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Will More Threads Make It Faster?
A service is slow. Someone suggests doubling the thread count. Will that make it faster?
The scenario
An order service runs a 32-thread pool on an 8-core box. p99 is 2.1s, throughput is 340 rps, CPU is 30%, and a profiler shows 55% of wall time inside one inventory mutex. The proposal on the table is 64 threads.
What it tests
- Whether the candidate refuses to answer without knowing the workload
- Whether they can name the mechanisms by which more threads make things slower
- Whether they propose a measurement rather than a guess
Answer it out loud first. Recognising a good answer is not the same skill as producing one.