GODRIVER-2628 optimize allocations in selectServerFromDescription

[isopov]

GODRIVER-2628

Summary

Optimize allocations in selectServerFromDescription

Background & Motivation

Same way as in #1108 and #1107 I want to reduce allocations in selectServerFromDescription func which flat allocations in my case are > 100GB and which possess the very first place by flat allocations in my long running job.

Benchmark attached without changes to the code:

BenchmarkSelectServerFromDescription
BenchmarkSelectServerFromDescription/AllFit
BenchmarkSelectServerFromDescription/AllFit-10         	   63248	     17195 ns/op	  111265 B/op	       8 allocs/op
BenchmarkSelectServerFromDescription/AllButOneFit
BenchmarkSelectServerFromDescription/AllButOneFit-10   	   65990	     18173 ns/op	  111265 B/op	       8 allocs/op
BenchmarkSelectServerFromDescription/HalfFit
BenchmarkSelectServerFromDescription/HalfFit-10        	  110966	      9053 ns/op	   53920 B/op	       7 allocs/op
BenchmarkSelectServerFromDescription/OneFit
BenchmarkSelectServerFromDescription/OneFit-10         	 3362158	       351.3 ns/op	     416 B/op	       1 allocs/op
PASS

And with changes:

BenchmarkSelectServerFromDescription
BenchmarkSelectServerFromDescription/AllFit
BenchmarkSelectServerFromDescription/AllFit-10         	 2548441	       480.4 ns/op	     896 B/op	       1 allocs/op
BenchmarkSelectServerFromDescription/AllButOneFit
BenchmarkSelectServerFromDescription/AllButOneFit-10   	  159066	      6740 ns/op	   41856 B/op	       2 allocs/op
BenchmarkSelectServerFromDescription/HalfFit
BenchmarkSelectServerFromDescription/HalfFit-10        	  295821	      3710 ns/op	   21376 B/op	       2 allocs/op
BenchmarkSelectServerFromDescription/OneFit
BenchmarkSelectServerFromDescription/OneFit-10         	 1814713	       639.6 ns/op	    1312 B/op	       2 allocs/op
PASS

[isopov]

BTW in all the cases it seems beneficial to use bool slice for filtering like

	allowedIndexes := make([]bool, len(desc.Servers))
	allowedIndexesCount := 0
	for i, s := range desc.Servers {
		if s.Kind != description.Unknown {
			allowedIndexes[i] = true
			allowedIndexesCount++
		}
	}
	var allowed []description.Server
	if allowedIndexesCount == len(desc.Servers) {
		allowed = desc.Servers
	} else {
		allowed = make([]description.Server, 0, allowedIndexesCount)
		for i, allowedIndex := range allowedIndexes {
			if allowedIndex {
				allowed = append(allowed, desc.Servers[i])
			}
		}
	}

In this case benchmark results are:

BenchmarkSelectServerFromDescription
BenchmarkSelectServerFromDescription/AllFit
BenchmarkSelectServerFromDescription/AllFit-10         	 4538070	       276.1 ns/op	     112 B/op	       1 allocs/op
BenchmarkSelectServerFromDescription/AllButOneFit
BenchmarkSelectServerFromDescription/AllButOneFit-10   	  146278	      7216 ns/op	   41072 B/op	       2 allocs/op
BenchmarkSelectServerFromDescription/HalfFit
BenchmarkSelectServerFromDescription/HalfFit-10        	  297627	      3904 ns/op	   20592 B/op	       2 allocs/op
BenchmarkSelectServerFromDescription/OneFit
BenchmarkSelectServerFromDescription/OneFit-10         	 2960940	       393.9 ns/op	     528 B/op	       2 allocs/op
PASS

Should I use this approach in all three places?

[prestonvasquez]

Thank you for this PR @isopov !

[prestonvasquez]

LGTM

[benjirewis]

LGTM assuming tests pass! This seems like a very reasonable addition given the previous changes in GODRIVER-2526. Thanks for your contribution, @isopov .