Add a Reserved option

[jcorbin]

Okay, after offline discussion with @prashantv and @akshayjshah, here's an addendum to #13:

• adds a Reserved(q float64) option that can be used to not allocate all of the CPU quota to GOMAXPROCS
• intentionally left room, per discussion with @akshayjshah, so that we can more easily choose to add an option that takes some sort of a custom strategy func (i.e. maybe a func(func(quota float64, config CPUQuotaConfig) (int, CPUQuotaStatus)) Option)
• based this branch off master, so we cane choose whether to have (some version) of it and/or Use floor(quota) rather than ceil(quota) #13

Questions I have for reviewers at the outset:

• would you prefer much of the prep work broken out from the introduction of Reserved?
• would you rather just externalize some rendition of the internal runtime types and jump to a more generic strategy-func option?

[codecov]

Codecov Report

Merging #14 into master will decrease coverage by 2%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   93.58%   91.57%   -2.01%     
==========================================
  Files           7        7              
  Lines         187      190       +3     
==========================================
- Hits          175      174       -1     
- Misses         10       14       +4     
  Partials        2        2

Impacted Files	Coverage Δ
maxprocs/maxprocs.go	91.11% <85.18%> (-8.89%)	⬇️

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[arkajit]

Sounds reasonable to have a separate reservation for resources used outside the go-runtime.

Left a couple small comments about whether we want to respect this reservation fully or whether the boundary between reserved and non-reserved quota is permeable. The answer depends on the previous discussion about whether we want to prevent throttling or under-utilization.

From the system perspective, it seems clear that we'd prefer throttling to under-utilization, but at the moment the throttling penalty is quite large (100ms) which can be severe for latency-critical applications. Ideally we'd allow applications to make this tradeoff themselves.

[arkajit]

Do we want Floor here after all? If we use Ceil, we will not be respecting the full reservation for the Reserved quota. Can Reserved ever be fractional?

[jcorbin]

Correct, we do want floor, this PR was just written orthogonally to #13.

[arkajit]

Is "rounded down" still the intention? This part of the comment contradicts the use of Ceil in CPUQuotaToGOMAXPROCS (commented there).

[jcorbin]

Yes it is, this doc comment is out of line with the code reality above, good catch.

[jasonlai]

Good stuff!

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