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Measure JVM and OS hiccups influencing your program
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README.md

jvm-hiccup-meter

A miniature library for measuring JVM and OS hiccups that affect your program. It is inspired by Gil Tene's jHiccup. In fact, this is a maximally trimmed jHiccup with the implementation taken straight out of the ancestor library.

jHiccup is a great tool that focuses on very precise on-demand measurement of platform-induced lags. It uses HdrHistogram to record high-fidelity measurement data which can later be queried for real percentile values on arbitrary time spans. However, jHiccup's API is not convenient for ongoing always-on measuring and reporting.

jvm-hiccup-meter contains the exact measuring code from jHiccup and exposes it via the most primitive callback API. This gives developers the utmost flexibility in consuming the measurements, be it writing them to a monitoring solution (Metrics, StatsD, etc.) or aggregating in any other way.

How it works

The idea behind jHiccup (and, as a consequence, this library) is trivially brilliant: if an idle thread that does nothing but sleeping is denied the CPU time by the system, then the real worker threads are likely to observe the similar effect. jvm-hiccup-meter starts a thread that sleeps for a defined interval (by default, 10 milliseconds) and measures how much of actual time has passed. The difference between the desired and actual sleep time becomes the length of the system "hiccup" for this cycle.

Of course, you can't expect Thread.sleep(10) to wake up strictly in 10 milliseconds even under ideal conditions when no hiccups are happening. But obsessing about small fluctuations is not the point, the point is to discover long pauses of hundreds of milliseconds or even seconds. Those can be caused by stop-the-world GC phases, stolen CPU cycles in virtualized environments, unpredicted hardware effects, and other application-independent reasons.

jvm-hiccup-meter is non-invasive — an extra thread that does a few arithmetic operations and goes to sleep a hundred times per second should not bring a noticeable overhead to your application.

Usage

(require '[clj-hiccup-meter.core :as hmeter])
(def hm (hmeter/start-hiccup-meter callback-fn :resolution-ms 10))

;; To stop the meter thread
(hm)

The only function start-hiccup-meter accepts an unary function as an argument and starts the measuring thread. On each measurement iteration, the callback will be called with the measured delay in nanoseconds. Optional resolution can be passed to change the default sleep period. start-hiccup-meter returns a nullary function calling which stops the thread.

Here's an example of combining jvm-hiccup-meter with Dropwizard Metrics:

(import 'com.codahale.metrics.Histogram
        'com.codahale.metrics.SlidingTimeWindowArrayReservoir
        'java.util.concurrent.TimeUnit)

(def hist (Histogram. (SlidingTimeWindowArrayReservoir. 10 TimeUnit/SECONDS)))
(def hm (hmeter/start-hiccup-meter #(.update ^Histogram hist %)))

;; Now, you can forward this histogram to Graphite, or check the values
;; manually, e.g:
(.getMax (.getSnapshot hist))

License

jHiccup was written by Gil Tene of Azul Systems, and released to the public domain, as explained at http://creativecommons.org/publicdomain/zero/1.0/


jvm-hiccup-meter is distributed under the Eclipse Public License. See LICENSE.

Copyright 2018 Alexander Yakushev

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