jvm_stat

Capturing JVM heap allocation and comparing it between two executions of a Java application can be super insightful when you're analyzing memory usage, leaks, or performance. Here’s a practical way to do it:

🚀 Step-by-Step Guide

✅ 1. Enable JVM Heap Monitoring

Use the following Java options when running your app to capture memory usage:

java -Xmx512m -Xms512m -XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:gc.log -jar your-app.jar

This:

Sets max and initial heap size.
Logs garbage collection and heap stats to gc.log.

✅ 2. Use jstat (Java Statistics Monitoring Tool)

Run jstat while your app is running:

jstat -gc <pid> 1000 10

Where:

<pid> = Process ID of the Java app.
1000 = delay between samples (in ms).
10 = number of samples.

This gives heap usage stats like:

S0C, S1C – survivor spaces
EC, EU – Eden space capacity and used
OC, OU – Old gen capacity and used
YGC, FGC – young/full GC counts

✅ 3. Heap Dump (Optional – for full analysis)

Capture a heap dump:

jmap -dump:format=b,file=heap1.hprof <pid>

Do this at both execution points (heap1.hprof and heap2.hprof).

You can analyze .hprof files using tools like:

Eclipse MAT (Memory Analyzer Tool)
VisualVM
JProfiler

✅ 4. Compare Between Runs

You can:

Diff the gc.log files.
Use Eclipse MAT to open both heap dumps and visually compare.
Look at jstat outputs side-by-side for patterns (like growing Old Gen memory or GC frequency).

🧠 Pro Tip

Automate it via a shell script:

#!/bin/bash
java -Xmx512m -Xms512m -Xloggc:gc_run1.log -jar your-app.jar
# then:
jstat -gc <pid> > jstat_run1.txt

Repeat for run2 and then use diff or Excel to compare jstat_run1.txt and jstat_run2.txt.

Want help writing a script to automate all this? Or comparing .hprof files with MAT?

rajeshpv/jvm_stat.md