What Makes System Calls Expensive: A Linux Internals Deep Dive

15 hours ago

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System calls in Linux are expensive due to both direct and indirect overheads.
Direct overhead includes the kernel entry/exit path, which involves switching page tables, stacks, and saving/restoring registers.
Indirect overhead comes from microarchitectural side-effects like pipeline draining, branch predictor state loss, and speculative execution mitigations.
The vDSO (virtual dynamic shared object) provides a user-space shortcut for certain system calls to avoid kernel entry overhead.
Benchmarks show that vDSO-based calls (e.g., clock_gettime) are significantly faster than traditional system calls.
System calls disrupt CPU optimizations such as instruction pipelining and branch prediction, leading to performance degradation.
Mitigations against speculative execution attacks (e.g., Spectre, Retbleed) add overhead by clearing branch history and return stack buffers.
Practical strategies to reduce system calls include using vDSO, caching values, batching I/O operations (e.g., io_uring), and leveraging eBPF for kernel-side processing.
The true cost of system calls extends beyond kernel execution time, impacting CPU state recovery and optimization rebuilding.

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