THE DESIGN OF THE MPS TELEMETRY MECHANISM
                          design.mps.telemetry
                           incomplete design
                           richard 1997-07-07


INTRODUCTION:

This documents the design of the telemetry mechanism within the MPS.

.readership: This document is intended for any MPS developer.

.source: Various meetings and brainstorms, including 
meeting.general.1997-03-04(0), mail.richard.1997-07-03.17-01(0), 
mail.gavinm.1997-05-01.12-40(0).


Document History

.hist.0: 1997-04-11  GavinM  Rewritten

.hist.1: 1997-07-07  GavinM  Rewritten again after discussion in Pool Hall.


OVERVIEW:

Telemetry permits the emission of events from the MPS.  These can be used to 
drive a graphical tool, or to debug, or whatever.  The system is flexible and 
robust, but doesn't require heavy support from the client.


REQUIREMENTS:

.req.simple: It must be possible to generate code both for the MPS and any tool 
without using complicated build tools.

.req.open: We must not constrain the nature of events before we are certain of 
what we want them to be.

.req.multi: We must be able to send events to multiple streams.

.req.share: It must be possible to share event descriptions between the MPS and 
any tool.

.req.version: It must be possible to version the set of events so that any tool 
can detect whether it can understand the MPS.

.req.back: Tools should be able to understand older and newer version of the 
MPS, so far as is appropriate.

.req.type: It must be possible to transmit a rich variety of types to the tool, 
including doubles, and strings.

.req.port: It must be possible to transmit and receive events between different 
platforms.

.req.control: It must be possible to control whether and what events are 
transmitted at least at a coarse level.

.req.examine: There should be a cheap means to examine the contents of logs.

.req.pm: The event mechanism should provide for post mortem to detect what 
significant events led up to death.

.req.perf: Events should not have a significant effect on performance when 
unwanted.

.req.small: Telemetry streams should be small.

.req.avail: Events should be available in all varieties, subject to performance 
requirements.

.req.impl: The plinth support for telemetry should be easy to write and 
flexible.

.req.robust: The telemetry protocol should be robust against some forms of 
corruption, e.g. packet loss.

.req.intern: It should be possible to support string-interning.


ARCHITECTURE:

.arch: Event annotations are scattered throughout the code, but there is a 
central registration of event types and properties.  Events are written to a 
buffer via a specialist structure, and are optionally written to the plinth.  
Events can take any number of parameters of a range of types, indicated as a 
format both in the annotation and the the registry.


ANALYSIS:

.anal: The proposed order of development, with summary of requirements impact 
is as follows:

                        v      c e 
               s        e      o x             r  i
               i   m  s r      n a      s  a   o  n
               m o u  h s t  p t m    p m  v i b  t b
               p p l  a i y  o r i    e a  a m u  e a
               l e t  r o p  r o n  p r l  i p s  r c
               e n i  e n e  t l e  m f l  l l t  n k

.sol.format    0 0 0  0 0 +  0 0 0  0 0 0  0 0 0  0 0  Merged.
.sol.struct    0 0 0  0 0 +  0 0 0  0 + -  0 0 0  0 0  Merged.
.sol.string    0 0 0  0 0 +  0 0 0  0 0 0  0 0 0  + 0  Merged.
.sol.relation  + 0 0  + 0 0  0 0 +  0 0 +  0 0 0  0 0  Merged.
.sol.dumper    0 0 0  0 0 0  0 0 +  0 0 0  0 0 0  0 0  Merged.
.sol.kind      0 - 0  0 0 0  0 + 0  + 0 0  0 0 0  0 0  Merged.
.sol.control   0 0 0  0 0 0  0 + 0  0 + 0  0 0 0  0 0  Merged.

.sol.variety   0 0 0  0 0 0  0 0 0  + + 0  + 0 0  0 0

[ Not yet ordered. ]

.sol.buffer    0 0 0  0 0 0  0 + 0  + + 0  0 0 0  0 0
.sol.traceback 0 0 0  0 0 0  0 0 0  + 0 0  0 0 0  0 0
.sol.client    0 0 0  0 0 0  0 0 0  0 0 0  0 0 0  + 0
.sol.head      0 0 0  0 0 0  + 0 0  0 0 0  0 0 0  0 0
.sol.version   0 0 0  0 + 0  0 0 0  0 0 0  0 0 0  0 +
.sol.exit      0 0 0  0 0 0  0 0 0  + 0 0  0 0 0  0 0
.sol.block     0 0 0  0 0 0  0 0 0  0 + -  0 0 +  0 0 
.sol.code      0 0 0  0 0 0  0 0 0  0 0 +  0 0 0  0 +
.sol.msg       0 0 +  0 0 0  + 0 0  0 0 0  0 + +  0 0

.file-format: One of the objectives of this plan is to minimise the impact of 
the changes to the log file format.  This is to be achieved firstly by 
completing all necessary support before changes are initiated, and secondly by 
performing all changes at the same time.


