thread_cache_size limits the reusable connection threads retained after clients disconnect. It is not a limit on active connections and should not be sized by copying your usual concurrent-connection count.
Check the cache and thread-handling mode
These read-only checks apply to Oracle MySQL 8.4. The MariaDB distinction below is separate from the MySQL example.
SELECT VERSION() AS server_version,
@@version_comment AS vendor_build,
@@GLOBAL.thread_handling AS thread_handling,
@@GLOBAL.thread_cache_size AS resolved_thread_cache_size;
SHOW GLOBAL STATUS
WHERE Variable_name IN (
'Uptime', 'Connections', 'Aborted_connects',
'Threads_created', 'Threads_cached', 'Threads_connected'
);With the usual one-thread-per-connection model, a cached thread can serve a later connection. Oracle MySQL 8.4 documents an autosized default; SQL reports the resolved cache size, not an instruction to assign the default sentinel -1.
MariaDB handles defaults differently, including limiting the cache by max_connections. When MariaDB uses its thread pool, thread_cache_size is ignored. Do not transfer the MySQL default or a cache recommendation between vendors without checking the actual mode.
MariaDB handles defaults differently, including limiting the cache by max_connections. When MariaDB uses its thread pool, thread_cache_size is ignored. Do not transfer the MySQL default or a cache recommendation between vendors without checking the actual mode.
Read counters and gauges correctly
- Connections counts connection attempts, including unsuccessful attempts.
- Threads_created counts threads created to handle connections.
- Aborted_connects records failed attempts to connect.
- Threads_cached and Threads_connected are current counts, not interval totals.
Compare snapshots across a representative interval on the same instance, with no restart or counter reset. Your diagnostic connection also affects the observations. A snapshot with zero cached threads does not prove the cache is disabled or ineffective.
An increasing lifetime Threads_created total alone does not establish a problem. Check new connection attempts, failed connections, thread creation and application connection latency together. These counters do not provide a universal cache-size recommendation.
Separate connection reuse from connection capacity
Review application connection pooling and connection churn before proposing a server setting. Active concurrency belongs to the separate max_connections limit.
For ongoing observations, see MySQL monitoring. For a configuration review, explore MySQL configuration tuning with Releem.
For ongoing observations, see MySQL monitoring. For a configuration review, explore MySQL configuration tuning with Releem.
Sources
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