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# 18. Telemetry

In its cool and hot varieties, the MPS is capable of outputting a configurable stream of events to assist with debugging and profiling.

The selection of events that appear in the stream is controlled by the environment variable `MPS_TELEMETRY_CONTROL` (by default none), and the stream is written to the file named by the environment variable `MPS_TELEMETRY_FILENAME` (by default `mpsio.log`).

The telemetry system writes blocks of binary output, and is fast enough to be left turned on in production code (the hot variety avoids emitting events on the critical path), which can be useful for diagnosing memory management problems in production environments.

The reporting of garbage collection statistics hasn’t always been suitable for deployment. John McCarthy described the first on-line demonstration of Lisp in an appendix to his paper “History of Lisp”:

> Everything was going well, if slowly, when suddenly the Flexowriter began to type (at ten characters per second)

>     THE GARBAGE COLLECTOR HAS BEEN CALLED.
>     SOME INTERESTING STATISTICS ARE AS FOLLOWS:

> and on and on and on. The garbage collector was quite new at the time, we were rather proud of it and curious about it, and our normal output was on a line printer, so it printed a full page every time it was called giving how many words were marked and how many were collected and the size of list space, etc. [...]

> Nothing had ever been said about the garbage collector, and I could only imagine the reaction of the audience. We were already behind time on a tight schedule, it was clear that typing out the garbage collector message would take all the remaining time allocated to the demonstration, and both the lecturer and the audience were incapacitated with laughter. I think some of them thought we were victims of a practical joker.

## 18.1. Telemetry utilities

The telemetry system relies on three utility programs:

- mpseventcnv decodes the machine-dependent binary event stream into a portable text format. It must be compiled for the same architecture as the MPS-linked program whose event stream it decodes.
- mpseventtxt takes the output of mpseventcnv and outputs it in a more human-readable form.
- mpseventsql takes the output of mpseventcnv and loads it into a SQLite database for further analysis.

You must build and install these programs as described in Building the Memory Pool System. Thee programs are described in more detail below.

## 18.2. Example

Here’s an example of turning on telemetry in the debugger and then encountering a corrupted object:

    $ gdb ./scheme
    GNU gdb 6.3.50-20050815 (Apple version gdb-1820) (Sat Jun 16 02:40:11 UTC 2012)
    [...]
    (gdb) set environment MPS_TELEMETRY_CONTROL=all
    (gdb) run
    Starting program: example/scheme/scheme
    Reading symbols for shared libraries +............................. done
    MPS Toy Scheme Example
    [...]
    7944, 0> (gc)
    [...]
    7968, 1> foo
    Assertion failed: (TYPE(frame) == TYPE_PAIR), function lookup_in_frame, file scheme.c, line 1066.

    Program received signal SIGABRT, Aborted.
    0x00007fff91aeed46 in __kill ()

At this point there’s still output in the MPS’s internal event buffers, which needs to be flushed. It would be a good idea to add a call to `mps_telemetry_flush()` to the error handler, but for now we can just call it directly from the debugger:

    (gdb) print mps_telemetry_flush()
    $1 = void

The MPS writes the telemetry to the log in an encoded form for speed. It can be decoded using the mpseventcnv and mpseventtxt programs:

    (gdb) shell mpseventcnv | mpseventtxt | sort > mpsio.txt

The `sort` is useful because the events are not necessarily written to the telemetry file in time order, but each event starts with a timestamp so sorting makes a time series. The decoded events look like this, with the timestamp in the first column, the event type in the second column, and then addresses or other data related to the event in the remaining columns. The source of the timestamp depends on the platform; it may be a low-cost high-resolution processor timer, such as the Time Stamp Counter on IA-32 and x86-64, if one is available. All numbers are given in hexadecimal.

