improve bucket.Get requests performance

[f41gh7]

This commit changes order of bucket structure fields and mu access
in order to improve CPU cache locality. Previously, mu and atomic counters were accessed with different CPU cache lines and strongly hit performance at older CPUs.

The following benchmark shows up to 30% performance increase:

go test -run=^$ -bench=BenchmarkCache

goos: linux
goarch: amd64
pkg: github.com/VictoriaMetrics/fastcache
cpu: AMD EPYC 7B12
              │    master    │               opt_v3                │
              │    sec/op    │   sec/op     vs base                │
CacheSet-8      4.666m ±  2%   4.674m ± 2%        ~ (p=0.739 n=10)
CacheGet-8      2.364m ±  4%   1.732m ± 7%  -26.74% (p=0.000 n=10)
CacheHas-8      2.171m ±  6%   1.652m ± 2%  -23.89% (p=0.000 n=10)
CacheSetGet-8   11.65m ± 11%   12.03m ± 3%        ~ (p=0.481 n=10)
geomean         4.087m         3.562m       -12.84%

              │    master     │                opt_v3                │
              │      B/s      │     B/s       vs base                │
CacheSet-8      13.39Mi ±  2%   13.37Mi ± 2%        ~ (p=0.755 n=10)
CacheGet-8      26.44Mi ±  5%   36.09Mi ± 7%  +36.50% (p=0.000 n=10)
CacheHas-8      28.80Mi ±  6%   37.83Mi ± 2%  +31.38% (p=0.000 n=10)
CacheSetGet-8   10.73Mi ± 10%   10.39Mi ± 3%        ~ (p=0.469 n=10)
geomean         18.19Mi         20.87Mi       +14.73%

              │    master     │                opt_v3                 │
              │     B/op      │     B/op       vs base                │
CacheSet-8      19.04Ki ± 10%   19.11Ki ±  7%        ~ (p=0.542 n=10)
CacheGet-8      9.598Ki ± 20%   7.006Ki ±  5%  -27.00% (p=0.000 n=10)
CacheHas-8      8.700Ki ± 23%   6.499Ki ±  3%  -25.31% (p=0.000 n=10)
CacheSetGet-8   41.59Ki ±  9%   40.87Ki ± 10%        ~ (p=0.362 n=10)
geomean         16.04Ki         13.73Ki        -14.36%

              │   master    │               opt_v3                │
              │  allocs/op  │  allocs/op   vs base                │
CacheSet-8      32.00 ±  9%   32.50 ±  8%        ~ (p=0.418 n=10)
CacheGet-8      16.00 ± 19%   12.00 ±  8%  -25.00% (p=0.000 n=10)
CacheHas-8      15.00 ± 20%   11.00 ±  9%  -26.67% (p=0.000 n=10)
CacheSetGet-8   71.00 ± 10%   70.00 ± 10%        ~ (p=0.396 n=10)
geomean         27.17         23.41        -13.85%

[Copilot]

Pull Request Overview

This PR optimizes CPU cache locality by reordering fields in the bucket struct and adjusting mutex acquisition order to improve performance of bucket.Get requests. Based on benchmark results, this change delivers up to 30% performance improvement for cache operations.

Key changes:

• Reordered atomic counter fields (getCalls, setCalls, misses) to be adjacent to the mutex
• Moved mutex acquisition before accessing chunks field in Set and Get methods

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.68%. Comparing base (66aca6e) to head (c311ee8).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #92   +/-   ##
=======================================
  Coverage   76.68%   76.68%           
=======================================
  Files           4        4           
  Lines         549      549           
=======================================
  Hits          421      421           
  Misses         73       73           
  Partials       55       55           

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