15. Eventlog encodings — Glasgow Haskell Compiler 9.0.1 User's Guide

# 15. Eventlog encodings

This section documents the encodings of the events emitted to GHC’s event log. These events can include information about the thread scheduling events, garbage collection statistics, profiling information, user-defined tracing events.

This section is intended for implementors of tooling which consume these events. GHC ships with a C header file (`EventlogFormat.h`) which provides symbolic names for the event type IDs described in this file.

## 15.1. Event log format

The log format is designed to be extensible: old tools should be able to parse (but not necessarily understand all of) new versions of the format, and new tools will be able to understand old log files.

- The format is endian-independent: all values are represented in big-endian order.
- The format is extensible:
  - The header describes each event type and its length. Tools that don’t recognise a particular event type can skip those events.
  - There is room for extra information in the event type specification, which can be ignored by older tools.
  - Events can have extra information added, but existing fields cannot be changed. Tools should ignore extra fields at the end of the event record.

The event-log stream begins with a header describing the event types present in the file. The header is followed by the event records themselves, each of which consist of a 64-bit timestamp

    log : EVENT_HEADER_BEGIN
          EventType*
          EVENT_HEADER_END
          EVENT_DATA_BEGIN
          Event*
          EVENT_DATA_END

    EventType :
          EVENT_ET_BEGIN
          Word16         -- unique identifier for this event
          Int16          -- >=0  size of the event in bytes (minus the header)
                         -- -1   variable size
          Word32         -- length of the next field in bytes
          Word8*         -- string describing the event
          Word32         -- length of the next field in bytes
          Word8*         -- extra info (for future extensions)
          EVENT_ET_END

    Event :
          Word16         -- event_type
          Word64         -- time (nanosecs)
          [Word16]       -- length of the rest (for variable-sized events only)
          ... extra event-specific info ...

There are two classes of event types:

> - Fixed size: All event records of a fixed-sized type are of the same length, given in the header event-log header.
> - Variable size: Each event record includes a length field.

## 15.2. Runtime system diagnostics

> - `ThreadId ~ Word32`
> - `CapNo ~ Word16`
> - `CapSetId ~ Word32`

### 15.2.1. Capability sets

TODO

### 15.2.2. Environment information

These events are typically produced during program startup and describe the environment which the program is being run in.

`RTS_IDENTIFIER`

  Tag: | 29
  Length: | variable
  Field CapSetId: | Capability set
  Field String: | Runtime system name and version.

  Describes the name and version of the runtime system responsible for the indicated capability set.

`PROGRAM_ARGS`

  Tag: | 30
  Length: | variable
  Field CapSetId: | Capability set
  Field [String]: | The command-line arguments passed to the program

  Describes the command-line used to start the program.

`PROGRAM_ENV`

  Tag: | 31
  Length: | variable
  Field CapSetId: | Capability set
  Field [String]: | The environment variable name/value pairs. (TODO: encoding?)

  Describes the environment variables present in the program’s environment.

### 15.2.3. Thread and scheduling events

`CREATE_THREAD`

  Tag: | 0
  Length: | fixed
  Field ThreadId: | thread id

  Marks the creation of a Haskell thread.

`RUN_THREAD`

  Tag: | 1
  Length: | fixed
  Field ThreadId: | thread id

  The indicated thread has started running.

`STOP_THREAD`

  Tag: | 2
  Length: | fixed
  Field ThreadId: | thread id
  Field Word16: | status 1: HeapOverflow 2: StackOverflow 3: ThreadYielding 4: ThreadBlocked 5: ThreadFinished 6: ForeignCall 7: BlockedOnMVar 8: BlockedOnBlackHole 9: BlockedOnRead 10: BlockedOnWrite 11: BlockedOnDelay 12: BlockedOnSTM 13: BlockedOnDoProc 16: BlockedOnMsgThrowTo
  Field ThreadId: | thread id of thread being blocked on (only for some status values)

  The indicated thread has stopped running for the reason given by `status`.

`THREAD_RUNNABLE`

  Tag: | 3
  Length: | fixed
  Field ThreadId: | thread id

  The indicated thread is has been marked as ready to run.

