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 (
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 EventType* 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.
15.2. Runtime system diagnostics¶
ThreadId ~ Word32
CapNo ~ Word16
CapSetId ~ Word32
15.2.1. Capability sets¶
15.2.2. Environment information¶
These events are typically produced during program startup and describe the environment which the program is being run in.
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.
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.
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¶
Tag: 0 Length: fixed Field ThreadId: thread id
Marks the creation of a Haskell thread.
Tag: 1 Length: fixed Field ThreadId: thread id
The indicated thread has started running.
- 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
thread id of thread being blocked on (only for some status values)
The indicated thread has stopped running for the reason given by
Tag: 3 Length: fixed Field ThreadId: thread id
The indicated thread is has been marked as ready to run.
Tag: 4 Length: fixed Field ThreadId: thread id Field CapNo: capability
The indicated thread has been migrated to a new capability.
Tag: 8 Length: fixed Field ThreadId: thread id Field CapNo: other capability
The indicated thread has been woken up on another capability.
Tag: 44 Length: fixed Field ThreadId: thread id Field String: label
The indicated thread has been given a label (e.g. with GHC.Conc.labelThread).
15.2.4. 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:
- 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
- As other capabilities reach yield points and suspend execution they emit
- When all capabilities have suspended execution, collection will begin,
marked by a
- As individual parallel GC threads commence with scavenging they will emit
- If a parallel GC thread runs out of work it will emit a
GC_IDLEevent. If it is later handed more work it will emit another
- Eventually when scavenging has finished a
GC_DONEevent will be emitted by each GC thread.
- A bit of book-keeping is performed.
GC_ENDevent will be emitted marking the end of the GC cycle.
HEAP_SIZEevent will be emitted giving the current size of the heap, in bytes, calculated by how many megablocks are allocated.
BLOCKS_SIZEevent will be emitted giving the current size of the heap, in bytes, calculated by how many blocks are allocated.
GC_STATS_GHCevent will be emitted containing various details of the collection and heap state.
- In the case of a major collection, a
HEAP_LIVEevent will be emitted describing the current size of the live on-heap data.
- In the case of the
SPARK_COUNTERSevent will be emitted giving details on how many sparks have been created, evaluated, and GC’d.
- As mutator threads resume execution they will emit
MEM_RETURNevent 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.
Tag: 9 Length: fixed
A garbage collection pass has been started.
Tag: 10 Length: fixed
A garbage collection pass has been finished.
Tag: 11 Length: fixed
A sequential garbage collection has been requested by a capability.
Tag: 12 Length: fixed
A parallel garbage collection has been requested by a capability.
Tag: 20 Length: fixed
An idle-time garbage collection has been started.
Tag: 21 Length: fixed
Marks the start of concurrent scavenging.
Tag: 22 Length: fixed
Marks the end of concurrent scavenging.
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 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 a major collection.
Tag: 54 Length: fixed
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.
15.2.5. Heap events and statistics¶
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.
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.
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.
Tag: 51 Length: fixed Field CapSetId: heap capability set Field Word64: heap size in bytes
Report the live heap size.
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¶
Tag: 15 Length: fixed
A thread has been created to perform spark evaluation.
Tag: 34 Length: fixed
A periodic reporting of various statistics of spark evaluation.
Tag: 35 Length: fixed
A spark has been added to the spark pool.
Tag: 36 Length: fixed
Tag: 37 Length: fixed
Tag: 38 Length: fixed
Evaluation has started on a spark.
Tag: 39 Length: fixed Field Word16: capability from which the spark was stolen
A spark has been stolen from another capability for evaluation.
Tag: 40 Length: fixed
A spark has been GC’d before being evaluated.
Tag: 41 Length: fixed
An unevaluated spark has been garbage collected.
15.2.7. Capability events¶
Tag: 45 Length: fixed Field CapNo: the capability number
A capability has been started.
Tag: 46 Length: fixed
A capability has been deleted.
Tag: 47 Length: fixed
A capability has been disabled.
Tag: 48 Length: fixed
A capability has been enabled.
15.2.8. Task events¶
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.
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¶
Tag: 16 Length: variable Field String: The message
A log message from the runtime system.
Tag: 18 Length: variable Field Word32: size Field Word64: end time in nanoseconds Field String: marker name
Tag: 19 Length: variable Field String: message
A user log message (from, e.g., Control.Concurrent.traceEvent).
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¶
188.8.131.52. Beginning of sample stream¶
A single fixed-width event emitted during program start-up describing the samples that follow.
sampling period in nanoseconds
sample breadown type. One of,
closure description filter
type description filter
cost centre filter
cost centre stack filter
184.108.40.206. Cost centre definitions¶
A variable-length packet produced once for each cost centre,
cost centre number
- bit 0: is the cost-centre a CAF?
220.127.116.11. Info Table Provenance definitions¶
A message which describes an approximate source position for
info tables. See
-finfo-table-map for more information.
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
18.104.22.168. 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
Length: fixed Field Word64: sample number
Marks the beginning of a heap profile sample.
Biographical profiling samples start with the
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.
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.
Tag: 165 Length: fixed Field Word64: sample number
Marks the end of a heap profile sample.
22.214.171.124. Cost-centre break-down¶
- A variable-length packet encoding a heap profile sample broken down by,
- cost-centre (
- 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)
126.96.36.199. String break-down¶
A variable-length event encoding a heap sample broken down by,
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¶
Tag: 168 Length: fixed Field Word64: tick interval, in nanoseconds
Marks the beginning of a time profile.
15.5. Biographical profile sample event¶
A variable-length packet encoding a profile sample.
15.6. Non-moving GC event output¶
These events mark various stages of the
non-moving collection lifecycle. These are enabled
+RTS -lg event-set.
A typical non-moving collection cycle will look something like the following:
- The preparatory phase of collection will emit the usual events associated with a moving collection. See Garbage collector events for details.
- 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_FLUSHevents, indicating that they have flushed their capability-local update remembered sets.
- Concurrent marking begins, denoted by a
- When the mark queue is depleted a
- If necessary (e.g. due to weak pointer marking), the marking process will continue, returning to step (3) above.
- 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
- Mutator threads will suspend execution and, if necessary, flush their update
remembered sets (indicated by
- The collector will do any final marking necessary (indicated by
- The collector will do a small amount of sweeping, disable the write barrier,
CONC_SYNC_ENDevent, and allow mutators to resume
- The collector will begin the concurrent sweep phase, indicated by a
- Once sweeping has concluded a
CONC_SWEEP_ENDevent will be emitted and the concurrent collector thread will terminate.
NONMOVING_HEAP_CENSUSevent will be emitted describing the fragmentation state of the non-moving heap.
Tag: 200 Length: fixed
Marks the beginning of marking by the concurrent collector.
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.
Tag: 202 Length: fixed
Marks the beginning of the concurrent garbage collector’s post-mark synchronization phase.
Tag: 203 Length: fixed
Marks the end of the concurrent garbage collector’s post-mark synchronization phase.
Tag: 204 Length: fixed
Marks the beginning of the concurrent garbage collector’s sweep phase.
Tag: 205 Length: fixed
Marks the end of the concurrent garbage collector’s sweep phase.
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
event-set) are intended to provide insight into fragmentation of the non-moving
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.
15.6.2. Ticky counters¶
Programs compiled with
-eventlog and invoked
+RTS -lT will emit periodic samples of the ticky
entry counters to the eventlog.
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.
Tag: 212 Length: fixed
Denotes the beginning of an atomic set of ticky-ticky profiler counter samples.
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
Records the number of “ticks” recorded by a ticky-ticky counter single the last sample.