perf: Optimize statCache expiration timestamp by using int64 instead of time.Time

[vedantdas-source]

Description

In the StatCache the entry struct tracks TTL expiration using a standard Go time.Time object. This time.Time struct consumes 24 bytes of memory which can be reduced to 8 bytes using int64 for the expiration timestamp. This saves 16 bytes of memory per object.

Changes Made

• Changed expiration field in the internal cache entry from time.Time to int64
• Cache insertion methods (Insert, InsertImplicitDir, AddNegativeEntry, etc.) now convert the incoming expiration time.Time parameter to seconds using .Unix()
• Lookup methods now evaluate expiration against now.Unix()

Benchmarking results

Using time.Time :

goos: linux
goarch: amd64
pkg: github.com/googlecloudplatform/gcsfuse/v3/internal/cache/metadata
cpu: Intel(R) Xeon(R) CPU @ 2.60GHz
BenchmarkStatCache_Insert-48                     3033651               409.2 ns/op           272 B/op          4 allocs/op
BenchmarkStatCache_AddNegativeEntry-48           7874757               149.2 ns/op            80 B/op          2 allocs/op
BenchmarkStatCache_LookUp-48                    19632806                61.15 ns/op           17 B/op          0 allocs/op

Using int64 :

goos: linux
goarch: amd64
pkg: github.com/googlecloudplatform/gcsfuse/v3/internal/cache/metadata
cpu: Intel(R) Xeon(R) CPU @ 2.60GHz
BenchmarkStatCache_Insert-48                     3174098               382.8 ns/op           256 B/op          4 allocs/op
BenchmarkStatCache_AddNegativeEntry-48           8443263               141.3 ns/op            64 B/op          2 allocs/op
BenchmarkStatCache_LookUp-48                    22825635                54.30 ns/op           11 B/op          0 allocs/op

All existing tests pass successfully.

Link to the issue in case of a bug fix.

Testing details

1. Manual - NA
2. Unit tests - Added stat_cache_benchmark_test.go to measure memory and CPU performance of StatCache operations.
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

[gemini-code-assist]

Code Review

This pull request optimizes the entry struct in stat_cache.go by changing the expiration field from time.Time to int64 (using Unix timestamps) and adds a benchmark test suite to measure cache performance. The reviewer suggests using UnixMilli() instead of Unix() across all insertions and lookups to preserve sub-second precision while still maintaining the memory benefits of the int64 representation.

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.68%. Comparing base (b61c988) to head (f2acd33).

Files with missing lines	Patch %	Lines
internal/cache/metadata/stat_cache.go	62.50%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4857       +/-   ##
===========================================
+ Coverage        0   83.68%   +83.68%     
===========================================
  Files           0      170      +170     
  Lines           0    20958    +20958     
===========================================
+ Hits            0    17539    +17539     
- Misses          0     2762     +2762     
- Partials        0      657      +657     

Flag	Coverage Δ
unittests	83.68% <62.50%> (?)

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