Introduction:
The user is experiencing a high CPU usage spike on their SQL Server instance, leading to performance degradation. The objective is to identify the root cause of the CPU usage hike and take appropriate actions to stabilize the server and optimize CPU utilization.
Preliminary OS, Database, and Task Manager Check
Before diving into SQL Server-specific analysis, perform the following preliminary checks at the OS and database level:
Check Server Resource Usage (CPU, Memory, Disk):
- Task Manager / Resource Monitor:
- Open Task Manager or Resource Monitor to check for system-level bottlenecks, such as high CPU, memory, or disk usage.
- Windows Performance Monitor (PerfMon):
- Monitor key metrics such as:
- Processor Time
- Available Memory
- Disk Queue Length
- Identify resource saturation points.
Verify SQL Server Memory Utilization:
Run the following query to check SQL Server memory usage:
SELECT
total_physical_memory_kb / 1024 AS [TotalPhysicalMemoryMB],
available_physical_memory_kb / 1024 AS [AvailablePhysicalMemoryMB],
system_memory_state_desc AS [MemoryState]
FROM sys.dm_os_sys_memory;
Action: Analyze the server’s total and available physical memory to detect memory pressure.
Step-by-Step Analysis
Step 1: Identify High CPU Consumers
Start by identifying which processes or queries consume the most CPU.
Query:
SELECT
r.session_id,
r.cpu_time,
r.total_elapsed_time,
r.logical_reads,
r.reads,
r.writes,
t.text AS Query_Text
FROM sys.dm_exec_requests r
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) t
WHERE r.cpu_time > 1000 — Filter for sessions with high CPU time
ORDER BY r.cpu_time DESC;
Action: Analyze high CPU-consuming queries for inefficiencies such as missing indexes, poor join strategies, or unnecessary loops.
Step 2: Check SQL Server Wait Statistics
Identify wait types causing CPU bottlenecks.
Query:
SELECT
wait_type,
wait_time_ms,
signal_wait_time_ms,
wait_time_ms – signal_wait_time_ms AS resource_wait_time_ms
FROM sys.dm_os_wait_stats
ORDER BY wait_time_ms DESC;
Action: Focus on SOS_SCHEDULER_YIELD waits, which indicate CPU pressure. Investigate resource contention if this wait type is prevalent.
Step 3: Review Execution Plans for Inefficiencies
Poorly optimized queries often have inefficient execution plans leading to high CPU usage.
Query:
SELECT
qp.query_plan,
qt.text AS Query_Text
FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt
WHERE qs.total_worker_time > 1000000 — Filter for queries using high CPU time
ORDER BY qs.total_worker_time DESC;
Action: Look for table scans, high estimated row counts, and missing indexes. Optimize queries with full table scans by creating appropriate indexes.
Step 4: Monitor Running Queries
Identify resource-intensive queries in real-time.
Query:
SELECT
session_id,
start_time,
status,
command,
cpu_time,
logical_reads,
reads,
writes
FROM sys.dm_exec_requests
WHERE status = ‘running’
ORDER BY cpu_time DESC;
Action: Determine if indexes are missing or if inefficient query logic is causing high CPU usage.
Step 5: Review Index and Statistics Maintenance
Ensure that statistics and indexes are up-to-date to avoid suboptimal query plans.
Actions:
- 1. Rebuild or Reorganize Indexes:
ALTER INDEX ALL ON [schema_name].[table_name] REBUILD;
- Update Statistics:
UPDATE STATISTICS [schema_name].[table_name];
Note: Regular maintenance ensures accurate optimizer decisions and better query performance.
Step 6: Optimize Parallelism Settings
Review the Max Degree of Parallelism (MAXDOP) and Cost Threshold for Parallelism settings to control CPU load from parallel executions.
Query:
EXEC sp_configure ‘show advanced options’, 1;
RECONFIGURE:
EXEC sp_configure ‘max degree of parallelism’;
EXEC sp_configure ‘cost threshold for parallelism’;
Action:
- Adjust MAXDOP (e.g., set to 4 or less for OLTP systems).
- Increase the Cost Threshold for Parallelism to limit parallel processing to complex queries.
Step 7: Identify CPU-Intensive Processes Outside SQL Server
Check if external processes are contributing to CPU pressure.
Actions:
- Use Task Manager or PerfMon to monitor CPU usage by external processes.
- Optimize or terminate non-essential processes competing for CPU resources.
Recommendations:
Short-Term Fixes:
- Throttle Expensive Queries:
Use Resource Governor to limit specific workloads.
- Kill Long-Running Sessions:
KILL <session_id>;
- Review Indexes:
Create missing indexes to optimize query performance.
Long-Term Fixes:
- Query Optimization:
Rewrite inefficient queries to minimize CPU cycles.
- Server Hardware Upgrade:
Upgrade CPU capacity if performance issues persist.
- Workload Balancing:
Distribute workloads across multiple servers to reduce CPU load on a single instance.
Continuous Monitoring:
Set up monitoring to proactively detect and manage CPU usage patterns.
Query:
SELECT
*
FROM sys.dm_os_performance_counters
WHERE counter_name LIKE ‘%CPU%’;
Action: Use SQL Server Agent to create alerts for high CPU usage thresholds.
Conclusion:
High CPU utilization in SQL Server often results from inefficient queries, missing indexes, outdated statistics, or inappropriate server settings. By following the steps outlined above, you can systematically identify and resolve CPU spikes. Regular monitoring and maintenance are essential for ensuring long-term system health and performance.