IDEAS:

.sol.format: Event annotations indicate the types of their arguments, e.g. 
EVENT_WD for a Word, and a double.  (.req.type)

.sol.struct: Copy event data into a structure of the appropriate type, e.g. 
EventWDStruct.  (.req.type, .req.perf, but not .req.small because of padding)

.sol.string: Permit at most one string per event, at the end, and use the char 
[1] hack, and specialised code; deduce the string length from the event length 
and also NUL-terminate  (.req.type, .req.intern)

.sol.buffer: Enter all events initially into internal buffers, and 
conditionally send them to the message stream. (.req.pm, .req.control, 
.req.perf)

.sol.variety: In optimized varieties, have internal events (see .sol.buffer) 
for a subset of events and no external events; in normal varieties have all 
internal events, and the potential for external events.  (.req.avail, .req.pm, 
.req.perf)

.sol.kind: Divide events by some coarse type into around 6 groups, probably 
related to frequency.  (.req.control, .req.pm, but not .req.open)

.sol.control: Hold flags to determine which events are emitted externally.  
(.req.control, .req.perf)

.sol.dumper: Write a simple tool to dump event logs as text.  (.req.examine)

.sol.msg: Redesign the plinth interface to send and receive messages, based on 
any underlying IPC mechanism, e.g. append to file, TCP/IP, messages, shared 
memory.  (.req.robust, .req.impl, .req.port, .req.multi)

.sol.block: Buffer the events and send them as fixed size blocks, commencing 
with a timestamp, and ending with padding.  (.req.robust, .req.perf, but not 
.req.small)

.sol.code: Commence each event with two bytes of event code, and two bytes of 
length.  (.req.small, .req.back)

.sol.head: Commence each event stream with a platform-independent header block 
giving information about the session, version (see .sol.version), and file 
format; file format will be sufficient to decode the (platform-dependent) rest 
of the file.  (.req.port)

.sol.exit: Provide a mechanism to flush events in the event of graceful sudden 
death.  (.req.pm)

.sol.version: Maintain a three part version number for the file comprising 
major (incremented when the format of the entire file changes (other than 
platform differences)), median (incremented when an existing event changes its 
form or semantics), and minor (incremented when a new event type is added); 
tools should normally fail when the median or major is unsupported.  
(.req.version, .req.back)

.sol.relation: Event types will be defined in terms of a relation specifying 
their name, code, optimised behaviour (see .sol.variety), kind (see .sol.kind), 
and format (see .sol.format); both the MPS and tool can use this by suitable 
#define hacks.  (.req.simple. .req.share, .req.examine, .req.small (no format 
information in messages))

.sol.traceback: Provide a mechanism to output recent events (see .sol.buffer) 
as a form of backtrace when AVERs fire or from a debugger, or whatever. 
(.req.pm)

.sol.client: Provide a mechanism for user events.  (.req.intern)



IMPLEMENTATION:

Annotation

.annot: An event annotation is of the form:
  EVENT3(FooCreate, pointer, address, word);

.annot.string: If there is a string in the format, it must be the last 
parameter (and hence there can be only one).  There is currrently a maximum 
string length, defined by EventMaxStringLength in impl.h.eventcom.

.annot.type: The event type should be given as the first parameter to the event 
macro, as registered in impl.h.eventdef.

.annot.param: The parameters of the event should be given as the remaining 
parameters of the event macro, in order as indicated in the event parameters definition in impl.h.eventdef.


Registration

.reg: All event types and parameters should
be registered in impl.h.eventdef, in the form of a higher-order list
macros.

.reg.just: This use of a higher-order macros enables great flexibility in the use of 
this file.

.reg.rel: The event type registration is of the form:
  EVENT(X, FooCreate, 0x1234, TRUE, Arena)

.reg.type: The first parameter of the relation is the event type.  This needs 
no prefix, and should correspond to that used in the annotation.

.reg.code: The second parameter is the event code, a 16-bit value used to 
represent this event type.  Codes should not be re-used for new event types, to allow interpretation of event
log files of all ages.

.reg.always: The third parameter is a boolean value indicating whether this 
event type should be implemented in all varieties.  See .control.buffer.  
Unless your event is on the critical path (typically per reference or per 
object), you will want this to be TRUE.

.reg.kind: The fourth parameter is a kind keyword indicating what category this 
event falls into.  See .control.  The possible values are:
  Arena -- per space or arena or global
  Pool -- pool-related
  Trace -- per trace or scan
  Seg -- per segment
  Ref -- per reference or fix
  Object -- per object or allocation
  User -- invoked by the user through the MPS interface
This list can be seen in impl.h.eventcom.

