performance regression with threadpool

Enabling the thread pool leads to about 5% performance loss in MariaDB 10.0 and 10.1, but not in MariaDB 5.5. I tested 5.5.49 vs. 10.0.25 vs. 10.1.14.

The benchmark is sysbench OLTP read-only with 1000 point-selects per transaction. The benchmark machine has 16 cores (32 hyperthreads).

my.cnf:

[mysqld]

max_connections = 1300

table_open_cache = 2600

query_cache_type = 0

innodb_buffer_pool_size = 512M

innodb_buffer_pool_instances = 10

innodb_adaptive_hash_index_partitions = 20

thread_handling=pool-of-threads

See attached spread sheet for numbers.