High CPU usage in IIS worker processes is typically caused by inefficient code, poor configuration, or unexpected traffic patterns. To diagnose the issue, first identify the specific w3wp.exe process using high CPU via Task Manager or Resource Monitor and determine its associated application pool. Next, use debugging tools like DebugDiag, PerfMon, or IIS Log Analyzer to analyze memory dumps, track performance counters, and examine log files for problematic request patterns. Then, inspect application code for long-running queries, infinite loops, or excessive garbage collection, and evaluate third-party modules that may run on every request. Additionally, consider external factors such as client-side polling, caching misconfigurations, or interference from anti-virus software. Finally, adjust application pool settings like recycling intervals, worker process limits, and overlapped recycling to temporarily mitigate issues while implementing long-term fixes.

If you're seeing high CPU usage tied to IIS worker processes, it’s often a sign that something on your web server isn’t performing as expected. This can lead to slow response times, timeouts, or even application crashes. Diagnosing the issue requires a methodical approach — starting from basic monitoring and moving into deeper analysis if needed.

Check Which Worker Process Is Using High CPU

Start by identifying which specific w3wp.exe process is consuming the most CPU. You can do this via Task Manager or more effectively using Resource Monitor or Process Explorer from Sysinternals.

• Open Task Manager > Go to the Processes tab > Look for multiple instances of "IIS Worker Process (w3wp)."
• Right-click one and select "Go to Details" to see its resource usage in more depth.
• In Resource Monitor, under the CPU tab, you can sort by CPU usage and find out exactly which w3wp instance is spiking.

Each IIS application pool runs in its own w3wp process, so once you’ve identified the high-CPU process, note its Application Pool name. This helps narrow down which site or service is causing the problem.

Use IIS Debugging Tools to Analyze the Issue

Once you know which application pool is problematic, use tools like DebugDiag, PerfMon, or IIS Log Analyzer to dig deeper.

Here’s what you can do:

• Attach DebugDiag to the relevant w3wp process and generate a memory dump when CPU usage spikes.
• Use Performance Monitor (PerfMon) to track .NET CLR exceptions, request execution time, or other counters that might indicate bottlenecks.
• Examine IIS logs for frequent requests or patterns — look for URLs with high time-taken values or those that are called repeatedly.

A common culprit is inefficient code — for example, tight loops, recursive calls, or blocking operations inside ASP.NET pages or Web APIs.

Review Application Code and Third-party Modules

After narrowing things down, it's time to inspect the actual code running in the affected application pool.

Some areas to check:

• Long-running database queries without proper indexing or timeout handling.
• Infinite loops or excessive garbage collection due to large object allocations.
• Poorly optimized third-party modules or HTTP handlers that run on every request.

If you’re using ASP.NET MVC/WebAPI, consider enabling MiniProfiler or similar profiling tools during development to trace slow actions.

Also, review any custom HttpModules or IHttpHandlers — these run per request and can silently eat up CPU if not optimized.

Consider External Factors and Configuration Settings

Sometimes, high CPU usage isn't directly caused by your application logic but by external dependencies or misconfigurations.

For example:

• Too many concurrent requests due to aggressive client-side polling.
• Misconfigured caching leading to repeated expensive computations.
• Anti-virus scanning or scheduled tasks interfering with IIS processes.

You can also adjust application pool settings to mitigate issues temporarily:

• Set a regular recycle schedule to prevent long-running processes from accumulating overhead.
• Limit the number of worker processes (especially in shared environments).
• Enable overlapped recycling to avoid downtime while recycling.

That’s the core of diagnosing high CPU usage in IIS worker processes. It starts simple and gets more technical depending on what you find. Most issues come down to either inefficient code, poor configuration, or unexpected traffic patterns. Once you've identified the source, targeted fixes usually bring things back under control.

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