When monitoring processes with top, the VIRT column shows the total virtual memory usage of a process. This includes:

• All memory the process has mapped (including shared libraries)
• Memory allocated but not necessarily used (committed but not resident)
• Memory-mapped files
• Swap space reserved but not used

Java applications typically show high virtual memory due to their memory management architecture:

// Example Java memory allocation
public class MemoryDemo {
    public static void main(String[] args) {
        // Allocate 500MB - will show in VIRT but may not be resident
        byte[] bigArray = new byte[500 * 1024 * 1024]; 
    }
}

Key factors affecting Java VIRT size:

• JVM heap size (-Xmx setting)
• PermGen/Metaspace allocation
• Thread stack sizes
• Memory-mapped files

Your Java processes showing 800MB-1GB VIRT with 0% swap usage indicates:

• Normal behavior for JVM processes
• Memory is allocated but not necessarily used
• Physical memory (RES) is more important for actual usage

Use this command for better Java memory analysis:

top -p $(pgrep -d',' java) -c
# Alternative with more details:
ps -p $(pgrep -d',' java) -o pid,vsz,rss,pmem,pcpu,cmd

Critical metrics to watch:

For Tomcat and Java daemons:

# Set appropriate JVM options in catalina.sh or startup script:
JAVA_OPTS="-Xms256m -Xmx768m -XX:MaxMetaspaceSize=256m"

Remember that Linux memory management is complex - unused RAM is wasted RAM. High VIRT alone isn't problematic unless accompanied by high RES with swapping.

When monitoring processes with top, the VIRT column (Virtual Memory Size) often causes confusion. Virtual memory represents the total address space a process has allocated, including:

• Memory-mapped files
• Shared libraries
• Heap allocations
• Stack space

For Java processes, high virtual memory usage (800MB-1GB) is completely normal due to JVM memory management:

# Sample top output showing Java process
PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
1234 tomcat    20   0  1.2g 450m  25m S  2.3  5.6  12:34.56 java

Your 0% swap usage indicates:

1. Physical RAM is sufficient for active memory (RES column)
2. Linux prefers caching in RAM over swapping
3. Java's memory management minimizes swap usage

Tomcat and Java daemons allocate virtual memory aggressively:

# Common JVM memory arguments affecting virtual size
JAVA_OPTS="-Xms512m -Xmx1024m -XX:MaxMetaspaceSize=256m"

Key metrics to watch instead of VIRT:

• RES (resident memory) - actual physical RAM used
• %MEM - percentage of system memory
• Java heap usage (via JMX or jstat)

Only investigate if you see:

# Signs of actual memory pressure
$ free -h
              total        used        free      shared  buff/cache   available
Mem:           7.7G        5.2G        1.2G        123M        1.3G        2.1G
Swap:          1.0G        0B          1.0G

Or these symptoms occur:

• OOM (Out of Memory) errors
• Severe swapping (si/so columns in vmstat)
• Performance degradation

For better Java process monitoring:

# Use jstat for real JVM heap metrics
jstat -gcutil <pid> 1000

# Alternative with pmap for detailed mapping
pmap -x <pid> | less

Remember that Linux's virtual memory management is designed to efficiently handle large address spaces. The 800MB-1GB virtual size you're seeing is completely normal for JVM processes and doesn't indicate any memory pressure, especially with your swap sitting at 0% usage.