`DISCARD_STAT` to check the syntax of the statement because it is expected to use fields that have been compiled away by :c:macro:`STATISTIC_DECL`, and these will cause compilation errors.) :mps:tag:`stat.gather.effect` The argument to the :c:macro:`STATISTIC` macro is not executed in non-statistical varieties and must have no side effects, except for updates to fields that are declared in :c:macro:`STATISTIC_DECL`, and telemetry output containing the values of such fields. .. c:macro:: STATISTIC_WRITE(format, arg) :mps:tag:`stat.write` The :c:macro:`STATISTIC_WRITE` macro is used in :c:func:`WriteF()` argument lists to output the values of statistics. Related systems ............... The MPS diagnostic system is separate from the following other MPS systems: - The telemetry-log-events system. This emits much more data, in a less human-readable form, requires MPS-aware external tools, and is more stable from release to release). In non-diagnostic telemetry varieties, the telemetry-log-events system emits events that log all normal MPS actions. In diagnostic telemetry varieties, it may emit additional events containing diagnostic information. Additionally, the telemetry-log-events stream might in future be available as a channel for emitting human-readable text diagnostics. See also design.mps.telemetry_. - The MPS message system. This is present in all varieties, and manages asynchronous communication from the MPS to the client program). However, the MPS message system might in future also be available as a channel for emitting diagnostics. See also design.mps.message_. .. _design.mps.message: message.html References ---------- .. [RHSK_2007-04-13] Richard Kistruck. 2007-04-13. "`diagnostic feedback from the MPS `_". .. [RHSK_2007-04-18] Richard Kistruck. 2007-04-18. "`Diverse types of diagnostic feedback `_".