High CPU usage in SQL Server can be caused by a variety of factors. Here are some common causes:

  1. Poorly Optimized Queries: Queries that are not optimized can consume a lot of CPU resources. This can include queries with missing indexes, large table scans, or complex calculations.
  2. High Volume of Queries: Even if individual queries are not resource-intensive, a high volume of queries can lead to high CPU usage.
  3. Parallelism: SQL Server can use multiple CPU cores for a single query operation, which can lead to high CPU usage. This is controlled by the “max degree of parallelism” setting.
  4. Inadequate Hardware: If your server’s hardware is not sufficient for the workload, it can lead to high CPU usage. This can include not having enough CPU cores, or the CPU being too slow.
  5. Blocking and Deadlocks: Blocking occurs when one process holds a lock on a resource that another process needs. Deadlocks are a specific type of blocking where two or more processes each hold a lock on a resource the other needs. Both can lead to high CPU usage.
  6. High Number of Active Connections: Each active connection to SQL Server uses some CPU resources. A high number of active connections can therefore lead to high CPU usage.
  7. Background Tasks: SQL Server performs a number of background tasks, such as index maintenance, statistics updates, and log flushing. These tasks can consume CPU resources.
  8. Inefficient Application Design: Applications that make inefficient use of the database can cause high CPU usage. This can include things like unnecessary data processing on the server side, or opening more connections than necessary.

To diagnose high CPU usage, you can use tools like SQL Server Profiler, Performance Monitor, and Activity Monitor. You might also need to examine your SQL Server settings, query design, and application design.