    000AE03973336E3C 002B VMCreate            vm:00000001003FC000 base:00000001003FD000 limit:00000001003FE000
    000AE0397333BC6D 002D VMMap               vm:00000001003FC000 base:00000001003FD000 limit:00000001003FE000
    000AE0397334DF9F 001A Intern              stringId:0000000000000002 string:"Reservoir"
    000AE0397334E0A0 001B Label               address:00000001078C85B8["Reservoir"] stringId:0000000000000002
    000AE03973352375 0015 PoolInit            pool:00000001003FD328 arena:00000001003FD000 poolClass:00000001078C85B8["Reservoir"]
    000AE039733592F9 002B VMCreate            vm:00000001003FE000 base:00000001003FF000 limit:000000010992F000
    000AE0397335C8B5 002D VMMap               vm:00000001003FE000 base:00000001003FF000 limit:0000000107930000
    000AE03973361D5A 0005 ArenaCreateVM       arena:00000001003FD000 userSize:0000000002000000 chunkSize:0000000002000000

You can search through the telemetry for events related to particular addresses of interest.

In the example, we might look for events related to the address of the corrupted `frame` object:

    (gdb) frame 3
    #3  0x0000000100003f55 in lookup_in_frame (frame=0x1003fa7d0, symbol=0x1003faf20) at scheme.c:1066
    1066            assert(TYPE(frame) == TYPE_PAIR);
    (gdb) print frame
    $2 = (obj_t) 0x1003fa7d0
    (gdb) shell grep -i 1003fa7d0 mpsio.txt || echo not found
    not found

There are no events related to this address, so in particular this address was never fixed (no `TraceFix` event).

Note

You may find it useful to add the command:

    set environment MPS_TELEMETRY_CONTROL=all

to your `.gdbinit`.

## 18.3. Event categories

The “bit” column gives the bit number in the telemetry filter. These numbers are liable to change, but the current meanings (zero being the least significant bit) are:

Bit | Name | Description
--- | --- | ---
0 | `Arena` | Per space or arena.
1 | `Pool` | Per pool.
2 | `Trace` | Per trace or scan.
3 | `Seg` | Per page (segment).
4 | `Ref` | Per reference or fix.
5 | `Object` | Per allocation, block, or object.
6 | `User` | User-invoked events: see `mps_telemetry_intern()`.

## 18.4. Environment variables

In the ANSI plinth (the plinth that comes as default with the MPS), these two environment variables control the behaviour of the telemetry feature.

`MPS_TELEMETRY_CONTROL`

  The event categories which should be included in the telemetry stream.

  If its value can be interpreted as a number, then this number represents the set of event categories as a bitmap. For example, this turns on the `Pool` and `Seg` event categories:

      MPS_TELEMETRY_CONTROL=6

  Otherwise, the value is split into words at spaces, and any word that names an event category turns it on. For example:

      MPS_TELEMETRY_CONTROL="arena pool trace"

  The special event category `all` turns on all events.

`MPS_TELEMETRY_FILENAME`

  The name of the file to which the telemetry stream should be written. Defaults to `mpsio.log`. For example:

      MPS_TELEMETRY_FILENAME=$(mktemp -t mps)

In addition, the following environment variable controls the behaviour of the mpseventsql program.

`MPS_TELEMETRY_DATABASE`

  The name of a SQLite database file that will be updated with the events from the decoded telemetry stream, if it is not specified with the `-d` option. If this variable is not assigned, `mpsevent.db` is used.

## 18.5. Decoding the telemetry stream

The MPS writes the telemetry stream in a binary encoded format for speed. The encoding is specific to the platform the program was running on, and so the output needs to be decoded before it can be processed.

The decoding takes place in two stages. First, the program mpseventcnv converts the binary encoded format into a portable text format suitable for input to one of the second-stage tools (mpseventtxt and mpseventsql).

`-f``<filename>`

  The name of the file containing the telemetry stream to decode. Defaults to `mpsio.log`.

`-h```

  Help: print a usage message to standard output.

Note

mpseventcnv can only read telemetry streams that were written by an MPS compiled on the same platform.

