Debug Determinism: The Sweet Spot for Replay-Based Debugging

Deterministic replay tools offer a compelling approach to debugging hard-to-reproduce bugs. Recent work on relaxed-deterministic replay techniques shows that replay debugging with low in-production overhead is possible. However, despite considerable progress, a replay-debugging system that offers not only low in-production runtime overhead but also high debugging utility, remains out of reach. To this end, we argue that the research community should strive for debug determinism —a new determinism model premised on the idea that effective debugging entails reproducing the same failure and the same root cause as the original execution. We present ideas on how to achieve and quantify debug determinism and give preliminary evidence that a debug deterministic system has potential to provide both low in-production overhead and high debugging utility

permission. Replay Debugging for the Datacenter

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Calot: a Constant-Diameter Low-Traffic Distributed Hash Table

Current Distributed Hash Tables (DHTs) keep a small routing table on each node, achieving low-cost maintenance at the expense of high lookup hop count and therefore high lookup traffic. Some recently proposed DHTs keep a complete routing table on every node to support efficient lookup. However, our analysis reveals that this approach cannot scale beyond a few thousand nodes due to its high maintenance traffic. We propose Calot, a lightweight enhancement to DHTs that resolves lookups in either one or two hops while reducing the total traffic of lookup and maintenance operations. Calot is designed for relatively stable environments---the niche we advocate for DHTs. For a medium-size DHT, our gCalot algorithm aggregates membership changes and propagates them to every node. Lookups are resolved in one hop. For a large-size DHT, our sCalot algorithm partitions the overlay into slices and runs gCalot inside each slice. Lookups are resolved in two hops. Calot inherits the robustness and simplicity of existing DHT protocols. The only change to their messaging protocol that Calot makes is to piggyback membership updates on keep-alive messages infrequently exchanged between neighbors. For a 1024-node DHT, gCalot eliminates 70% of traffic and delivers 99.5% of lookups in one hop. For a 10 -node DHT, sCalot eliminates traffic by 40% and delivers 97.9% of lookups in two hops