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.
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.
A single fixed-width event emitted during program start-up describing the samples that follow.
EVENT_HEAP_PROF_BEGIN
Word8: Profile ID
Word64: Sampling period in nanoseconds
Word32: Sample break-down 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)
String: Module filter
String: Closure description filter
String: Type description filter
String: Cost centre filter
String: Cost centre stack filter
String: Retainer filter
String: Biography filter
A variable-length packet produced once for each cost centre,
- EVENT_HEAP_PROF_COST_CENTRE
- Word32: cost centre number
- String: label
- String: module
- String: source location
- Word8: flags
- bit 0: is the cost-centre a CAF?
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 mark the beginning of a new sample with an EVENT_HEAP_PROF_SAMPLE_BEGIN event,
- EVENT_HEAP_PROF_SAMPLE_BEGIN
- Word64: sample number
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.
A variable-length event encoding a heap sample broken down by,
- type description (-hy)
- closure description (-hd)
- module (-hm)
- EVENT_HEAP_PROF_SAMPLE_STRING
- Word8: Profile ID
- Word64: heap residency in bytes
- String: type or closure description, or module name