When dealing with IIS6 on Windows Server 2003 x86, we're bound by the 32-bit architecture's fundamental limitations:

// Theoretical memory limits:
Total addressable space: 4GB
Kernel mode space: ~2GB (default)
User mode space: ~2GB 
Per-process limit: ~1.2-1.4GB (practical limit)

The 500MB limitation you're observing isn't a hard IIS6 cap but rather a combination of factors:

• Application Pool Recycling Settings: Default recycling occurs at 60% of available memory
• .NET GC Behavior: Managed heaps tend to stay below process limits
• Fragmentation: Address space fragmentation in long-running processes

Modify the following in your application pool properties:

// Set in applicationHost.config or IIS Manager
<applicationPools>
    <add name="YourAppPool" 
         memoryLimit="800"  // MB
         privateMemoryLimit="0" // Disables recycling
    />
</applicationPools>

When dealing with memory-intensive .NET apps:

// Example web.config modification for large object heaps
<configuration>
    <runtime>
        <gcAllowVeryLargeObjects enabled="true"/>
        <gcServer enabled="true"/>
    </runtime>
</configuration>

Use Performance Monitor to track these key counters:

Process(w3wp)\Private Bytes
Process(w3wp)\Virtual Bytes
.NET CLR Memory\% Time in GC
Memory\Available MBytes

For deeper analysis, enable user mode dump collection:

// Using Debug Diagnostic Tool
adplus.exe -hang -pn w3wp.exe -o c:\dumps

If you consistently hit memory limits:

• Implement application partitioning across multiple servers
• Consider x64 migration (requires OS upgrade)
• Evaluate memory optimization techniques like object pooling

// Example object pooling implementation
public class ObjectPool<T> where T : new()
{
    private readonly ConcurrentBag<T> _items = new();
    
    public T Get() => _items.TryTake(out T item) ? item : new T();
    
    public void Return(T item) => _items.Add(item);
}

On Windows Server 2003 x86 (even with SP2), the fundamental limitation comes from the 32-bit architecture. Despite having 4GB physical RAM, the OS can only address approximately 2.8-3.2GB due to:

• Memory-mapped hardware addresses
• System reserved memory space
• Kernel-mode memory requirements

The 500MB limit you're observing isn't an arbitrary IIS6 restriction - it's the practical outcome of several interacting factors:

// Example of memory threshold detection in .NET
private static void MonitorMemory() {
    long maxMemory = Process.GetCurrentProcess().MaxWorkingSet.ToInt64();
    Console.WriteLine($"Process memory limit: {maxMemory/1024/1024}MB");
    
    if (Environment.Is64BitProcess) {
        throw new PlatformNotSupportedException("32-bit limitation detected");
    }
}

Per-process memory fragmentation: The .NET heap becomes fragmented in long-running processes, preventing efficient memory usage even when theoretical space exists.

Thread Stack Allocation: Each thread consumes ~1MB (default stack size), which counts against your process limit.

// Example showing thread stack impact
public class ThreadTest {
    public static void Main() {
        for(int i=0; i<500; i++) {
            new Thread(() => Thread.Sleep(Timeout.Infinite)).Start();
            Console.WriteLine($"Threads: {i}, Memory: {Process.GetCurrentProcess().WorkingSet64/1024/1024}MB");
        }
    }
}

1. Application Pool Recycling Configuration:

cscript.exe adsutil.vbs SET W3SVC/AppPools/YourAppPool/PeriodicRestartMemory 1000000

2. Enable 3GB Switch in boot.ini:

multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Windows Server 2003" /3GB /USERVA=3030

3. Implement Memory-Optimized Coding:

// Bad practice - holds large dataset in memory
public List<Customer> GetAllCustomers() {
    return _context.Customers.ToList();
}

// Better - streams data
public IEnumerable<Customer> StreamCustomers() {
    return _context.Customers.AsNoTracking();
}

Common cases where you get OOM exceptions despite available memory:

• GDI object leaks (each handle consumes USER space)
• Too many assemblies loaded in AppDomain
• Memory-mapped file limitations

// Diagnose GDI handles
[DllImport("user32.dll")]
public static extern int GetGuiResources(IntPtr hProcess, int uiFlags);

var gdiCount = GetGuiResources(Process.GetCurrentProcess().Handle, 0);
var userCount = GetGuiResources(Process.GetCurrentProcess().Handle, 1);