How to Fix CPU Waits: SOS_SCHEDULER_YIELD

How to Fix CPU Waits: SOS_SCHEDULER_YIELD in SQL Server

When SQL Server Performance issues arise, one of the most challenging problems to diagnose and resolve is high CPU usage. Among the various wait types that indicate CPU bottlenecks, SOS_SCHEDULER_YIELD is one of the most common. This post will explore how SQL Server schedules CPU, what the SOS_SCHEDULER_YIELD wait type signifies, and practical steps to address this issue.

Understanding SQL Server’s CPU Scheduling

SQL Server uses a cooperative scheduling mechanism called Non-Preemptive Scheduling to manage how tasks (queries, stored procedures, etc.) utilize the CPU. This scheduling happens through two main components: workers (threads) and schedulers (logical CPU cores).

Here’s a breakdown of how SQL Server handles CPU scheduling:

• Worker Threads: These are the execution threads SQL Server uses to process tasks. Each thread operates on a scheduler.
• Schedulers: Represent logical CPUs. Each scheduler manages a queue of tasks assigned to its CPU.
• Context Switching: SQL Server determines when a thread should yield the CPU for others to execute, ensuring fair CPU usage.

Each worker thread is allocated a time slice of 4 milliseconds by default. After this quantum expires, the thread voluntarily yields the CPU, allowing other threads in the queue to execute. This cooperative approach prevents any single thread from monopolizing the CPU.

What Is SOS_SCHEDULER_YIELD?

The SOS_SCHEDULER_YIELD wait type occurs when a worker thread has yielded its time slice but must wait to resume execution because other threads are ahead in the scheduler’s runnable queue. This wait type signifies CPU contention—essentially, there’s more demand for CPU time than the available capacity.

Common symptoms of SOS_SCHEDULER_YIELD waits include:

• High CPU usage on the server.
• Queries taking longer to execute than expected.
• Increased application response times.

Diagnosing SOS_SCHEDULER_YIELD Waits

Before implementing any solutions, it’s essential to confirm that CPU contention is the root cause of the problem. Use the following techniques to diagnose the issue:

1. Check Wait Statistics

The first step is to check SQL Server’s Wait Statistics for evidence of high SOS_SCHEDULER_YIELD waits. Run the following query:

                    SELECT                 wait_type,                 SUM(wait_time_ms) AS total_wait_time_ms,                SUM(waiting_tasks_count) AS waiting_tasks_count            FROM sys.dm_os_wait_stats            WHERE wait_type = 'SOS_SCHEDULER_YIELD'            GROUP BY wait_type;            

If SOS_SCHEDULER_YIELD is one of the top wait types, it indicates significant CPU scheduling issues.

2. Examine Scheduler Usage

To dive deeper, analyze the workload on individual schedulers using the following query:

                    SELECT                 scheduler_id,                 current_tasks_count,                 runnable_tasks_count,                 work_queue_count,                 active_workers_count            FROM sys.dm_os_schedulers            WHERE scheduler_id < 255; -- Exclude hidden schedulers            

Pay attention to the runnable_tasks_count. A high value indicates threads are queuing for CPU time.

3. Monitor CPU Usage

Use tools such as Performance Monitor (PerfMon) or Database Health Monitor to observe the following key metrics:

• High CPU utilization (typically above 80%).
• High context switching rates.
• Processor queue length exceeding the number of logical CPUs.

How to Fix SOS_SCHEDULER_YIELD Waits

Once you've confirmed the issue, apply the following strategies to resolve CPU contention:

1. Optimize Queries

Poorly optimized queries are often the primary cause of high CPU usage. Focus on:

• Indexes: Add missing indexes and remove unused ones. Leverage the Database Engine Tuning Advisor or analyze the sys.dm_db_missing_index_* DMVs to identify index opportunities.
• Query Plans: Identify and optimize queries with inefficient execution plans using tools like Query Store or by examining Execution Plans.
• Statistics: Ensure statistics are up to date to improve query performance.

2. Reduce Parallelism

Excessive parallelism can cause additional CPU overhead. Adjust these settings to mitigate the impact:

• MAXDOP: Lower the maximum degree of parallelism to limit the number of threads used for parallel queries. For example:

                    EXEC sp_configure 'max degree of parallelism', 4; -- Limit to 4 cores            RECONFIGURE;            

• Cost Threshold for Parallelism: Increase this setting to reduce the number of queries that go parallel.

3. Scale Your Server

If CPU contention is persistent, it may be time to upgrade your server's resources:

• Add more physical or logical CPU cores.
• Invest in faster processors.

4. Offload Non-Critical Work

Reduce the overall CPU workload by offloading non-critical tasks:

• Use Resource Governor to cap resource usage for specific workloads.
• Schedule maintenance jobs, such as BACKUPs or index rebuilds, during off-peak hours.

5. Monitor Continuously

Use tools like Database Health Monitor to monitor your SQL Server environment and detect emerging issues early.

Let Stedman Solutions Help

Addressing SOS_SCHEDULER_YIELD waits often requires expert analysis and continuous tuning. At Stedman Solutions, our SQL Server Managed Services provide proactive monitoring, query optimization, and comprehensive Performance Tuning to prevent CPU bottlenecks and other critical issues.

Our Team of experienced DBAs uses tools like Database Health Monitor to ensure your SQL Server environment operates efficiently, so you can focus on your business.

Conclusion

High SOS_SCHEDULER_YIELD waits indicate CPU contention, which can severely impact SQL Server Performance. By understanding SQL Server's scheduling mechanism, analyzing Wait Statistics, and applying targeted solutions, you can resolve these waits and improve system responsiveness.

If you need assistance resolving performance issues, don't hesitate to contact Stedman Solutions. We're here to help you achieve peace of mind with your SQL Server environment!