mysql thread not executing. Still in sleep state - mysql

Question

I was running benchmark test in mysql server using mysqlslap. But sometimes the last thread in the pool is not executing and closing the connection. It keeps in sleep state.
mysql> show full processlist;
+-----+------+-----------------------+-----------+---------+------+-------+-----------------------+
| Id | User | Host | db | Command | Time | State | Info |
+-----+------+-----------------------+-----------+---------+------+-------+-----------------------+
| 2 | root | 122.178.231.xxx:15832 | mysqlslap | Sleep | 740 | | NULL |
| 106 | root | localhost | NULL | Query | 0 | NULL | show full processlist |
+-----+------+-----------------------+-----------+---------+------+-------+-----------------------+
2 rows in set (0.00 sec)
Any idea? It happens often :(
This was the command I executed.
mysqlslap -h IP_address -u root --auto-generate-sql --concurrency=100
-pPASSWORD -vvv --number-of-queries=2500
my.cnf file is as follows,
[mysqld]
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
user=mysql
skip-bdb
max_connections = 500
query_cache_type = 1
query_cache_size = 128M
sort_buffer_size = 64M
key_buffer_size = 425M
query_cache_limit = 1024M
thread_cache_size = 4M
innodb_buffer_pool_size = 1360M
innodb_log_file_size = 512M
long_query_time = 5
innodb_thread_concurrency = 8
innodb_lock_wait_timeout = 300
max_allowed_packet = 128M
net_read_timeout = 240
net_write_timeout = 240
table_lock_wait_timeout =240
connect_timeout = 30
log-warnings = 2
log = /var/log/mysqld.log
log-error = /var/log/mysqld.error.log
log-slow-queries = /var/log/mysql_slow_queries.log