`MIGRATE_THREAD`

  Tag: | 4
  Length: | fixed
  Field ThreadId: | thread id
  Field CapNo: | capability

  The indicated thread has been migrated to a new capability.

`THREAD_WAKEUP`

  Tag: | 8
  Length: | fixed
  Field ThreadId: | thread id
  Field CapNo: | other capability

  The indicated thread has been woken up on another capability.

`THREAD_LABEL`

  Tag: | 44
  Length: | fixed
  Field ThreadId: | thread id
  Field String: | label

  The indicated thread has been given a label (e.g. with Control.Concurrent.setThreadLabel).

### 15.2.4. Garbage collector events

`GC_START`

  Tag: | 9
  Length: | fixed

  A garbage collection pass has been started.

`GC_END`

  Tag: | 10
  Length: | fixed

  A garbage collection pass has been finished.

`REQUEST_SEQ_GC`

  Tag: | 11
  Length: | fixed

  A sequential garbage collection has been requested by a capability.

`REQUEST_PAR_GC`

  Tag: | 12
  Length: | fixed

  A parallel garbage collection has been requested by a capability.

`GC_IDLE`

  Tag: | 20
  Length: | fixed

  An idle-time garbage collection has been started.

`GC_WORK`

  Tag: | 21
  Length: | fixed

  Marks the start of concurrent scavenging.

`GC_DONE`

  Tag: | 22
  Length: | fixed

  Marks the end of concurrent scavenging.

`GC_STATS_GHC`

  Tag: | 53
  Length: | fixed
  Field CapSetId: | heap capability set
  Field Word16: | generation of collection
  Field Word64: | bytes copied
  Field Word64: | bytes of slop found
  Field Word64: | TODO
  Field Word64: | number of parallel garbage collection threads
  Field Word64: | maximum number of bytes copied by any single collector thread
  Field Word64: | total bytes copied by all collector threads

  Report various information about the heap configuration. Typically produced during RTS initialization..

`GC_GLOBAL_SYNC`

  Tag: | 54
  Length: | fixed

  TODO

### 15.2.5. Heap events and statistics

`HEAP_ALLOCATED`

  Tag: | 49
  Length: | fixed
  Field CapSetId: | heap capability set
  Field Word64: | allocated bytes

  A new chunk of heap has been allocated by the indicated capability set.

`HEAP_SIZE`

  Tag: | 50
  Length: | fixed
  Field CapSetId: | heap capability set
  Field Word64: | heap size in bytes

  Report the heap size.

`HEAP_LIVE`

  Tag: | 51
  Length: | fixed
  Field CapSetId: | heap capability set
  Field Word64: | heap size in bytes

  Report the live heap size.

`HEAP_INFO_GHC`

  Tag: | 52
  Length: | fixed
  Field CapSetId: | heap capability set
  Field Word16: | number of garbage collection generations
  Field Word64: | maximum heap size
  Field Word64: | allocation area size
  Field Word64: | MBlock size
  Field Word64: | Block size

  Report various information about the heap configuration. Typically produced during RTS initialization..

### 15.2.6. Spark events

`CREATE_SPARK_THREAD`

  Tag: | 15
  Length: | fixed

  A thread has been created to perform spark evaluation.

`SPARK_COUNTERS`

  Tag: | 34
  Length: | fixed

  A periodic reporting of various statistics of spark evaluation.

`SPARK_CREATE`

  Tag: | 35
  Length: | fixed

  A spark has been added to the spark pool.

`SPARK_DUD`

  Tag: | 36
  Length: | fixed

  TODO

`SPARK_OVERFLOW`

  Tag: | 37
  Length: | fixed

  TODO

`SPARK_RUN`

  Tag: | 38
  Length: | fixed

  Evaluation has started on a spark.

`SPARK_STEAL`

  Tag: | 39
  Length: | fixed
  Field Word16: | capability from which the spark was stolen

  A spark has been stolen from another capability for evaluation.

`SPARK_FIZZLE`

  Tag: | 40
  Length: | fixed

  A spark has been GC’d before being evaluated.

`SPARK_GC`

  Tag: | 41
  Length: | fixed

  An unevaluated spark has been garbage collected.

### 15.2.7. Capability events

`CAP_CREATE`

  Tag: | 45
  Length: | fixed
  Field CapNo: | the capability number

  A capability has been started.

