Why mysql 5.5 slower than 5.1 (linux,using mysqlslap) my.cnf (5.5 and 5.1 is the same) :

back_log=200
max_connections=512
max_connect_errors=999999
key_buffer=512M
max_allowed_packet=8M
table_cache=512
sort_buffer=8M
read_buffer_size=8M
thread_cache=8
thread_concurrency=4
myisam_sort_buffer_size=128M
interactive_timeout=28800
wait_timeout=7200


mysql 5.5:

..mysql5.5/bin/mysqlslap -a --concurrency=10 --number-of-queries 5000 
      --iterations=5 -S /tmp/mysql_5.5.sock --engine=innodb
Benchmark
        Running for engine innodb
        Average number of seconds to run all queries: 15.156 seconds
        Minimum number of seconds to run all queries: 15.031 seconds
        Maximum number of seconds to run all queries: 15.296 seconds
        Number of clients running queries: 10
        Average number of queries per client: 500


mysql5.1:

..mysql5.5/bin/mysqlslap -a --concurrency=10 --number-of-queries 5000 
      --iterations=5 -S /tmp/mysql_5.1.sock --engine=innodb
Benchmark
        Running for engine innodb
        Average number of seconds to run all queries: 13.252 seconds
        Minimum number of seconds to run all queries: 13.019 seconds
        Maximum number of seconds to run all queries: 13.480 seconds
        Number of clients running queries: 10
        Average number of queries per client: 500


Why mysql 5.5 slower than 5.1 ?

BTW:I'm tried mysql5.5/bin/mysqlslap and mysql5.1/bin/mysqlslap,result is the same
You may find this surprising, but MySQL 5.1 may outperform MySQL 5.5 under certain circumstances.

Percona performed a bake-off among multiple releases of MySQL


MySQL 4.1
MySQL 5.0
MySQL 5.1 (with built-in InnoDB)
MySQL 5.1 with InnoDB-plugin
MySQL 5.5
MySQL 5.6


All tests were performed with MySQL unconfigured (In other words, no my.cnf was made). The results?


MySQL 4.1 performs the best single-threaded
MySQL 5.1 with InnoDB plug-in scales on multiple cores better than 5.1 InnoDB built, 5.5 and 5.6


If you want newer versions of MySQL to perform better, you must tune for it. In fact, I described in the DBA StackExchange the idea of performing a MySQL Bakeoff.

What do I mean tune for it?

In MySQL 5.5, there are new InnoDB options for utilizing more dedicated read threads, write threads, and overall I/O capacity. This can engage more CPUs in multicore servers. Left unconfigured, MySQL 5.5 would operate on the same level playing field, in most cases, as older versions of MySQL. Sometimes, it could perform worse.