Here’s some example output. The first column contains the timestamp of the event, the second column contains the event type, and remaining columns contain parameters related to the event.

    000AE03973336E3C 2B 1003FC000 1003FD000 1003FE000
    000AE0397333BC6D 2D 1003FC000 1003FD000 1003FE000
    000AE0397334DF9F 1A 2 "Reservoir"
    000AE0397334E0A0 1B 1078C85B8 2
    000AE03973352375 15 1003FD328 1003FD000 1078C85B8
    000AE039733592F9 2B 1003FE000 1003FF000 10992F000
    000AE0397335C8B5 2D 1003FE000 1003FF000 107930000
    000AE03973361D5A 5 1003FD000 2000000 2000000

## 18.6. Making the telemetry stream readable

The output of mpseventcnv can be made more readable by passing it through mpseventtxt, which takes the following options:

`-l``<filename>`

  The name of a file containing telemetry events that have been decoded by mpseventcnv. Defaults to standard input.

`-h```

  Help: print a usage message to standard output.

For example, here’s the result of passing the output shown above through mpseventtxt:

    000AE03973336E3C 002B VMCreate            vm:00000001003FC000 base:00000001003FD000 limit:00000001003FE000
    000AE0397333BC6D 002D VMMap               vm:00000001003FC000 base:00000001003FD000 limit:00000001003FE000
    000AE0397334DF9F 001A Intern              stringId:0000000000000002 string:"Reservoir"
    000AE0397334E0A0 001B Label               address:00000001078C85B8["Reservoir"] stringId:0000000000000002
    000AE03973352375 0015 PoolInit            pool:00000001003FD328 arena:00000001003FD000 poolClass:00000001078C85B8["Reservoir"]
    000AE039733592F9 002B VMCreate            vm:00000001003FE000 base:00000001003FF000 limit:000000010992F000
    000AE0397335C8B5 002D VMMap               vm:00000001003FE000 base:00000001003FF000 limit:0000000107930000
    000AE03973361D5A 0005 ArenaCreateVM       arena:00000001003FD000 userSize:0000000002000000 chunkSize:0000000002000000

## 18.7. Loading the telemetry stream into SQLite

The decoded telemetry stream (as output by mpseventcnv) can be loaded into a SQLite database for further analysis by running mpseventsql.

mpseventsql takes the following options:

`-i``<filename>`

  The name of a file containing a decoded telemetry stream. Defaults to standard input.

`-o``<filename>`

  The name of a SQLite database file that will be updated with the events from the decoded telemetry stream specified by the `-l` option. The database will be created if it does not exist. If not specified, the file named by the environment variable `MPS_TELEMETRY_DATABASE` is used; if this variable is not assigned, `mpsevent.db` is used.

  Updating a database with events from a file is idempotent unless the `-f` option is specified.

`-d```

  Delete the database before importing.

`-f```

  Forces the database to be updated with events from the decoded telemetry stream specified by the `-i` option, even if those events have previously been added.

`-v```

  Increase the verbosity. With one or more `-v` options, mpseventsql prints informative messages to standard error. Verbosity levels up to 3 (`-vvv`) produce successively more detailed information.

  This option implies `-p`.

`-p```

  Show progress by printing a dot to standard output for every 100,000 events processed.

`-t```

  Run internal tests.

`-r```

  Rebuild the tables `event_kind`, `event_type`, and `event_param`. (This is necessary if you changed the event descriptions in `eventdef.h`.)

## 18.8. Telemetry interface

mps_word_t `mps_telemetry_control`(mps_word_t reset_mask, mps_word_t flip_mask)

  Deprecated

  starting with version 1.111.

  Use `mps_telemetry_get()`, `mps_telemetry_reset()`, and `mps_telemetry_set()` instead.

  Update and return the telemetry filter.

  `reset_mask` is a bitmask indicating the bits in the telemetry filter that should be reset.