`CAP_DELETE`

  Tag: | 46
  Length: | fixed

  A capability has been deleted.

`CAP_DISABLE`

  Tag: | 47
  Length: | fixed

  A capability has been disabled.

`CAP_ENABLE`

  Tag: | 48
  Length: | fixed

  A capability has been enabled.

### 15.2.8. Task events

`TASK_CREATE`

  Tag: | 55
  Length: | fixed
  Field TaskId: | task id
  Field CapNo: | capability number
  Field ThreadId: | TODO

  Marks the creation of a task.

`TASK_MIGRATE`

  Tag: | 56
  Length: | fixed
  Field TaskId: | task id
  Field CapNo: | old capability
  Field CapNo: | new capability

  Marks the migration of a task to a new capability.

### 15.2.9. Tracing events

`LOG_MSG`

  Tag: | 16
  Length: | variable
  Field String: | The message

  A log message from the runtime system.

`BLOCK_MARKER`

  Tag: | 18
  Length: | variable
  Field Word32: | size
  Field Word64: | end time in nanoseconds
  Field String: | marker name

  TODO

`USER_MSG`

  Tag: | 19
  Length: | variable
  Field String: | message

  A user log message (from, e.g., Control.Concurrent.traceEvent).

`USER_MARKER`

  Tag: | 58
  Length: | variable
  Field String: | marker name

  A user marker (from Debug.Trace.traceMarker).

## 15.3. Heap profiler event log output

The heap profiler can produce output to GHC’s event log, allowing samples to be correlated with other event log events over the program’s lifecycle.

This section defines the layout of these events. The `String` type below is defined to be a UTF-8 encoded NUL-terminated string.

### 15.3.1. Metadata event types

#### 15.3.1.1. Beginning of sample stream

A single fixed-width event emitted during program start-up describing the samples that follow.

`HEAP_PROF_BEGIN`

  Tag: | 160
  Length: | variable
  Field Word8: | profile ID
  Field Word64: | sampling period in nanoseconds
  Field Word32: | sample breadown type. One of, `HEAP_PROF_BREAKDOWN_COST_CENTER` (output from `-hc`) `HEAP_PROF_BREAKDOWN_CLOSURE_DESCR` (output from `-hd`) `HEAP_PROF_BREAKDOWN_RETAINER` (output from `-hr`) `HEAP_PROF_BREAKDOWN_MODULE` (output from `-hm`) `HEAP_PROF_BREAKDOWN_TYPE_DESCR` (output from `-hy`) `HEAP_PROF_BREAKDOWN_BIOGRAPHY` (output from `-hb`) `HEAP_PROF_BREAKDOWN_CLOSURE_TYPE` (output from `-hT`)
  Field String: | module filter
  Field String: | closure description filter
  Field String: | type description filter
  Field String: | cost centre filter
  Field String: | cost centre stack filter
  Field String: | retainer filter
  Field String: | biography filter

#### 15.3.1.2. Cost centre definitions

A variable-length packet produced once for each cost centre,

`HEAP_PROF_COST_CENTRE`

  Tag: | 161
  Length: | fixed
  Field Word32: | cost centre number
  Field String: | label
  Field String: | module
  Field String: | source location
  Field Word8: | flags: bit 0: is the cost-centre a CAF?

#### 15.3.1.3. Sample event types

A sample (consisting of a list of break-down classes, e.g. cost centres, and heap residency sizes), is to be encoded in the body of one or more events.

We normally mark the beginning of a new sample with an `EVENT_HEAP_PROF_SAMPLE_BEGIN` event,

`HEAP_PROF_SAMPLE_BEGIN`

  Length: | fixed
  Field Word64: | sample number

  Marks the beginning of a heap profile sample.

Biographical profiling samples start with the `EVENT_HEAP_BIO_PROF_SAMPLE_BEGIN` event. These events also include a timestamp which indicates when the sample was taken. This is because all these samples will appear at the end of the eventlog due to how the biographical profiling mode works. You can use the timestamp to reorder the samples relative to the other events.

