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I'm using Asp.Net together with MySQL. In the .Net connection string, I have set Max Pool Size to 150.

If I run the following I get these values:

SHOW GLOBAL STATUS LIKE 'max_used_connections'; gives 66
SHOW GLOBAL STATUS LIKE 'Threads_created'; gives 66
SHOW GLOBAL STATUS LIKE 'connections'; gives 474

Which gives Threads_created / Connections = 0,1392. 

So from that it seems like I need to increase thread_cache_size .

But if I run SHOW PROCESSLIST I always see that I have a lot of connections open (most of them sleeping) because of the pool created by .Net. Do I still need to set the thread_cache_size as I still will reuse the connections from the connection pool? If the Pool Size is 150 do you think a good value would be to set thread_cache_size to 150+? Would this affect CPU and memory a lot?

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Based on the info in the MySQL Documentation you should do the following: Find out what the highest number of simultaneous connections mysqld has had using Connections, Threads_created, and Max_used_connections,

  • SHOW GLOBAL STATUS LIKE 'Connections';
  • SHOW GLOBAL STATUS LIKE 'Threads_created';
  • SHOW GLOBAL STATUS LIKE 'Max_used_connections';

Try calculating the following

Threads_created / Connections : If this is over 0.01, then increase thread_cache_size. At the very least, thread_cache_size should be greater than Max_used_connections.

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Thanks for great comment! I updated my question a little bit. – Martin Jul 19 '12 at 6:59
I think you were intending mysql> SHOW GLOBAL STATUS LIKE 'connections'; (you have written max_used_connections twice) – Alekc Oct 11 '12 at 7:15

According to the MySQL docs, you should set thread_cache_size so that most new connections use threads from the cache rather than newly created threads. This saves some thread-creation overhead, though normally does not create a significant performance improvement:

Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. Normally, this does not provide a notable performance improvement if you have a good thread implementation. However, if your server sees hundreds of connections per second you should normally set thread_cache_size high enough so that most new connections use cached threads. (source)

This would mean that you should set your thread_cache_size so that Threads_created / Connections (the % of connections that lead to the creation of new threads) is rather low. If you take the MySQL docs literally ("most"), the value should be < 50%. RolandoMySQLDBA's answer says < 1%. I don't know who's closer to the truth.

You should not set thread_cache_size higher than Max_used_connections. The final sentence in RolandoMySQLDBA's answer ("At the very least, thread_cache_size should be greater than Max_used_connections") doesn't seem sensible because it says that you should keep more threads in the cache than your server ever uses. MySQL will never put that many threads in the cache anyway -- it does not pre-emptively put threads in the cache -- it only puts them there after a client creates a thread and disconnects. If you never have X clients connecting at the same time, you will never have X threads in the cache:

When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there. (source)

See also this answer by Michael:

Setting thread_cache_size to a value larger than max_connections seems like tremendously unhelpful advice... the cache can't possibly grow larger than max_connections and even a cache anywhere close to that size could only make sense if you have a tremendous amount of churn on your threads... which, in a well-behaved application, won't be the case.

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