```go
type interInst struct {
x int
}
type inter interface {
}
var sink []inter
func BenchmarkX(b *testing.B) {
sink = make([]inter, b.N)
for i := 0; i < b.N; i++ {
sink[i] = &interInst{}
}
clear(sink)
sink = sink[:0]
runtime.GC()
var ms runtime.MemStats
runtime.ReadMemStats(&ms)
b.Log(b.N, ms.Frees) // Frees is the cumulative count of heap objects freed.
}
```
```
clear:
ioz_test.go:35: 1 589
ioz_test.go:35: 100 711
ioz_test.go:35: 10000 10729
ioz_test.go:35: 1000000 1010750 <-- 1m+ freed
ioz_test.go:35: 16076874 17087643
ioz_test.go:35: 19514749 36602412
```
```
no clear:
ioz_test.go:35: 1 585
ioz_test.go:35: 100 606
ioz_test.go:35: 10000 725
ioz_test.go:35: 1000000 10745 <-- some "overhead" objects freed, not the slice.
ioz_test.go:35: 16391445 1010765
ioz_test.go:35: 21765238 17402230
```
This is documented at https://go.dev/wiki/SliceTricks:
> NOTE If the type of the element is a pointer or a struct with pointer
fields, which need to be garbage collected, the above implementations of
Cut and Delete have a potential memory leak problem: some elements with
values are still referenced by slice a’s underlying array, just not
“visible” in the slice. Because the “deleted” value is referenced in the
underlying array, the deleted value is still “reachable” during GC, even
though the value cannot be referenced by your code. If the underlying
array is long-lived, this represents a leak.
Followed by examples of how zeroing out the slice elements solves
the problem. This PR does the same.
Follow-up to
https://github.com/open-telemetry/opentelemetry-go/pull/5861. This is an
attempt to:
* Limit the API surface of the aggregate package
* Try to use predefined types (e.g. exemplar.Filter) over custom
functions where possible.
* Avoid using nil, and use No-Ops where it makes sense
This makes `aggregate.NewFilteredExemplarReservoir` no longer exported,
removes the `aggregate.FilteredExemplarReservoir` interface, and removes
the `aggregate.dropReservoir`.
Resolve https://github.com/open-telemetry/opentelemetry-go/issues/5249
### Spec
> exemplar_reservoir: A functional type that generates an exemplar
reservoir a MeterProvider will use when storing exemplars. This
functional type needs to be a factory or callback similar to aggregation
selection functionality which allows different reservoirs to be chosen
by the aggregation.
> Users can provide an exemplar_reservoir, but it is up to their
discretion. Therefore, the stream configuration parameter needs to be
structured to accept an exemplar_reservoir, but MUST NOT obligate a user
to provide one. If the user does not provide an exemplar_reservoir
value, the MeterProvider MUST apply a [default exemplar
reservoir](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar-defaults).
Also,
> the reservoir MUST be given the Attributes associated with its
timeseries point either at construction so that additional sampling
performed by the reservoir has access to all attributes from a
measurement in the "offer" method.
### Changes
In sdk/metric/exemplar, add:
* `exemplar.ReservoirProvider`
* `exemplar.HistogramReservoirProvider`
* `exemplar.FixedSizeReservoirProvider`
In sdk/metric, add:
* `metric.ExemplarReservoirProviderSelector` (func Aggregation ->
ReservoirProvider)
* `metric.DefaultExemplarReservoirProviderSelector` (our default
implementation)
* `ExemplarReservoirProviderSelector` added to `metric.Stream`
Note: the only required public types are
`metric.ExemplarReservoirProviderSelector` and
`ExemplarReservoirProviderSelector` in `metric.Stream`. Others are for
convenience and readability.
### Alternatives considered
#### Add ExemplarReservoirProvider directly to metric.Stream, instead of
ExemplarReservoirProviderSelector
This would mean users can configure a `func() exemplar.Reservoir`
instead of a `func(Aggregation) func() exemplar.Reservoir`.
I don't think this complies with the statement: `This functional type
needs to be a factory or callback similar to aggregation selection
functionality which allows different reservoirs to be chosen by the
aggregation.`. I'm interpreting "allows different reservoirs to be
chosen by the aggregation" as meaning "allows different reservoirs to be
chosen **based on the** aggregation", rather than meaning that the
aggregation is somehow choosing the reservoir.
### Future work
There is some refactoring I plan to do after this to simplify the
interaction between the internal/aggregate and exemplar package. I've
omitted that from this PR to keep the diff smaller.
---------
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Robert Pająk <pellared@hotmail.com>
Part of https://github.com/open-telemetry/opentelemetry-go/issues/5249
This makes all existing types designed to implement the public Exemplar
API public by moving most of `internal/exemplar` to `exemplar`. The only
types that are not being made public are `exemplar.Drop`, and
`exemplar.FilteredReservoir`. Those types are moved to
`internal/aggregate`, and are renamed to `DropReservoir` and
`FilteredExemplarReservoir`.
The following types are made public:
* `exemplar.Exemplar`
* `exemplar.Filter`
* `exemplar.SampledFilter`
* `exemplar.AlwaysOnFilter`
* `exemplar.HistogramReservoir`
* `exemplar.FixedSizeReservoir`
* `exemplar.Reservoir`
* `exemplar.Value`
* `exemplar.ValueType`
