remove reallocs and overallocs for s.lset in bucket.go

[ppanyukov]

Changes

• append to s.lset caused reallocs and overallocs
• this is now fixed by properly setting capacity on s.lset
• pprof shows "-316.22MB, 5.00% of 6322.82MB total" for sample data set and query I used

Verification

• Sample data generated with thanosbench using realistic-k8s-1w-small profile
• Query executed: count({__name__=~".+"}) by (__name__)
• Instrumented BucketStore.Series to dump heap profile at the start and end of the function call.
• Compared the profiles using pprof:

go tool pprof -sample_index=alloc_space -edgefraction=0 -lines -base heap-series-1-after-base.pb.gz  -http=:8080 heap-series-1-after-change.pb.gz

• PNG attached

• Heap profiles attached

heap-series-1-after-base.pb.gz
heap-series-1-after-change.pb.gz

---

Edit

For what it's worth, here is my HeapDump func I used, in case anyone wants to instrument anything. Might save you looking for pprof docs :)

func HeapDump(name string) {
	var fName = fmt.Sprintf("/Users/philip/thanos/github.com/ppanyukov/thanos-oom/scripts/%s.pb.gz", name)
	f, _ := os.Create(fName)
	defer f.Close()
	runtime.GC()
	pprof.WriteHeapProfile(f)
	fmt.Printf("WRITTEN HEAP DUMP TO %s\n", fName)
}

[bwplotka]

This is intense! =D

I wish we had some nice benchmark suite.. on my todo list. (:

I think we copy those labels not efficiently more than just here.. Thanks! small steps (:

[GiedriusS]

LGTM, thanks!