`HEAP_BIO_PROF_SAMPLE_BEGIN`

  Tag: | 166
  Length: | fixed
  Field Word64: | sample number
  Field Word64: | eventlog timestamp in ns

A heap residency census will follow. Since events may only be up to 2^16^ bytes in length a single sample may need to be split among multiple `EVENT_HEAP_PROF_SAMPLE` events. The precise format of the census entries is determined by the break-down type.

At the end of the sample period the `EVENT_HEAP_PROF_SAMPLE_END` event if emitted. This is useful to properly delimit the sampling period and to record the total time spent profiling.

`HEAP_PROF_SAMPLE_END`

  Tag: | 165
  Length: | fixed
  Field Word64: | sample number

  Marks the end of a heap profile sample.

#### 15.3.1.4. Cost-centre break-down

A variable-length packet encoding a heap profile sample broken down by,
  - cost-centre (`-hc`)

`HEAP_PROF_SAMPLE_COST_CENTRE`

  Tag: | 163
  Length: | variable
  Field Word8: | profile ID
  Field Word64: | heap residency in bytes
  Field Word8: | stack depth
  Field Word32[]: | cost centre stack starting with inner-most (cost centre numbers)

#### 15.3.1.5. String break-down

A variable-length event encoding a heap sample broken down by,

> - type description (`-hy`)
> - closure description (`-hd`)
> - module (`-hm`)

`HEAP_PROF_SAMPLE_STRING`

  Tag: | 164
  Length: | variable
  Field Word8: | profile ID
  Field Word64: | heap residency in bytes
  Field String: | type or closure description, or module name

## 15.4. Time profiler event log output

The time profiling mode enabled by `-p` also emits sample events to the eventlog. At the start of profiling the tick interval is emitted to the eventlog and then on each tick the current cost centre stack is emitted. Together these enable a user to construct an approximate track of the executation of their program.

### 15.4.1. Profile begin event

`PROF_BEGIN`

  Tag: | 168
  Length: | fixed
  Field Word64: | tick interval, in nanoseconds

  Marks the beginning of a time profile.

### 15.4.2. Profile sample event

A variable-length packet encoding a profile sample.

`PROF_SAMPLE_COST_CENTRE`

  Tag: | 167
  Length: | variable
  Field Word32: | capability
  Field Word64: | current profiling tick
  Field Word8: | stack depth
  Field Word32[]: | cost centre stack starting with inner-most (cost centre numbers)

## 15.5. Biographical profile sample event

A variable-length packet encoding a profile sample.

`BIO_PROF_SAMPLE_BEGIN`

  Tag: | 166

  TODO

## 15.6. Non-moving GC event output

These events mark various stages of the `non-moving collection` lifecycle. These are enabled with the `+RTS -lg` event-set.

`CONC_MARK_BEGIN`

  Tag: | 200
  Length: | fixed

  Marks the beginning of marking by the concurrent collector.

`CONC_MARK_END`

  Tag: | 201
  Length: | fixed

  Marks the end of marking by the concurrent collector.

`CONC_SYNC_BEGIN`

  Tag: | 202
  Length: | fixed

  Marks the beginning of the concurrent garbage collector’s post-mark synchronization phase.

`CONC_SYNC_END`

  Tag: | 203
  Length: | fixed

  Marks the end of the concurrent garbage collector’s post-mark synchronization phase.

`CONC_SWEEP_BEGIN`

  Tag: | 204
  Length: | fixed

  Marks the beginning of the concurrent garbage collector’s sweep phase.

`CONC_SWEEP_END`

  Tag: | 205
  Length: | fixed

  Marks the end of the concurrent garbage collector’s sweep phase.

`CONC_UPD_REM_SET_FLUSH`

  Tag: | 206
  Length: | fixed

  Marks a capability flushing its local update remembered set accumulator.

### 15.6.1. Non-moving heap census

The non-moving heap census events (enabled with the `+RTS -ln` event-set) are intended to provide insight into fragmentation of the non-moving heap.

`NONMOVING_HEAP_CENSUS`

  Tag: | 207
  Length: | fixed
  Field Word8: | base-2 logarithm of blk_sz.
  Field Word32: | number of active segments.
  Field Word32: | number of filled segments.
  Field Word32: | number of live blocks.

  Describes the occupancy of the blk_sz sub-heap.