Package metrics provides a stable interface to access implementation-defined metrics exported by the Go runtime. This package is similar to existing functions like runtime.ReadMemStats and debug.ReadGCStats, but significantly more general.
The set of metrics defined by this package may evolve as the runtime itself evolves, and also enables variation across Go implementations, whose relevant metric sets may not intersect.
Metrics are designated by a string key, rather than, for example, a field name in a struct. The full list of supported metrics is always available in the slice of Descriptions returned by All. Each Description also includes useful information about the metric.
Thus, users of this API are encouraged to sample supported metrics defined by the slice returned by All to remain compatible across Go versions. Of course, situations arise where reading specific metrics is critical. For these cases, users are encouraged to use build tags, and although metrics may be deprecated and removed, users should consider this to be an exceptional and rare event, coinciding with a very large change in a particular Go implementation.
Each metric key also has a "kind" that describes the format of the metric's value. In the interest of not breaking users of this package, the "kind" for a given metric is guaranteed not to change. If it must change, then a new metric will be introduced with a new key and a new "kind."
Metric key format
As mentioned earlier, metric keys are strings. Their format is simple and well-defined, designed to be both human and machine readable. It is split into two components, separated by a colon: a rooted path and a unit. The choice to include the unit in the key is motivated by compatibility: if a metric's unit changes, its semantics likely did also, and a new key should be introduced.
For more details on the precise definition of the metric key's path and unit formats, see the documentation of the Name field of the Description struct.
A note about floats
This package supports metrics whose values have a floating-point representation. In order to improve ease-of-use, this package promises to never produce the following classes of floating-point values: NaN, infinity.
Below is the full list of supported metrics, ordered lexicographically.
/gc/cycles/automatic:gc-cycles Count of completed GC cycles generated by the Go runtime. /gc/cycles/forced:gc-cycles Count of completed GC cycles forced by the application. /gc/cycles/total:gc-cycles Count of all completed GC cycles. /gc/heap/allocs-by-size:bytes Distribution of heap allocations by approximate size. Note that this does not include tiny objects as defined by /gc/heap/tiny/allocs:objects, only tiny blocks. /gc/heap/allocs:bytes Cumulative sum of memory allocated to the heap by the application. /gc/heap/allocs:objects Cumulative count of heap allocations triggered by the application. Note that this does not include tiny objects as defined by /gc/heap/tiny/allocs:objects, only tiny blocks. /gc/heap/frees-by-size:bytes Distribution of freed heap allocations by approximate size. Note that this does not include tiny objects as defined by /gc/heap/tiny/allocs:objects, only tiny blocks. /gc/heap/frees:bytes Cumulative sum of heap memory freed by the garbage collector. /gc/heap/frees:objects Cumulative count of heap allocations whose storage was freed by the garbage collector. Note that this does not include tiny objects as defined by /gc/heap/tiny/allocs:objects, only tiny blocks. /gc/heap/goal:bytes Heap size target for the end of the GC cycle. /gc/heap/objects:objects Number of objects, live or unswept, occupying heap memory. /gc/heap/tiny/allocs:objects Count of small allocations that are packed together into blocks. These allocations are counted separately from other allocations because each individual allocation is not tracked by the runtime, only their block. Each block is already accounted for in allocs-by-size and frees-by-size. /gc/pauses:seconds Distribution individual GC-related stop-the-world pause latencies. /memory/classes/heap/free:bytes Memory that is completely free and eligible to be returned to the underlying system, but has not been. This metric is the runtime's estimate of free address space that is backed by physical memory. /memory/classes/heap/objects:bytes Memory occupied by live objects and dead objects that have not yet been marked free by the garbage collector. /memory/classes/heap/released:bytes Memory that is completely free and has been returned to the underlying system. This metric is the runtime's estimate of free address space that is still mapped into the process, but is not backed by physical memory. /memory/classes/heap/stacks:bytes Memory allocated from the heap that is reserved for stack space, whether or not it is currently in-use. /memory/classes/heap/unused:bytes Memory that is reserved for heap objects but is not currently used to hold heap objects. /memory/classes/metadata/mcache/free:bytes Memory that is reserved for runtime mcache structures, but not in-use. /memory/classes/metadata/mcache/inuse:bytes Memory that is occupied by runtime mcache structures that are currently being used. /memory/classes/metadata/mspan/free:bytes Memory that is reserved for runtime mspan structures, but not in-use. /memory/classes/metadata/mspan/inuse:bytes Memory that is occupied by runtime mspan structures that are currently being used. /memory/classes/metadata/other:bytes Memory that is reserved for or used to hold runtime metadata. /memory/classes/os-stacks:bytes Stack memory allocated by the underlying operating system. /memory/classes/other:bytes Memory used by execution trace buffers, structures for debugging the runtime, finalizer and profiler specials, and more. /memory/classes/profiling/buckets:bytes Memory that is used by the stack trace hash map used for profiling. /memory/classes/total:bytes All memory mapped by the Go runtime into the current process as read-write. Note that this does not include memory mapped by code called via cgo or via the syscall package. Sum of all metrics in /memory/classes. /sched/goroutines:goroutines Count of live goroutines. /sched/latencies:seconds Distribution of the time goroutines have spent in the scheduler in a runnable state before actually running.
func Read(m Sample)
Read populates each Value field in the given slice of metric samples.
Desired metrics should be present in the slice with the appropriate name. The user of this API is encouraged to re-use the same slice between calls for efficiency, but is not required to do so.
Note that re-use has some caveats. Notably, Values should not be read or manipulated while a Read with that value is outstanding; that is a data race. This property includes pointer-typed Values (for example, Float64Histogram) whose underlying storage will be reused by Read when possible. To safely use such values in a concurrent setting, all data must be deep-copied.
It is safe to execute multiple Read calls concurrently, but their arguments must share no underlying memory. When in doubt, create a new Sample from scratch, which is always safe, though may be inefficient.
Sample values with names not appearing in All will have their Value populated as KindBad to indicate that the name is unknown.
▹ Example (ReadingAllMetrics)
▾ Example (ReadingAllMetrics)