[.reg.doc: Add a docstring column.  RB 2012-09-03]

.reg.params: The event parameters registration is of the form:

  #define EVENT_FooCreate_PARAMS(PARAM, X) \
    PARAM(X,  0, P, firstParamPointer) \
    PARAM(X,  1, U, secondParamUnsigned)

.reg.param.index: The first column is the index, and must start at zero
and increase by one for each row.

.reg.param.sort: The second column is the parameter "sort", which, when
appended to EventF, yields a type for the parameter.  It is a letter from the following list:
  P -- void *
  A -- Addr
  W -- Word
  U -- unsigned int
  S -- char *
  D -- double
  B -- Bool
The corresponding event parameter must be assignment compatible with the type.

.param.types: When an event has parameters whose type is not in the above 
list, use the following guidelines: All C pointer types not representing 
strings use P; Size, Count, Index use W; others should be obvious.

.reg.param.name: The third column is the parameter name.  It should be a valid C identifier
and is used for debugging display and human readable output.

[.reg.param.doc: Add a docstring column.  RB 2012-09-03]

.reg.dup: It is permissible for the one event type to be used for more than one 
annotation.  There are generally two reasons for this:
  - Variable control flow for successful function completion;
  - Platform/Otherwise-dependent implementations of a function.
Note that all annotations for one event type must have the same format (as 
implied by .reg.format).


Control

.control: There are two types of event control, buffer and output.

.control.buffer: Buffer control affects whether particular events implemented 
at all, and is controlled statically by variety using the always value (see 
.reg.always) for the event type.  The hot variety does compiles out annotations
with always=FALSE.  The cool variety does not, so always buffers a complete
set of events.

.control.output: Output control affects whether events written to the internal 
buffer are output via the plinth.  This is set on a per-kind basis (see 
.reg.kind), using a control bit table stored in EventKindControl.  By default, 
all event kinds are off.  You may switch some kinds on using a 
debugger.

For example, to enable Pool events using gdb (see impl.h.eventcom for numeric 
codes):

    rb@silverbird$ gdb ./xci3gc/cool/amcss
    (gdb) break GlobalsInit
    (gdb) run
    ...
    (gdb) print EventKindControl |= 2
    $2 = 2
    (gdb) continue
    ...
    (gdb) quit
    rb@silverbird$ ./xci3gc/cool/eventcnv -v | sort | head
    0000178EA03ACF6D PoolInit                9C1E0    9C000 0005E040
    0000178EA03C2825 PoolInitMFS             9C0D8    9C000     1000        C
    0000178EA03C2C27 PoolInitMFS             9C14C    9C000     1000       44
    0000178EA03C332C PoolInitMV              9C080    9C000     1000       20    10000
    0000178EA03F4DB4 BufferInit             2FE2C4   2FE1B0        0
    0000178EA03F4EC8 BufferInitSeg          2FE2C4   2FE1B0        0
    0000178EA03F57DA AMCGenCreate           2FE1B0   2FE288
    0000178EA03F67B5 BufferInit             2FE374   2FE1B0        0
    0000178EA03F6827 BufferInitSeg          2FE374   2FE1B0        0
    0000178EA03F6B72 AMCGenCreate           2FE1B0   2FE338

.control.env: The initial value of EventKindControl is read from the C environment
when the ANSI Plinth is used, and so event output can be controlled like this:

  MPS_TELEMETRY_CONTROL=127 amcss

or like this

  MPS_TELEMETRY_CONTROL="Pool Arena" amcss

where the variable is set to a space-separated list of names defined by EventKindENUM.

.control.just: These controls are coarse, but very cheap.

.control.external: The MPS interface function mps_telemetry_control
can be used to change EventKindControl.

.control.tool: The tools will be able to control EventKindControl.


Debugging

.debug.buffer: Each event kind is logged in a separate buffer, EventBuffer[kind].

.debug.buffer.reverse: The events are logged in reverse order from the top of the buffer, with the last logged
event at EventLast[kind].  This allows recovery of the list of recent events using the event->any.size field.

.debug.dump: The contents of all buffers can be dumped with the EventDump function from a debugger, e.g.

  gdb> print EventDump(mps_lib_get_stdout())

.debug.describe: Individual events can be described with the EventDescribe function, e.g.

  gdb> print EventDescribe(EventLast[3], mps_lib_get_stdout())

.debug.core: The event buffers are preserved in core dumps and can be used
to work out what the MPS was doing before a crash.  Since the kinds correspond
to frequencies, ancient events may still be available in some buffers, even
if they have been flushed to the output stream.  Some digging may be required.


Dumper Tool

.dumper: A primitive dumper tool is available in impl.c.eventcnv.  For details, 
see guide.mps.telemetry.


Allocation Replayer Tool

.replayer: A tool for replaying an allocation sequence from a log is available 
in impl.c.replay.  For details, see design.mps.telemetry.replayer.