thread_cache_size

Query Thread Cache Size

The number of threads in the cache can be queried by using the SHOW STATUS statement to query the value of the Threads_cached status variable:

SHOW GLOBAL STATUS
   LIKE 'Threads_cached';

Compute Thread Cache Miss Rate

The thread cache miss rate can be computed using the Threads_created and Connections status variables:

• The Threads_created status variable shows how many threads have been created since the server was brought online

• The Connections status variable shows how many new connection attempts have been made since the server was brought online

If you divide Threads_created by Connections, that shows the thread cache miss rate:

 SELECT threads_created, connections,
    (threads_created / connections) AS thread_cache_miss_rate
 FROM
    (SELECT gs1.VARIABLE_VALUE AS threads_created
    FROM information_schema.GLOBAL_STATUS gs1
    WHERE gs1.VARIABLE_NAME LIKE 'Threads_created') tc
JOIN
    (SELECT gs2.VARIABLE_VALUE AS connections
    FROM information_schema.GLOBAL_STATUS gs2
    WHERE gs2.VARIABLE_NAME LIKE 'Connections') c;

If the above query is executed at different times, the results can be interpreted in the following way:

• If thread_cache_miss_rate stays constant or grows and approaches 1, the current thread cache size is not large enough to keep up with new connection requests.

• If thread_cache_miss_rate shrinks and approaches 0, the current thread cache size is either equal to or larger than needed to keep up with new connection requests. To determine whether it is larger than needed, the Threads_cached status variable can also be monitored to determine whether the thread cache consistently contains too many cached threads.

Interactions with Thread Pool

When the thread pool is enabled by setting thread_handling=pool-of-threads, the thread cache is unused, so the thread_cache_size system variable has no effect.

Interactions with max_connections

If the thread_cache_size system variable is set to a value greater than the value of the max_connections system variable, the thread_cache_size value will be set to the max_connections value.

PARAMETERS

SKYSQL

Show Value with SHOW VARIABLES

SHOW GLOBAL VARIABLES can show the global value:

SHOW GLOBAL VARIABLES
   LIKE 'thread_cache_size';

Since this system variable is only global scope, SHOW VARIABLES and SHOW SESSION VARIABLES also show the global value.

Query Value using @@global and @@session

In SELECT and other SQL statements, @@global can be queried for the global value:

SELECT @@global.thread_cache_size;

Since this system variable is only global scope, @@session can't be queried for this variable.

Information Schema

The information_schema.SYSTEM_VARIABLES table can be queried to obtain details about the system variable:

SELECT *
FROM information_schema.SYSTEM_VARIABLES
WHERE VARIABLE_NAME LIKE 'thread_cache_size';

The information_schema.GLOBAL_VARIABLES table can be queried to obtain the global value:

SELECT *
FROM information_schema.GLOBAL_VARIABLES
WHERE VARIABLE_NAME LIKE 'thread_cache_size';

This system variable is only global scope, so the information_schema.SESSION_VARIABLES table can also be queried to obtain the global value:

SELECT *
FROM information_schema.SESSION_VARIABLES
WHERE VARIABLE_NAME LIKE 'thread_cache_size';