Segfault troubleshooting for gemma4 MoE model | Generated by AI
Question: Should I rebuild llama.cpp first, or attempt a run with Flash Attention disabled using the --no-flash-attn flag to troubleshoot the consistent segfaults and OOM issues with the gemma4 MoE model?
Answer:
Given the consistent segfault at the same memory offset (+231000), you are likely looking at a specific code path failure rather than a random hardware glitch. Here is the recommended path forward:
1. Try --no-flash-attn first
Running with Flash Attention disabled is the fastest way to isolate the variable. Since you suspect the Sliding Window Attention (SWA) and MoE indexing logic, bypassing the FA kernels will tell you immediately if the issue lies in the optimized attention implementation or the underlying KV cache management.
- If it works: The bug is likely in the FA kernels’ handling of gemma4’s specific tensor shapes or SWA logic.
- If it still segfaults: The issue is likely in the broader MoE expert routing or the way the KV cache is being addressed in
libllama.so, regardless of the attention mechanism.
2. The Rebuild (with a twist)
If you decide to rebuild, don’t just git pull. If the segfault is consistent on CPU 31, it might be worth checking if your current build used aggressive compiler optimizations (like -march=native) that are producing unstable code for that specific core’s instruction set.
- Try a “Clean” build:
rm -rf build && mkdir build && cd build && cmake .. && make. - Check if there are any pending PRs in the
llama.cpprepository specifically mentioning “gemma4” or “MoE” indexing, as these architectures often require specific logic for expert-level KV management.
3. Monitoring the V-cache Padding
The fact that your V-cache is padding to 2048 without FA is a strong hint. If the segfault happens during the transition between SWA and non-SWA layers, the “padding” logic might be calculating an out-of-bounds pointer. Keep an eye on the syslog for any “address alignment” or “illegal instruction” errors that might accompany the next crash.
Next Step: Would you like me to help you draft a specific gdb command to trace exactly what is happening at that +231000 offset during the next run?