  `flip_mask` is a bitmask indicating the bits in the telemetry filter whose value should be flipped after the resetting.

  Returns the previous value of the telemetry filter, prior to the reset and the flip.

  The parameters `reset_mask` and `flip_mask` allow the specification of any binary operation on the filter control. For typical operations, the parameters should be set as follows:

  Operation | `reset_mask` | `flip_mask`
  --- | --- | ---
  `set(M)` | `M` | `M`
  `reset(M)` | `M` | `0`
  `flip(M)` | `0` | `M`
  `read()` | `0` | `0`

void `mps_telemetry_flush`(void)

  Flush the internal event buffers into the telemetry stream.

  This function also calls `mps_io_flush()` on the event stream itself. This ensures that even the latest events are now properly recorded, should the client program terminate (uncontrollably as a result of a bug, for example) or some interactive tool require access to the telemetry stream. You could even try calling this from a debugger after a problem.

  Note

  Unless all arenas are properly destroyed (by calling `mps_arena_destroy()`), there are likely to be unflushed telemetry events when the program finishes. So in the case of abnormal program termination such as a fatal exception, you may want to call `mps_telemetry_flush()` explicitly.

mps_word_t `mps_telemetry_get`(void)

  Return the telemetry filter.

void `mps_telemetry_set`(mps_word_t set_mask)

  Set bits in the telemetry filter.

  `set_mask` is a bitmask indicating the bits in the telemetry filter that should be set.

void `mps_telemetry_reset`(mps_word_t reset_mask)

  Reset bits in the telemetry filter.

  `reset_mask` is a bitmask indicating the bits in the telemetry filter that should be reset.

## 18.9. Telemetry labels

Telemetry labels allow the client program to associate strings with addresses in the telemetry stream. The string must first be interned by calling `mps_telemetry_intern()`, returning a label, and then the address can be associated with the label by calling `mps_telemetry_label()`.

Typical uses of telemetry labels include:

- labelling pools with a human-meaningful name;
- labelling allocated objects with their type, class, or other description.

It is necessary to enable `User` events in the telemetry filter in order for telemetry labels to work. For example:

    mps_label_t label;
    mps_telemetry_set(1 << 6);
    label = mps_telemetry_intern("symbol pool");
    mps_telemetry_label(symbol_pool, label);

Labels are represented by the type `mps_label_t`. These are unsigned integers. After processing by mpseventsql, the association of addresses with labels appears in the `EVENT_Label` table, and the association of labels with strings appears in the `EVENT_Intern` table. These can then be used in queries, for example:

    /* Pool name and creation time */
    SELECT I.string, P.time
    FROM EVENT_PoolInit AS P,
         EVENT_Label AS L,
         EVENT_Intern AS I
    WHERE I.stringId = L.stringId AND L.address = P.pool;

mps_label_t `mps_telemetry_intern`(const char *label)

  Registers a string with the MPS, and receives a telemetry label, suitable for passing to `mps_telemetry_label()`.

  `label` is a NUL-terminated string. Its length should not exceed 256 characters, including the terminating NUL.

  Returns a telemetry label: a unique identifier that may be used to represent the string in future.

  The intention of this function is to provide an identifier that can be used to concisely represent a string for the purposes of `mps_telemetry_label()`.

  Note

  If the `User` event category is not turned on in the telemetry filter (via `mps_telemetry_control()`) then the string is not sent to the telemetry stream. A label is still returned in this case, but it is useless.

void `mps_telemetry_label`(mps_addr_t addr, mps_label_t label)

  Associate a telemetry label returned from `mps_telemetry_intern()` with an address.

  `addr` is an address.

  `label` is a telemetry label returned from `mps_telemetry_intern()`.

  The label will be associated with the address when it appears in the telemetry stream.

  Note

  If the `User` event category is not turned on in the telemetry filter (via `mps_telemetry_control()`) then calling this function has no effect.

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