When you are tuning the application’s memory & Garbage Collection settings, you should take well-informed decisions based on the key performance indicators. But there are overwhelming amount of metrics reported; which one to choose and which one to leave? This article intends to explain the right KPIs and right tools to source them.

What are the right KPIs?

1. Throughput
2. Latency
3. Footprint

Throughput

Throughput is the amount of productive work done by your application in a given time period. This brings the question what is productive work? what is non-productive work?

Productive Work: This is basically the amount of time your application spends in processing your customer’s transactions.

Non-Productive Work: This is basically the amount of time your application spend in house-keeping work, primarily Garbage collection.

Let’s say your application runs for 60 minutes. In this 60 minutes let’s say 2 minutes is spent on GC activities.

It means application has spent 3.33% on GC activities (i.e. 2 / 60 * 100)

It means application throughput is 96.67% (i.e. 100 – 3.33).

Now the question is: What is the acceptable throughput %? It depends on the application and business demands. Typically one should target for more than 95% throughput.

Latency

This is the amount of time taken by one single Garbage collection event to run. This indicator should be studied from 3 fronts.

1. Average GC time: What is the average amount of time spent on GC?
2. Maximum GC time: What is the maximum amount of time spent on a single GC event? Your application may have service level agreements such as “no transaction can run beyond 10 seconds”. In such cases, your maximum GC pause time can’t be running for 10 seconds. Because during GC pauses, entire JVM freezes – no customer transactions will be processed. So it’s important to understand the maximum GC pause time.
3. GC Time Distribution: You should also understand how many GC events are completing with in what time range (i.e. within 0 – 1 second, 200 GC events are completed, between 1 – 2 second 10 GC events are completed …)

Footprint

Footprint is basically the amount CPU consumed. Based on your GC algorithm, based on your memory settings, CPU consumption will vary. Some GC algorithms will consume more CPU (like Parallel, CMS), whereas other algorithms such as Serial will consume less CPU.

According to memory tuning Gurus, you can pick only 2 of them at a time.

• If you want good throughput and latency, then footprint will degrade.
• If you want good throughput and footprint, then latency will degrade.
• If you want good latency and footprint, then throughput will degrade.

Right Tools

Throughput and Latency can be obtained from analyzing Garbage collection Logs. Upload your application’s Garbage Collection log file in http://gceasy.io/ tool. This tool can parse Garbage Collection logs and generates Throughput and Latency indicators for you. Below is the screen shot from the http://gceasy.io/ tool showing the throughput and latency:

             Fig: KPI section from GCEasy.io report

Footprint (i.e. CPU consumption) can be obtained from the monitoring tools – Nagios, NewRelic, AppDynamics,…

其它

想一下：三元悖論、CAP、GC: KEY PERFORMANCE INDICATORS 是不是否很像？

• 三元悖論
  三元悖論（Mundellian Trilemma），也稱三難選擇(The Impossible Trinity)，它是由美國經濟學家保羅·克魯格曼（一說蒙代爾）就開放經濟下的政策選擇問題所提出的，其含義是：在開放經濟條件下，本國貨幣政策的獨立性(Monetary policy)，固定匯率(Exchange rate)，資本的自由進出(Capital mobility)不能同時實現，最多隻能同時滿足兩個目標，而放棄另外一個目標來實現調控的目的。
  
• CAP原則又稱CAP定理，指的是在一個分散式系統中，Consistency（一致性）、 Availability（可用性）、Partition tolerance（分割槽容錯性），三者不可兼得
• GC: KEY PERFORMANCE INDICATORS

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