Comment 0 for bug 1258097

The default value (500) for the thread_pool_max_threads is rather meaningless and provides an easy way for users to shoot themselves in the foot.

Limiting the total number of threads in the threadpool can result in deadlocks and uneven distribution of worker threads between thread groups in case of stalled connections.

Making it unlimited by default makes much more sense. This is to change both the maximum and default values for thread_pool_max_threads to 100k(the maximum max_connections value).