When managing IIS servers, controlling memory consumption is crucial for system stability. IIS provides two primary memory limit settings for application pools:

• Private Memory Limit: The maximum amount of physical RAM (working set) an application pool can use
• Virtual Memory Limit: The maximum virtual address space an application pool can consume

In your scenario with Private Memory Limit set to 500MB and Virtual Memory Limit to 3GB, the application pool will recycle when either threshold is reached. This means:

• If the application uses 500MB of physical RAM, it will recycle
• If the application's virtual memory reaches 3GB, it will recycle
• Whichever limit is hit first triggers the recycling

Here are three ways to configure these limits:

1. Using IIS Manager GUI

1. Open IIS Manager
2. Navigate to Application Pools
3. Right-click your pool and select Advanced Settings
4. Set values for:

   Private Memory Limit (KB)
   Virtual Memory Limit (KB)

2. Using appcmd.exe

Command line configuration example:

appcmd set apppool /apppool.name:"YourAppPool" /processModel.privateMemoryLimit:512000
appcmd set apppool /apppool.name:"YourAppPool" /processModel.virtualMemoryLimit:3145728

3. Programmatic Configuration (C#)

Using Microsoft.Web.Administration namespace:

using (ServerManager serverManager = new ServerManager())
{
    ApplicationPool appPool = serverManager.ApplicationPools["YourAppPool"];
    appPool.ProcessModel.PrivateMemoryLimit = 512000; // 500MB in KB
    appPool.ProcessModel.VirtualMemoryLimit = 3145728; // 3GB in KB
    serverManager.CommitChanges();
}

• Monitor first: Use Performance Monitor to track actual memory usage before setting limits
• Gradual adjustment: Start with higher limits and gradually reduce while monitoring stability
• Consider overlapped recycling: Enable to prevent service interruptions during recycling
• 32-bit vs 64-bit: On 32-bit systems, virtual memory space is limited (~2GB per process)

If your application pool recycles unexpectedly:

1. Check Event Viewer for recycling events
2. Verify no other limits are being hit (e.g., Regular Time Interval, Request Limit)
3. Examine memory dump if available (DebugDiag tool can help)

For more granular control:

• Implement application-level memory management
• Consider using Windows System Resource Manager (WSRM) for system-wide control
• Evaluate splitting high-memory applications into separate pools

When configuring memory limits for IIS application pools, two critical settings require special attention:

• Private Memory Limit: Controls the working set (physical RAM usage) of the w3wp.exe process
• Virtual Memory Limit: Governs the total address space (physical + paged memory) available

For your specific case (500MB private / 3GB virtual):

<applicationPools>
    <add name="MyAppPool">
        <recycling>
            <privateMemory limit="500000" />
            <virtualMemory limit="3000000" />
        </recycling>
    </add>
</applicationPools>

The application pool will recycle when either threshold is reached. The private memory limit (500MB) will likely trigger first in most scenarios.

Use PowerShell to track memory usage in real-time:

Get-Process w3wp | 
Where-Object { $_.ProcessName -eq 'w3wp' } |
Select-Object ProcessName, Id, 
    @{Name="Private(MB)";Expression={[math]::Round($_.PrivateMemorySize64/1MB,2)}},
    @{Name="Virtual(MB)";Expression={[math]::Round($_.VirtualMemorySize64/1MB,2)}}

For optimal performance:

• Set private memory limit to ~80% of expected peak usage
• Configure virtual memory limit at least 3x private limit
• Implement gradual recycling with overlapping processes

appcmd set apppool /apppool.name:MyAppPool /recycling.disallowOverlappingRotation:false

When memory limits are too restrictive:

1. Check Windows Event Log for ASP.NET memory-related errors
2. Monitor Application Pool Recycling events (Event ID 2289)
3. Adjust limits incrementally (10-15% changes per iteration)