Understanding CPU and steps to troubleshoot high CPU issues in SQL Server
1. What is a CPU?
?CPU (Central Processing Unit) is the primary component of a computer that performs most of the processing inside a system. It is often referred to as the "brain" of the computer, responsible for executing instructions from programs and handling calculations, data processing, and control functions.
- Key Responsibilities of the CPU:
? - Fetching instructions from memory
? - Decoding and executing instructions
? - Managing data input/output to/from memory and other devices
2. What are CPU Cores?
?A CPU core is an individual processing unit within the CPU. In the early days of computing, processors had only one core, meaning they could process only one instruction thread at a time. Modern CPUs, however, are multi-core, meaning they can process multiple tasks concurrently by handling multiple instruction threads in parallel.
?- Single-core CPUs: Can handle only one instruction stream at a time.
- Multi-core CPUs: Can handle multiple instruction streams, one per core. For example, a 4-core CPU can handle 4 tasks or threads simultaneously.
?Each core functions like its own mini-CPU. A CPU with multiple cores can perform more operations at once, improving system performance, especially for multi-threaded applications like SQL Server, where different processes (queries, backups, etc.) can be executed in parallel.
3. What are Logical Processors?
?Logical processors are virtual or logical representations of physical cores in a CPU. They are primarily the result of Hyper-Threading or Simultaneous Multi-Threading (SMT), technologies that allow a single core to run multiple threads of execution simultaneously. In systems with these technologies enabled, each physical core can be divided into two or more logical processors.
- Physical Core: The actual hardware unit within the CPU.
- Logical Processor: A virtual representation of a physical core that enables running multiple instruction threads simultaneously.
For example:
- A CPU with 4 physical cores that supports Hyper-Threading will appear to have 8 logical processors. Each core will be able to handle two threads simultaneously.
?Logical processors improve performance in multi-threaded applications by allowing better utilization of CPU resources, although they don’t double the performance since they share the same physical resources (cache, execution units) on each core.
Troubleshooting High CPU issues in SQL Server
High CPU utilization in SQL Server can be an indication of many issues, including inefficient queries, blocking, excessive compilation, or hardware limitations. This training material will provide a structured approach to identify and troubleshoot the root cause of high CPU usage in SQL Server environments.
?Contents:
1. Understanding CPU Bottlenecks in SQL Server
2. Tools and Methods to Diagnose High CPU Usage
3. Common Causes of High CPU Usage
4. Scripts to Identify the Cause
5. Performance Tuning for High CPU Scenarios
6. Proactive Measures to Prevent High CPU Issues
1. Understanding CPU Bottlenecks in SQL Server
Before troubleshooting, it's important to understand what constitutes "high" CPU usage. SQL Server is a CPU-intensive application, and the usage patterns may vary based on the workload.
?Key Metrics to Monitor:
- Processor Time (%): This measures the percentage of elapsed time the processor spends executing non-idle threads. Consistently above 80% may indicate CPU pressure.
- Queue Length: A CPU queue length of more than 2 per CPU core is considered problematic.
- Context Switches/sec: High context switches could indicate that threads are spending too much time waiting on CPU.
2. Tools and Methods to Diagnose High CPU Usage
?SQL Server provides several built-in tools and DMVs (Dynamic Management Views) to diagnose high CPU usage:
?- Task Manager/Performance Monitor (PerfMon):
? Monitor CPU usage, processor queue length, and SQL Server specific counters like Batch Requests/sec, Compilations/sec, and Recompilations/sec.
?- SQL Server Management Studio (SSMS):
? Use Activity Monitor to get an overview of processes and queries consuming the most resources.
?- SQL Server Profiler/Extended Events:
? These tools can be used to trace the SQL queries that may be contributing to the high CPU.
?- Dynamic Management Views (DMVs):
? Several DMVs provide insights into query performance, execution stats, CPU usage, and wait statistics.
?3. Common Causes of High CPU Usage
?a. Inefficient Queries
Poorly written or non-optimized queries are often the leading cause of high CPU usage. Examples include:
- Missing or incorrect indexes
- Cartesian joins (cross joins)
- Functions on columns in WHERE clauses
?b. High Compilation or Recompilation
Excessive query compilation and recompilation can also cause high CPU usage. You can identify this using PerfMon to monitor "SQL Compilations/sec" and "SQL Recompilations/sec".
?c. Parallelism
Queries running with high degrees of parallelism can sometimes cause high CPU usage. This is often noticeable when queries split work across many threads but cause contention and overuse of resources.
?d. Blocking and Locking
Long-running or blocked queries can cause increased CPU usage as processes wait for resources to become available.
