Use low precision Clock#now when computing timestamp for exemplars

[jack-berg]

Resolves #6403.

Introduces a new Clock#now(boolean highPrecision) overload, which uses a performance optimized millisecond precision when highPrecision=false. Updates ReservoirCell to obtain measurement times using Clock#now(false), which shaves a good chunk of time off the time to record each exemplar (~50ns on my machine).

This has a greater impact the more frequent measurements are candidates to be stored in ReservoirCell. The savings are especially impactful for the histogram exemplar reservoir, where the algorithm requires us to take the last measurement for each bucket. This means we compute the time for every measurement recorded.

Here's a benchmark output comparing the high precision and low precision Clock#now implementations. Benchmark run using OpenJDK Runtime Environment Temurin-17.0.3+7.

Benchmark                                                     Mode  Cnt   Score    Error   Units
ExemplarClockBenchmarks.now_highPrecision                     avgt   10  66.006 ±  0.110   ns/op
ExemplarClockBenchmarks.now_highPrecision:gc.alloc.rate       avgt   10   0.001 ±  0.001  MB/sec
ExemplarClockBenchmarks.now_highPrecision:gc.alloc.rate.norm  avgt   10  ≈ 10⁻⁴             B/op
ExemplarClockBenchmarks.now_highPrecision:gc.count            avgt   10     ≈ 0           counts
ExemplarClockBenchmarks.now_lowPrecision                      avgt   10  17.982 ±  0.152   ns/op
ExemplarClockBenchmarks.now_lowPrecision:gc.alloc.rate        avgt   10   0.001 ±  0.001  MB/sec
ExemplarClockBenchmarks.now_lowPrecision:gc.alloc.rate.norm   avgt   10  ≈ 10⁻⁵             B/op
ExemplarClockBenchmarks.now_lowPrecision:gc.count             avgt   10     ≈ 0           counts

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.11%. Comparing base (9845ac9) to head (c31f282).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6417      +/-   ##
============================================
- Coverage     91.12%   91.11%   -0.01%     
- Complexity     5856     5868      +12     
============================================
  Files           636      636              
  Lines         17062    17082      +20     
  Branches       1733     1741       +8     
============================================
+ Hits          15548    15565      +17     
- Misses         1019     1021       +2     
- Partials        495      496       +1     

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[trask]

👍

[jkwatson]

what JVM version were you using to benchmark with? (not that it matters much..but might be nice to include in the PR description for posterity).

[jkwatson]

Thanks!

[jack-berg]

what JVM version were you using to benchmark with? (not that it matters much..but might be nice to include in the PR description for posterity).

Updated!