Work Stealing and Fair Scheduling
Symptom You parallelize a recursive algorithm. Quicksort, or a tree traversal, or a divide-and-conquer numeric kernel. You have 16 cores and a central task queue, and you measure a speedup of 3. ...
Performance
Balls Into Bins and the Power of Two Choices
Symptom You have 100 backends and a load balancer hashing request IDs to pick one. Expected load per backend is exactly 1%, and you have checked the hash is good. ...
Gustafson's Law and the Universal Scalability Law
Symptom Two measurements that both look like they break the previous post. The first: your Spark job takes four hours on ten nodes. You move it to a hundred nodes and it still takes four hours — but it is now processing ten times the data. T081 said the ceiling was 20×, and nobody hit a ceiling. ...
Amdahl's Law
Symptom The profile was unambiguous: 94% of wall-clock time in one loop, and the loop’s iterations are independent. You parallelise it across sixteen cores, the flame graph flattens exactly as predicted, and the end-to-end time goes from 40 seconds to 17. ...