17. Eventlog encodings — Glasgow Haskell Compiler 9.10.0.20240426 User's Guide

# 17. 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.

## 17.1. 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 (`EventType`) present in the file. The header is followed by the event records (`Event`) themselves, each of which start with the event type id and a 64-bit timestamp:

    EventLog :
          EVENT_HEADER_BEGIN
          EVENT_HET_BEGIN -- header event types begin
          EventType*
          EVENT_HET_END -- header event types end
          EVENT_HEADER_END
          EVENT_DATA_BEGIN
          Event*
          EVENT_DATA_END

    EventType :
          EVENT_ET_BEGIN
          Word16         -- event type id, unique identifier for this event
          Int16          -- >=0  size of the event record in bytes (minus the event type id and timestamp fields)
                         -- -1   variable size
          Word32         -- size of the event description in bytes
          Word8*         -- event description, UTF8 encoded string describing the event
          Word32         -- size of the extra info in bytes
          Word8*         -- extra info (for future extensions)
          EVENT_ET_END

    Event :
          Word16         -- event type id, as included in the event log header
          Word64         -- timestamp (nanoseconds)
          [Word16]       -- length of the rest (optional, for variable-sized events only)
          ... 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, the size given in the header event-log header.
> - Variable size: Each event record includes a length field.

## 17.2. Runtime system diagnostics

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

### 17.2.1. Capability sets

TODO

### 17.2.2. 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.

`WALL_CLOCK_TIME`

  Tag: | 43
  Length: | fixed
  Field CapSetId: | Capability set
  Field Word64: | Unix epoch seconds
  Field Word32: | Nanoseconds

  Records the wall clock time to make it possible to correlate events from elsewhere with the eventlog.

### 17.2.3. 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 20: BlockedOnMVarRead
  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: | variable
  Field ThreadId: | thread id
  Field String: | label

  The indicated thread has been given a label (e.g. with GHC.Conc.labelThread).

### 17.2.4. Garbage collector events

The following events mark various points of the lifecycle of a moving garbage collection.

A typical garbage collection will look something like the following:

1. A capability realizes that it needs a garbage collection (e.g. as a result of running out of nursery) and requests a garbage collection. This is marked by `REQUEST_SEQ_GC` or `REQUEST_PAR_GC`.
2. As other capabilities reach yield points and suspend execution they emit `STOP_THREAD` events.
3. When all capabilities have suspended execution, collection will begin, marked by a `GC_START` event.
4. As individual parallel GC threads commence with scavenging they will emit `GC_WORK` events.
5. If a parallel GC thread runs out of work it will emit a `GC_IDLE` event. If it is later handed more work it will emit another `GC_WORK` event.
6. Eventually when scavenging has finished a `GC_DONE` event will be emitted by each GC thread.
7. A bit of book-keeping is performed.
8. A `GC_END` event will be emitted marking the end of the GC cycle.
9. A `HEAP_SIZE` event will be emitted giving the current size of the heap, in bytes, calculated by how many megablocks are allocated.
10. A `BLOCKS_SIZE` event will be emitted giving the current size of the heap, in bytes, calculated by how many blocks are allocated.
11. A `GC_STATS_GHC` event will be emitted containing various details of the collection and heap state.
12. In the case of a major collection, a `HEAP_LIVE` event will be emitted describing the current size of the live on-heap data.
13. In the case of the `-threaded` RTS, a `SPARK_COUNTERS` event will be emitted giving details on how many sparks have been created, evaluated, and GC’d.
14. As mutator threads resume execution they will emit `RUN_THREAD` events.
15. A `MEM_RETURN` event will be emitted containing details about currently live mblocks, how many we think we need and whether we could return excess to the OS.

Note that in the case of the concurrent non-moving collector additional events will be emitted during the concurrent phase of collection. These are described in Non-moving GC event output.

`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: | bytes of fragmentation, the difference between total mblock size and total block size. When all mblocks are full of full blocks, this number is 0.
  Field Word32: | 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
  Field Word64: | the amount of balanced data copied by all threads

  Report various information about a major collection.

`GC_GLOBAL_SYNC`

  Tag: | 54
  Length: | fixed

  TODO

`MEM_RETURN`

  Tag: | 90
  Length: | fixed
  Field CapSetId: | heap capability set
  Field Word32: | currently allocated mblocks
  Field Word32: | the number of mblocks we would like to retain
  Field Word32: | the number of mblocks which we returned to the OS

  Report information about currently allocation megablocks and attempts made to return them to the operating system. If your heap is fragmented then the current value will be greater than needed value but returned will be less than the difference between the two.

### 17.2.5. 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, calculated by the number of megablocks currently allocated.

`BLOCKS_SIZE`

  Tag: | 91
  Length: | fixed
  Field CapSetId: | heap capability set
  Field Word64: | heap size in bytes

  Report the heap size, calculated by the number of blocks currently allocated.

