storegateway: avoid unnecessary allocation in seriesForRefCacheKey

[ortuman]

Signed-off-by: Miguel Ángel Ortuño ortuman@gmail.com

What this PR does

According to a storegateway memory profile from the ops cluster, the strconv.FormatUint function used in seriesForRefCacheKey is taking roughly 1.37% of total allocated objects. That seems quite a lot for just an id generator function.

This PR adapts seriesForRefCacheKey to parse series ref id using a stack a buffer to avoid unnecessary allocations.

Below are the benchmark results comparing against the old implementation:

Previous:

BenchmarkStringCacheKeys/series_ref-8         	14691523	        81.81 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14615520	        81.87 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14474442	        81.29 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14345236	        81.50 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14785486	        81.70 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14608633	        81.88 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14584176	        81.90 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14644542	        81.68 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14593413	        81.64 ns/op	      88 B/op	       2 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	14616150	        81.88 ns/op	      88 B/op	       2 allocs/op

Current:

BenchmarkStringCacheKeys/series_ref-8         	16587760	        72.40 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16571724	        72.29 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16491350	        72.28 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16540212	        72.34 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16564346	        72.53 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16501989	        72.40 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16519273	        72.47 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16515446	        72.50 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	16552038	        72.55 ns/op	      64 B/op	       1 allocs/op
BenchmarkStringCacheKeys/series_ref-8         	15845139	        72.78 ns/op	      64 B/op	       1 allocs/op

Comparison:

name                          old time/op    new time/op    delta
StringCacheKeys/series_ref-8    81.7ns ± 1%    72.5ns ± 0%  -11.33%  (p=0.000 n=10+10)

name                          old alloc/op   new alloc/op   delta
StringCacheKeys/series_ref-8     88.0B ± 0%     64.0B ± 0%  -27.27%  (p=0.000 n=10+10)

name                          old allocs/op  new allocs/op  delta
StringCacheKeys/series_ref-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.000 n=10+10)

Which issue(s) this PR fixes or relates to

Fixes N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[colega]

Very good catch, thank you!