Lessons Learned

Lessons Learned

Performance

• Measure before optimizing: 62.55µs is excellent, don't over-optimize without reason
• Simplicity wins: static array > complex slab allocator for the ring buffer
• Zero-copy >> everything: eliminating copies is more effective than any "smart" allocator
• SoA >> AoS: memory coalescence is measurable, not theoretical

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Linux Kernel Development

• GFP_ATOMIC is mandatory in Netfilter hooks (network interrupt context)
• SpinLock = contention: RCU is preferable for frequent reads in the kernel
• Critical error handling: cascade of goto on initialization failure (standard kernel pattern)
• NF_DROP vs NF_ACCEPT: conscious design — intercepted packets don't go up the network stack

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Architecture

• One SpinLock per chunk, not a global lock: distributes contention
• Backward iteration for swap-and-pop: bug fixed, memorable lesson
• Double buffers cost 2× in hot memory only: acceptable compromise
• Transparent PinnedAllocator: the #ifdef __CUDACC__ makes g++ compilation possible without modifications

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Methodology

• README ≠ contract: the roadmap is a guide, not an absolute obligation
• Question the premises: "why this optimization now?"
• Validate empirically: performance data before any added complexity
• Document decisions: every architectural choice must be justified with technical reasons

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Networking

• Unify server and client networking: a single Network class eliminates code duplication and protocol drift
• Header-only = simpler builds: removing the separate .cpp/.o eliminates link order issues
• Socket fallback is essential: developing/testing without the kernel module (WSL, CI) greatly speeds up iteration
• Batch TX via kernel thread: one ioctl wake for N outbound packets is far cheaper than N sendto() syscalls
• Identify clients by src_ip/src_port in RxPacket: the kernel module can extract sender info from the IP header

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Modularization

• Flat module architecture scales: 20 independent lpl-<name> libraries with explicit deps is cleaner than one monolithic engine/
• xmake > Make for C++ projects: native dependency resolution, package management, and build modes eliminate Makefile complexity
• Archive legacy, don't delete: the previous-generation prototype was kept (out of the build tree, with git history) for reference and feature-parity checks rather than deleted outright
• Code quality requires consistency: Doxygen style (/**@), .inl files, guard clauses — must be enforced from the start, not retrofitted

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