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Performance Workloads

Turning observed production traffic into a runnable workload, then reading what comes back. Interviewers probe it because an average response time hides almost every problem worth finding.

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questions

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Response times climb across a long steady-load run while per-request resource use stays flat - what do you check?

level: seniorimportance: should knowfreq 42%

basics

~20 s

Check whether the climb advances with work done or with elapsed time. Replot it against cumulative completed work, run a low-rate control, probe a path the run never writes to, and restart the process keeping its data.

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When extra capacity arrives minutes after a surge, how do you keep a performance run from hiding that delay?

level: seniorimportance: should knowfreq 38%

basics

~20 s

Start at the size the system would reach under baseline demand, never pre-raised to peak, and timestamp two series: offered demand and capacity actually serving. The interval between them is the exposure window, and what happened inside it is the result.

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How do you prove across a demand surge that no unit of work was lost or done twice?

level: seniorimportance: should knowfreq 44%

basics

~20 s

Tag every submitted unit with a unique identifier before demand rises, then reconcile after the observation window: submitted equals completed plus refused plus pending. Separately compare distinct completed identifiers against total completions, because totals alone hide loss and repetition together.

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How would you build a workload model from real production traffic and keep it representative over time?

level: principalimportance: should knowfreq 42%

basics

~20 s

Derive the transaction mix, arrival shape and data profile from observed traffic rather than opinion, state the reference period, and keep the model a versioned, owned artefact that is re-derived on a schedule instead of ageing into fiction.

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When is adopting a new performance reference run legitimate rather than moving the goalposts?

level: principalimportance: should knowfreq 38%

basics

~20 s

It is legitimate when the shift in level has an identified, intentional cause, was reproduced across repeats, and still meets the obligation the team owes. It is a moved goalpost when the reason is that meeting the old figure became inconvenient.

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A slowdown reproduces only when requests overlap. How do you design the follow-up performance run that confirms it?

level: seniorimportance: nice to knowfreq 34%

basics

~20 s

Vary overlap alone. Run the same arrival rate with few busy clients and then with many idle ones: if duration differs, overlap is the cause. Repeat with requests spread across distinct records to locate what is shared.

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In a performance run, how much of each reply can you afford to verify in flight?

level: seniorimportance: nice to knowfreq 30%

basics

~20 s

Only checks cheap enough to fit inside the applying side's own budget: a length floor, a required field, a marker. Deep parsing per reply steals the capacity that generates demand, so sample it instead and report what share was verified.

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When a 99th percentile is computed from bucketed latency counts, what error do the bucket boundaries introduce?

level: seniorimportance: nice to knowfreq 22%

basics

~20 s

The read-out locates the bucket holding the ranked request, not its time, so the figure carries that bucket's width as uncertainty. Interpolating inside assumes a spread the slow end does not have, and an unbounded top bucket reports nothing at all.

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How do you decide whether a long sustained run should hit scheduled and rotational work or avoid it?

level: seniorimportance: nice to knowfreq 26%

basics

~20 s

Decide from what the hold must prove. Arrange the run to hit periodic work - scheduled jobs, credential rotation, cache expiry, index maintenance - when that interaction is the risk; otherwise disable it and record that the trend excludes it.

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How do you establish that a second, larger request peak after an applied surge was self-inflicted rather than real demand?

level: seniorimportance: nice to knowfreq 27%

basics

~20 s

Compare what the load generator offered against what the service received: the offered rate is known and unchanged, so any excess was manufactured inside the system. Repeats of identifiers already submitted, arriving synchronised at a fixed delay, confirm it.

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