When working with the top command in Linux, you'll notice two primary memory columns:

PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND

The VIRT column shows virtual memory usage (VSZ) while RES shows resident memory (RSS). By default, pressing M sorts by RSS, but sometimes you need to analyze VSZ instead.

For immediate VSZ sorting in an active top session:

1. Launch top: top
2. Press Shift+O to open the sort field menu
3. Type virt and press Enter

This will instantly re-sort processes by virtual memory consumption.

To make VSZ sorting the default:

1. Create or edit ~/.toprc
2. Add these lines:

RCfile for "top with windows"       # shameless bullshit line
global
    "Mem_sort=2"                    # 0=pid, 1=res, 2=virt, etc.

3. Save and restart top

For scripting or one-off analysis, use ps with custom sorting:

ps aux --sort=-vsz | head -n 10

This shows the top 10 processes by virtual memory usage. The -vsz parameter sorts in descending order.

Create an alias for VSZ-focused monitoring:

alias vsztop='top -o VIRT -c -n 1 -b | head -n 20'

This shows a static snapshot of the top 20 processes sorted by VSZ.

Remember that VSZ includes:

• Allocated but unused memory
• Shared libraries
• Memory-mapped files

High VSZ doesn't necessarily indicate a memory leak - it might just be pre-allocated address space.

Use this bash snippet to track VSZ growth:

watch -n 5 'ps -eo pid,vsz,cmd --sort=-vsz | head -n 5'

Updates the top 5 VSZ consumers every 5 seconds.

When investigating high VSZ processes:

pmap -x [PID] | less

This breaks down memory allocation by the process, helping identify memory-hungry components.

---

When analyzing process memory usage in Linux, it's crucial to distinguish between:

• RES (Resident Memory): Physical RAM currently used
• VSZ (Virtual Memory): Total address space allocated (RAM + swap)

The default M sort in top only sorts by RES. To sort by VSZ (Virtual Size), we need deeper control.

1. Run top command
2. Press Shift + F to enter field management
3. Navigate to VIRT (Virtual Memory) column using arrow keys
4. Press s to select as sort field
5. Press Enter to confirm
6. Press Esc to return to main view

For persistent VSZ sorting, create or modify ~/.toprc:

RCfile for "top with windows"       # shameless bullshit line
Id:a, Mode_altscr=0, Mode_irixps=1, Delay_time=3.0, Curwin=0
Def     fieldscur=AEHIOQTWKNMbcdfgjplrsuvyzX
        winflags=30427, sortindx=10, maxtasks=0
        summclr=1, msgsclr=1, headclr=3, taskclr=1

Key parameters:

• sortindx=10: Sets VIRT as default sort column (adjust index based on your fieldscur)
• Verify column order with top -O to list available fields

For scriptable solutions, use ps with VSZ sort:

ps aux --sort=-vsz | head -n 10

Or more detailed output:

ps -eo pid,ppid,cmd,%mem,%cpu,vsz --sort=-vsz | head -n 20

When troubleshooting JVM memory leaks, VSZ provides better visibility:

top -p $(pgrep -d',' java) -o VIRT

This shows all Java processes sorted by virtual memory allocation.

For comprehensive analysis, combine with pmap:

top -o VIRT -b -n1 | grep high_memory_process | awk '{print $1}' | xargs pmap -x

This reveals detailed memory mapping of high-VSZ processes.