A deep dive into an OpenJDK commit that replaced slow /proc file parsing with a single syscall, revealing obscure Linux kernel internals and a 20-year-old optimization opportunity.

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A deep dive into an OpenJDK commit that replaced slow /proc file parsing with a single syscall for getting thread CPU time. The old implementation opened and parsed /proc/self/task/<tid>/stat files, involving multiple syscalls, VFS machinery, and string parsing. The fix uses clock_gettime() with a Linux-specific clockid bit hack (CPUCLOCK_VIRT) to request user-time-only directly from the kernel. Benchmarks show 40x latency reduction (11μs to 279ns), with potential for further optimization by encoding PID=0 in the clockid to skip radix tree lookups. The change demonstrates how revisiting old assumptions and understanding kernel internals can unlock massive performance gains without new features.

How a 40-Line Fix Eliminated a 400x Performance Gap