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Table of Contents
Check for the execution of slow query ( SHOW PROCESSLIST )
Analyze Slow Query Log
Check if there are temporary tables or excessive file sorting
Check index usage ( EXPLAIN )
Other possible factors
Home Database Mysql Tutorial How to troubleshoot 100% CPU usage by the MySQL server?

How to troubleshoot 100% CPU usage by the MySQL server?

Jun 19, 2025 am 12:45 AM
mysql CPU Usage

Common causes of MySQL occupancy of 100% CPU include slow queries, missing indexes, temporary tables or excessive sorting, and configuration issues. 1. First use SHOW PROCESSLIST to check active connections and resource-consuming operations; 2. Enable and analyze slow query log location historical issues; 3. Check Created_tmp_tables and Sort_merge_passes to judge temporary tables and sorting; 4. Use EXPLAIN to analyze SQL execution plans and optimize full table scanning and file sorting; 5. Check other factors such as connection count, statistical information updates, and timing tasks. The above steps can gradually narrow the scope of problems and optimize performance bottlenecks.

How to troubleshoot 100% CPU usage by the MySQL server?

MySQL occupies 100% CPU and is not uncommon in server operation, especially when high concurrency or query design is unreasonable. The core of the problem is usually the inefficiency of one or some SQL queries, missing indexes, severe lock waiting, or improper configuration.

Here are some common troubleshooting directions and solutions:


Check for the execution of slow query ( SHOW PROCESSLIST )

This is the first step to see which queries are currently being executed. After connecting to MySQL, execute:

 SHOW FULL PROCESSLIST;

You will see all the currently active connections and the statements they are executing. Focus on querying states such as Sending data , Copying to tmp table , Sorting result , etc.

  • If you find a query running for a long time, you can try to copy the SQL first and then analyze whether it is missing an index.
  • Note whether there are a large number of duplicate queries, which may be because the cache is not working or there is a problem with the program logic.

Analyze Slow Query Log

If the problem does not occur in real time, you need to rely on the slow query log to locate historical problems.

The following methods to enable slow query logs (in the MySQL configuration file):

 slow_query_log = 1
slow_query_log_file = /var/log/mysql/mysql-slow.log
long_query_time = 1
log_queries_not_using_indexes = 1
  • long_query_time set to 1 means that queries that have been recorded for more than 1 second.
  • Turning on log_queries_not_using_indexes can help you discover queries that do not use indexes.

After collecting for a period of time, you can use tools such as mysqldumpslow or pt-query-digest to analyze these logs and find out the "culprit".


Check if there are temporary tables or excessive file sorting

MySQL may create internal temporary tables or use Using filesort when executing complex queries, which can significantly consume CPU resources.

You can check it by:

 SHOW STATUS LIKE 'Created_tmp%';
SHOW STATUS LIKE 'Sort%';
  • If Created_tmp_tables or Sort_merge_passes value is high, it means there are a large number of temporary tables or sorting operations.
  • Solutions include optimizing the SQL query structure, adding appropriate indexes, and reducing unnecessary ORDER BY and GROUP BY fields.

Check index usage ( EXPLAIN )

Use the EXPLAIN command for SQL with suspected problems, for example:

 EXPLAIN SELECT * FROM orders WHERE user_id = 123;

Check out type and Extra columns in the output:

  • type should be ref or range . If it is ALL , it means full table scanning.
  • If you appear Using filesort or Using temporary in Extra , you need to pay attention.

According to the results of EXPLAIN , appropriately increasing the index or adjusting the query structure can usually significantly reduce CPU usage.


Other possible factors

  • Too high connections : Too many concurrent connections may also cause CPU overload. You can check whether max_connections is set properly.
  • Automatic statistics update frequently : Some versions of MySQL frequently update statistics when they are written in large quantities, affecting performance.
  • Regular task interference : For example, the statistical tasks suddenly start running the entire table in the early morning of every day, which can easily cause CPU explosions.

Basically, these common check points. The key is to gradually narrow the scope based on the actual running SQL and combine logs and monitoring data. Don't rush to adjust the configuration, find out the specific bottleneck first.

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