? e. Ad Hoc Workloads
Large numbers of single-use, ad-hoc queries can overwhelm SQL Server's query cache, leading to memory and CPU overhead.
4. Scripts to Identify the Cause
?a. Identifying CPU-bound queries
The following DMV query shows which SQL queries are consuming the most CPU:
SELECT TOP 10
r.session_id,
r.cpu_time AS CPU_MS,
r.total_elapsed_time / 1000 AS Elapsed_Time_sec,
r.reads AS Logical_Reads,
r.writes AS Writes,
r.logical_reads,
r.wait_time,
SUBSTRING(qt.text, (r.statement_start_offset / 2) + 1,
((CASE r.statement_end_offset
WHEN -1 THEN DATALENGTH(qt.text)
ELSE r.statement_end_offset
END - r.statement_start_offset) / 2) + 1) AS QueryText,
DB_NAME(r.database_id) AS DatabaseName
FROM sys.dm_exec_requests r
JOIN sys.dm_exec_sessions s ON r.session_id = s.session_id
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) AS qt
WHERE r.cpu_time > 0
ORDER BY r.cpu_time DESC;?
b. Checking for High Compilation/Recompilation
To see if high CPU is related to compilations or recompilations, use this query:
SELECT *
FROM sys.dm_os_performance_counters
WHERE counter_name IN('Batch Requests/sec', 'SQL Compilations/sec', 'SQL Re-Compilations/sec')
c. Identify Missing Indexes
Missing or poorly designed indexes can cause table scans, which lead to high CPU usage. Use the following query to identify missing indexes:
SELECT
migs.avg_total_user_cost * (migs.avg_user_impact / 100.0) * (migs.user_seeks + migs.user_scans) AS improvement_measure,
mid.statement AS table_name,
mid.equality_columns,
mid.inequality_columns,
mid.included_columns
FROM sys.dm_db_missing_index_groups mig
JOIN sys.dm_db_missing_index_group_stats migs ON mig.index_group_handle = migs.group_handle
JOIN sys.dm_db_missing_index_details mid ON mig.index_handle = mid.index_handle
ORDER BY improvement_measure DESC;
d. Identify Parallelism Issues
If high CPU is being caused by parallelism issues, the following query will help identify them:
?SELECT
[Worker_Count] = r.scheduler_id,
[Request_ID] = r.request_id,
[Session_ID] = r.session_id,
[Query] = st.text
FROM
sys.dm_os_tasks AS r
JOIN
sys.dm_exec_requests AS er ON r.task_address = er.task_address
CROSS APPLY
sys.dm_exec_sql_text(er.sql_handle) AS st
5. Performance Tuning for High CPU Scenarios
?After identifying the cause of high CPU, it's important to take appropriate actions:
?a. Query Optimization
- Indexes: Create or adjust indexes to reduce the number of logical reads.
- Refactor Queries: Avoid using functions in WHERE clauses, reduce joins, and use subqueries judiciously.
b. Reduce Compilations/Recompilations
- Use parameterized queries to avoid constant recompilations.
- If you have dynamic SQL, consider using sp_executesql to improve cache utilization.
?c. Control Parallelism
- Use the MAXDOP (Maximum Degree of Parallelism) query hint to limit the number of threads a query can use.
- Set the Cost Threshold for Parallelism to an appropriate value (typically, values between 25-50 are suggested).
d. Improve Ad-hoc Query Workloads
- Enable 'Optimize for Ad hoc Workloads' in the server settings to avoid caching every single query.
6. Proactive Measures to Prevent High CPU Issues
?- Index Maintenance: Regularly rebuild and reorganize indexes.
- Query Optimization Plan: Continuously review and optimize frequently run queries.
- Monitoring: Set up SQL Server alerts for CPU spikes, long-running queries, and blocked processes.
- Capacity Planning: Ensure that the hardware (CPU cores, memory, storage) matches the workload requirements.
Conclusion
High CPU usage in SQL Server is often a symptom of underlying issues, such as inefficient queries, missing indexes, or excessive compilations. By following a structured approach—monitoring, diagnosing with the right tools and DMVs, and applying tuning techniques—you can mitigate CPU bottlenecks and ensure optimal performance.
With regular monitoring, proactive tuning, and best practices, SQL Server environments can be optimized to prevent recurring high CPU issues.
Reference Links:
SQL DBA and Azure SQL PaaS Specialist | Microsoft Certified Azure Database Administration Associate
4 天前Good one...??
MS SQL Database Administrator |specializing in Performance Tuning & HA/DR planning|Expertise in SQL SERVER|Dedicated to enhancing Database performance, security and Backup strategy and Recovery and Server automation
1 周Very helpful. Thanks for sharing.
Sr SQL Server DBA
1 个月Very helpful