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rajeshpv/jvm_stat.md

Created March 27, 2025 13:54
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  1. rajeshpv created this gist Mar 27, 2025.
    81 changes: 81 additions & 0 deletions jvm_stat.md
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    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:

    ```bash
    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:

    ```bash
    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:

    ```bash
    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:
    ```bash
    #!/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?