What value of thread_cache_size should I use?

Question

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?

RolandoMySQLDBA · Answer 1 · 2012-10-11T07:16:57.117

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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.

edited Oct 11 '12 at 07:16

answered Jul 18 '12 at 18:08

RolandoMySQLDBA

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Thanks for great comment! I updated my question a little bit. – Martin Jul 19 '12 at 06:59
I think you were intending mysql> SHOW GLOBAL STATUS LIKE 'connections'; (you have written max_used_connections twice) – Alekc Oct 11 '12 at 07:15
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I just want to add that by referring to this http://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_thread_cache_size you should add 8 to Max_used_connections for the thread_cache_size value (without exceeding 100) – CME64 Nov 23 '16 at 11:37
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Actually, the manual suggests a default of (1% of the mac_used_connections) + 8 .... or 100...whichever is less. – Christopher McGowan Apr 20 '17 at 22:36
For me the #'s are 264/103134 which works out to 0.0026 which is way less than 0.01 But my MaxUsedConnections is 72, so the last sentence is terribly conflicted – boatcoder Nov 01 '17 at 18:57
My max connections is 828 and created/connections = 9.15. My question is, is it safe to set my thread_cache_size to 1000? What are the implications of that? – Vincent Apr 02 '18 at 16:59

score 13 · Answer 2 · edited Apr 25 '19 at 04:03

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)

What value of thread_cache_size should I use?

3 Answers3