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Why Is Eight Cores Not Eight Times Faster?
You parallelise a job across eight cores and get a 3x speedup. Why, and what do you do about it?
The scenario
A nightly ETL job was parallelised from 1 to 8 workers. Wall time went from 96 minutes to 31. The team wants to move to 32 workers and expects roughly 8 minutes.
What it tests
- Whether the candidate reasons about the serial fraction as the ceiling
- Whether they know the other costs beyond the serial fraction
- Whether they can distinguish fixed-size from scaled-size workloads
Answer it out loud first. Recognising a good answer is not the same skill as producing one.