`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..

### 17.2.6. 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.

### 17.2.7. 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.

### 17.2.8. Task events

`TASK_CREATE`

  Tag: | 55
  Length: | fixed
  Field TaskId: | task id
  Field CapNo: | capability number
  Field KernelThreadId:
  | The thread-id of the kernel thread which created the task.

  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.

`TASK_DELETE`

  Tag: | 57
  Length: | fixed
  Field TaskId: | task id

  Marks the deletion of a task.

### 17.2.9. Tracing events

`LOG_MSG`

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

  A log message from the runtime system.

`BLOCK_MARKER`

  Tag: | 18
  Length: | fixed
  Field Word32: | block size
  Field Word64: | end time in nanoseconds
  Field Word16: | capability number, invalid if `0xffff`.

  Marks a chunk of events. The events that fit in the next `block size` bytes all belong to the block marker capability.

`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).

## 17.3. 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.

### 17.3.1. Metadata event types

#### 17.3.1.1. 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

#### 17.3.1.2. 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?

#### 17.3.1.3. Info Table Provenance definitions

A message which describes an approximate source position for info tables. See `-finfo-table-map` for more information.

`IPE`

  Tag: | 169
  Length: | fixed
  Field Word64: | info table address
  Field String: | table name
  Field String: | closure type
  Field String: | type
  Field String: | source position label
  Field String: | source position module
  Field String: | source position location

#### 17.3.1.4. 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.

#### 17.3.1.5. 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)

#### 17.3.1.6. 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

## 17.4. 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.

### 17.4.1. Profile begin event

`PROF_BEGIN`

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

  Marks the beginning of a time profile.

### 17.4.2. 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)

## 17.5. Biographical profile sample event

A variable-length packet encoding a profile sample.

`BIO_PROF_SAMPLE_BEGIN`

  Tag: | 166

  TODO

## 17.6. 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.

A typical non-moving collection cycle will look something like the following:

1. The preparatory phase of collection will emit the usual events associated with a moving collection. See Garbage collector events for details.
2. The concurrent write barrier is enabled and the concurrent mark thread is started. From this point forward mutator threads may emit `CONC_UPD_REM_SET_FLUSH` events, indicating that they have flushed their capability-local update remembered sets.
3. Concurrent marking begins, denoted by a `CONC_MARK_BEGIN` event.
4. When the mark queue is depleted a `CONC_MARK_END` is emitted.
5. If necessary (e.g. due to weak pointer marking), the marking process will continue, returning to step (3) above.
6. When the collector has done as much concurrent marking as it can it will enter the post-mark synchronization phase of collection, denoted by a `CONC_SYNC_BEGIN` event.
7. Mutator threads will suspend execution and, if necessary, flush their update remembered sets (indicated by `CONC_UPD_REM_SET_FLUSH` events).
8. The collector will do any final marking necessary (indicated by `CONC_MARK_BEGIN` and `CONC_MARK_END` events).
9. The collector will do a small amount of sweeping, disable the write barrier, emit a `CONC_SYNC_END` event, and allow mutators to resume
10. The collector will begin the concurrent sweep phase, indicated by a `CONC_SWEEP_BEGIN` event.
11. Once sweeping has concluded a `CONC_SWEEP_END` event will be emitted and the concurrent collector thread will terminate.
12. A `NONMOVING_HEAP_CENSUS` event will be emitted describing the fragmentation state of the non-moving heap.

`CONC_MARK_BEGIN`

  Tag: | 200
  Length: | fixed

  Marks the beginning of marking by the concurrent collector.

`CONC_MARK_END`

  Tag: | 201
  Length: | fixed
  Field Word32: | number of objects which were marked in this marking phase.

  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.

### 17.6.1. 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 Word16: | blk_sz in bytes.
  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.

### 17.6.2. Ticky counters

Programs compiled with `-ticky` and `-eventlog` and invoked with `+RTS -lT` will emit periodic samples of the ticky entry counters to the eventlog.

`TICKY_COUNTER_DEF`

  Tag: | 210
  Length: | variable
  Field Word64: | counter ID
  Field Word16: | arity/field count
  Field String: | argument kinds. This is the same as the synonymous field in the textual ticky summary.
  Field String: | counter name

  Defines a ticky counter.

`TICKY_COUNTER_BEGIN_SAMPLE`

  Tag: | 212
  Length: | fixed

  Denotes the beginning of an atomic set of ticky-ticky profiler counter samples.

`TICKY_COUNTER_SAMPLE`

  Tag: | 211
  Length: | fixed
  Field Word64: | counter ID
  Field Word64: | number of times closures of this type has been entered.
  Field Word64: | number of allocations (words)
  Field Word64: | number of times this has been allocated (words). Only produced for modules compiled with `-ticky-allocd`.

  Records the number of “ticks” recorded by a ticky-ticky counter single the last sample.