Joe McDonnell created IMPALA-15088:
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Summary: Improve interoperability of IntGauge and FunctionGauge
for read-only code
Key: IMPALA-15088
URL: https://issues.apache.org/jira/browse/IMPALA-15088
Project: IMPALA
Issue Type: Improvement
Components: Backend
Affects Versions: Impala 5.0.0
Reporter: Joe McDonnell
The IntGauge type is an alias for AtomicMetric<TMetricKind::GAUGE>, which has
an atomic int64_t that can be updated. However, we also use IntGauge for many
other things that are read only. Here are some examples:
* MemTracker can take an IntGauge consumption metric to integrate it into the
MemTracker hierarchy (e.g. tcmalloc overhead)
* SumGauge reads multiple IntGauges and returns the sum
* NegatedGauge reads a single IntGauge and negates it
* Memory metrics override IntGauge but get the values from gperftools
MallocExtension (or other equivalents)
These are read only uses of IntGauge or situations where the IntGauge behaves
more like a FunctionGauge. Since FunctionGauge doesn't inherit from IntGauge,
it makes it hard to use FunctionGauge with any of these pieces of code.
Since these are all read-only uses, it might make sense to create a
ReadOnlyIntGauge alias that is the base class of FunctionGauge and IntGauge.
i.e.
{noformat}
using ReadOnlyIntGauge = ScalarMetric<int64_t, TMetricKind::GAUGE>;{noformat}
Read only uses work properly with this base class, and it makes it possible for
IntGauges and FunctionGauges to interoperate with this code.
This would be very useful for the memory metrics. In particular, the derived
metric classes like TotalBytesReservedMetric or the OverheadBytesMetric from
the MallocUtil change would be unnecessary if they could be expressed as
FunctionGauges.
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