This change is made in attempt to reduce memory usage by vmalert when
parsing big instant responses from VM/Prometheus.
In
a5c427bac4
vmalert switched from std json lib to fastjson lib in order to reduce
amount of allocations, as according to highloaded profiles of vmalert
the CPU is mostly spent on GC.
But switching to fastjson resulted into excessive memory usage for cases
when vmalert has to parse long json lines, which usually happens when
instant response contains many `metric` objects.
In this change we do a mixed parsing:
1. Slice of `metric` objects is parsed with std lib to keep mem low
2. Each `metric` object is parsed with fastjson to reduce allocs
The benchmark results are the following:
```
pkg: github.com/VictoriaMetrics/VictoriaMetrics/app/vmalert/datasource
BenchmarkParsePrometheusResponse/Instant_std+fastjson-10 1760 668959 ns/op 280147 B/op 5781 allocs/op
MBs allocated at heap: 493.078392
mallocs: 18655472
BenchmarkParsePrometheusResponse/Instant_fastjson-10 6109 198258 ns/op 172839 B/op 5548 allocs/op
MBs allocated at heap: 1056.384464
mallocs: 34457184
BenchmarkParsePrometheusResponse/Instant_std-10 1287 950987 ns/op 451677 B/op 9619 allocs/op
MBs allocated at heap: 580.802976
mallocs: 13351636
```
The benchmark function code with mem measurement is available here
https://gist.github.com/hagen1778/b9c3ca7f8ca7d6b21aec9777112c5810
The benchmark contains 3 results:
1. Instant_std+fastjson is the implementation in this change
2. Instant_fastjson-10 is the implementation from
a5c427bac4
3. BenchmarkParsePrometheusResponse/Instant_std-10 is implementation
before
a5c427bac4
According to these results, this new implementation is slower than
previous, but faster than before switching to fastjson. It also has
lower number of allocations and roughly the same memory allocation on
heap with GC turned off.
---------
Other changes:
1. rm BenchmarkMetrics as it doesn't measure anything
2. simplify BenchmarkParsePrometheusResponse into
BenchmarkPromInstantUnmarshal
### Describe Your Changes
Please provide a brief description of the changes you made. Be as
specific as possible to help others understand the purpose and impact of
your modifications.
### Checklist
The following checks are **mandatory**:
- [ ] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
Signed-off-by: hagen1778 <roman@victoriametrics.com>
Moving changelog-related docs to a separate dir should make it easier to
navigate in `docs/` folder.
-----------
The change shouldn't have any visual changes or changes to the links.
Signed-off-by: hagen1778 <roman@victoriametrics.com>
(cherry picked from commit 9dc8d1debd)
