Tail Latency

Tail latency refers to the slow end of the response-time distribution, typically measured as the 95th, 99th, or 99.9th percentile. Most requests may finish quickly while a small fraction takes many times longer.

Why it matters

Averages hide the tail. If the median is 80 ms but p99 is 3 seconds, one request in a hundred is painfully slow. That matters more than it sounds: a single page load often makes dozens of requests, so the chance that a user hits at least one slow call is much higher than 1 percent. In systems that fan out to many backends, the slowest backend sets the pace for the whole response.

Example

p50:  82 ms
p90:  140 ms
p99:  3100 ms
max:  9800 ms

Common causes of a long tail are garbage collection pauses, lock contention, cold caches, noisy neighbors on shared hosts, retries after a timeout, and a slow database replica. Useful mitigations include hedged requests (send a duplicate after a delay and use the first reply), tighter timeouts, and load shedding.

Common mistakes

• Alerting on average response time only.
• Averaging percentiles across servers, which is mathematically meaningless. Merge histograms instead.
• Setting timeouts so long the tail has no ceiling.

Related

• Percentile
• Latency
• Apdex
• Golden signals

यह पेज सामान्य जानकारी है, जिसे हमारी संपादकीय टीम ने लिखा है और दस्तावेज़ीकरण बदलने पर अपडेट किया जाता है। सर्वर और प्रदाता अलग-अलग होते हैं, इसलिए बदलाव पहले सुरक्षित तरीके से जाँचें। देखें: संपादकीय नीति और अस्वीकरण.