Breakdown generic writeOffsetTable

[codesome]

This is a broken down piece of #627

With this, I am avoiding allocating slice headers for postings by having the name and value as separate fields. This saves quite some allocs.

[Don't merge until Prometheus 2.11 is out]

[cstyan]

This saves quite some allocs.

Any sense of how much? 10%? 20%?

[codesome]

I don't have numbers for individual optimizations. All combined numbers is in this comment #627 (comment). I will try to get number for each optimizations.

[codesome]

So the gains are mostly allocs.

benchmark                                                                                 old ns/op       new ns/op       delta
BenchmarkCompaction/type=normal,blocks=2,series=1000000,samplesPerSeriesPerBlock=51-8     12824719385     12761476707     -0.49%

benchmark                                                                                 old allocs     new allocs     delta
BenchmarkCompaction/type=normal,blocks=2,series=1000000,samplesPerSeriesPerBlock=51-8     15845398       14839951       -6.35%

benchmark                                                                                 old bytes      new bytes      delta
BenchmarkCompaction/type=normal,blocks=2,series=1000000,samplesPerSeriesPerBlock=51-8     2074569456     2074402112     -0.01%

[gouthamve]

LGTM, but I am not sure the minor improvement in allocs is worth this change.

[gouthamve]

I'm just surprised that this small change reduces allocs so much.

[bwplotka]

Quite interesting indeed, but keep in mind that slice header is like ~16 bytes overall, so in some cases might be bigger than name or key (:

[codesome]

I am not sure the minor improvement in allocs is worth this change.

I would still like to have it. Multiple small improvements will add up to a big gain. And the absolute gain from this PR will be bigger in different scenarios (more number of label-values and series than the benchmark).

[bwplotka]

Make sesne to me. Might be small improvement but not much harm in code complexity.