You can determine the default buffer sizes used by the mysqld server with this command (prior to MySQL 4.1, omit --verbose):
shell> mysqld --verbose --help
This command produces a list of all mysqld options and configurable system variables. The output includes the default variable values and looks something like this:
back_log current value: 5 bdb_cache_size current value: 1048540 binlog_cache_size current value: 32768 connect_timeout current value: 5 delayed_insert_limit current value: 100 delayed_insert_timeout current value: 300 delayed_queue_size current value: 1000 flush_time current value: 0 interactive_timeout current value: 28800 join_buffer_size current value: 131072 key_buffer_size current value: 1048540 long_query_time current value: 10 lower_case_table_names current value: 0 max_allowed_packet current value: 1048576 max_binlog_cache_size current value: 4294967295 max_connect_errors current value: 10 max_connections current value: 100 max_delayed_threads current value: 20 max_heap_table_size current value: 16777216 max_join_size current value: 4294967295 max_sort_length current value: 1024 max_tmp_tables current value: 32 max_write_lock_count current value: 4294967295 myisam_sort_buffer_size current value: 8388608 net_buffer_length current value: 16384 net_read_timeout current value: 30 net_retry_count current value: 10 net_write_timeout current value: 60 read_buffer_size current value: 131072 read_rnd_buffer_size current value: 262144 slow_launch_time current value: 2 sort_buffer current value: 2097116 table_cache current value: 64 thread_concurrency current value: 10 thread_stack current value: 131072 tmp_table_size current value: 1048576 wait_timeout current value: 28800
If there is a mysqld server currently running, you can see what values it actually is using for the system variables by connecting to it and issuing this statement:
mysql> SHOW VARIABLES;
You can also see some statistical and status indicators for a running server by issuing this statement:
mysql> SHOW STATUS;
System variable and status information also can be obtained using mysqladmin:
shell> mysqladmin variables shell> mysqladmin extended-status
You can find a full description for all system and status variables in Section 4.2.3, "Server System Variables," and Section 4.2.4, "Server Status Variables."
MySQL uses algorithms that are very scalable, so you can usually run with very little memory. However, normally you will get better performance by giving MySQL more memory.
When tuning a MySQL server, the two most important variables to configure are key_buffer_size and table_cache. You should first feel confident that you have these set appropriately before trying to change any other variables.
The following examples indicate some typical variable values for different runtime configurations. The examples use the mysqld_safe script and use --var_name=value syntax to set the variable var_name to the value value. This syntax is available as of MySQL 4.0. For older versions of MySQL, take the following differences into account:
If you have at least 256MB of memory and many tables and want maximum performance with a moderate number of clients, you should use something like this:
shell> mysqld_safe --key_buffer_size=64M --table_cache=256 \ --sort_buffer_size=4M --read_buffer_size=1M &
If you have only 128MB of memory and only a few tables, but you still do a lot of sorting, you can use something like this:
shell> mysqld_safe --key_buffer_size=16M --sort_buffer_size=1M
If there are very many simultaneous connections, swapping problems may occur unless mysqld has been configured to use very little memory for each connection. mysqld performs better if you have enough memory for all connections.
With little memory and lots of connections, use something like this:
shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=100K \ --read_buffer_size=100K &
Or even this:
shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=16K \ --table_cache=32 --read_buffer_size=8K \ --net_buffer_length=1K &
If you are doing GROUP BY or ORDER BY operations on tables that are much larger than your available memory, you should increase the value of read_rnd_buffer_size to speed up the reading of rows after sorting operations.
When you have installed MySQL, the support-files directory will contain some different my.cnf sample files: my-huge.cnf, my-large.cnf, my-medium.cnf, and my-small.cnf. You can use these as a basis for optimizing your system.
Note that if you specify an option on the command line for mysqld or mysqld_safe, it remains in effect only for that invocation of the server. To use the option every time the server runs, put it in an option file.
To see the effects of a parameter change, do something like this (prior to MySQL 4.1, omit --verbose):
shell> mysqld --key_buffer_size=32M --verbose --help
The variable values are listed near the end of the output. Make sure that the --verbose and --help options are last. Otherwise, the effect of any options listed after them on the command line will not be reflected in the output.
For information on tuning the InnoDB storage engine, see Section 9.12, "InnoDB Performance Tuning Tips."
6.5.3 How Compiling and Linking Affects the Speed of MySQL
Most of the following tests were performed on Linux with the MySQL benchmarks, but they should give some indication for other operating systems and workloads.
You get the fastest executables when you link with -static.
On Linux, you will get the fastest code when compiling with pgcc and -O3. You need about 200MB memory to compile sql_yacc.cc with these options, because gcc/pgcc needs a lot of memory to make all functions inline. You should also set CXX=gcc when configuring MySQL to avoid inclusion of the libstdc++ library, which is not needed. Note that with some versions of pgcc, the resulting code will run only on true Pentium processors, even if you use the compiler option indicating that you want the resulting code to work on all x586-type processors (such as AMD).
By just using a better compiler and better compiler options, you can get a 10-30% speed increase in your application. This is particularly important if you compile the MySQL server yourself.
We have tested both the Cygnus CodeFusion and Fujitsu compilers, but when we tested them, neither was sufficiently bug-free to allow MySQL to be compiled with optimizations enabled.
The standard MySQL binary distributions are compiled with support for all character sets. When you compile MySQL yourself, you should include support only for the character sets that you are going to use. This is controlled by the --with-charset option to configure.
Here is a list of some measurements that we have made:
Binary MySQL distributions for Linux that are provided by MySQL AB used to be compiled with pgcc. We had to go back to regular gcc due to a bug in pgcc that would generate code that does not run on AMD. We will continue using gcc until that bug is resolved. In the meantime, if you have a non-AMD machine, you can get a faster binary by compiling with pgcc. The standard MySQL Linux binary is linked statically to make it faster and more portable.
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