We start with system-level factors, because some of these decisions must be made very early to achieve large performance gains. In other cases, a quick look at this section may suffice. However, it is always nice to have a sense of how much can be gained by changing factors that apply at this level.
The operating system to use is very important. To get the best use of multiple-CPU machines, you should use Solaris (because its threads implementation works well) or Linux (because the 2.4 and later kernels have good SMP support). Note that older Linux kernels have a 2GB filesize limit by default. If you have such a kernel and a need for files larger than 2GB, you should get the Large File Support (LFS) patch for the ext2 filesystem. Other filesystems such as ReiserFS and XFS do not have this 2GB limitation.
Before using MySQL in production, we advise you to test it on your intended platform.
If you have enough RAM, you could remove all swap devices. Some operating systems use a swap device in some contexts even if you have free memory.
Avoid external locking. Since MySQL 4.0, the default has been for external locking to be disabled on all systems. The
--skip-external-lockingoptions explicitly enable and disable external locking.
Note that disabling external locking does not affect MySQL's functionality as long as you run only one server. Just remember to take down the server (or lock and flush the relevant tables) before you run myisamchk. On some systems it is mandatory to disable external locking because it does not work, anyway.
The only case in which you cannot disable external locking is when you run multiple MySQL servers (not clients) on the same data, or if you run myisamchk to check (not repair) a table without telling the server to flush and lock the tables first. Note that using multiple MySQL servers to access the same data concurrently is generally not recommended, except when using MySQL Cluster.
UNLOCK TABLESstatements use internal locking, so you can use them even if external locking is disabled.
You can determine the default buffer sizes used by the mysqld server using this command:
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 50 binlog_cache_size 32768 bulk_insert_buffer_size 8388608 connect_timeout 5 date_format (No default value) datetime_format (No default value) default_week_format 0 delayed_insert_limit 100 delayed_insert_timeout 300 delayed_queue_size 1000 expire_logs_days 0 flush_time 1800 ft_max_word_len 84 ft_min_word_len 4 ft_query_expansion_limit 20 ft_stopword_file (No default value) group_concat_max_len 1024 innodb_additional_mem_pool_size 1048576 innodb_autoextend_increment 8 innodb_buffer_pool_awe_mem_mb 0 innodb_buffer_pool_size 8388608 innodb_concurrency_tickets 500 innodb_file_io_threads 4 innodb_force_recovery 0 innodb_lock_wait_timeout 50 innodb_log_buffer_size 1048576 innodb_log_file_size 5242880 innodb_log_files_in_group 2 innodb_mirrored_log_groups 1 innodb_open_files 300 innodb_sync_spin_loops 20 innodb_thread_concurrency 8 innodb_thread_sleep_delay 10000 interactive_timeout 28800 join_buffer_size 131072 key_buffer_size 8388600 key_cache_age_threshold 300 key_cache_block_size 1024 key_cache_division_limit 100 long_query_time 10 lower_case_table_names 1 max_allowed_packet 1048576 max_binlog_cache_size 4294967295 max_binlog_size 1073741824 max_connect_errors 10 max_connections 100 max_delayed_threads 20 max_error_count 64 max_heap_table_size 16777216 max_join_size 4294967295 max_length_for_sort_data 1024 max_relay_log_size 0 max_seeks_for_key 4294967295 max_sort_length 1024 max_tmp_tables 32 max_user_connections 0 max_write_lock_count 4294967295 multi_range_count 256 myisam_block_size 1024 myisam_data_pointer_size 6 myisam_max_extra_sort_file_size 2147483648 myisam_max_sort_file_size 2147483647 myisam_repair_threads 1 myisam_sort_buffer_size 8388608 net_buffer_length 16384 net_read_timeout 30 net_retry_count 10 net_write_timeout 60 open_files_limit 0 optimizer_prune_level 1 optimizer_search_depth 62 preload_buffer_size 32768 query_alloc_block_size 8192 query_cache_limit 1048576 query_cache_min_res_unit 4096 query_cache_size 0 query_cache_type 1 query_cache_wlock_invalidate FALSE query_prealloc_size 8192 range_alloc_block_size 2048 read_buffer_size 131072 read_only FALSE read_rnd_buffer_size 262144 div_precision_increment 4 record_buffer 131072 relay_log_purge TRUE relay_log_space_limit 0 slave_compressed_protocol FALSE slave_net_timeout 3600 slave_transaction_retries 10 slow_launch_time 2 sort_buffer_size 2097144 sync-binlog 0 sync-frm TRUE sync-replication 0 sync-replication-slave-id 0 sync-replication-timeout 10 table_cache 64 thread_cache_size 0 thread_concurrency 10 thread_stack 196608 time_format (No default value) tmp_table_size 33554432 transaction_alloc_block_size 8192 transaction_prealloc_size 4096 updatable_views_with_limit 1 wait_timeout 28800
For a mysqld server that is currently running, you can see the current values of its system variables by connecting to it and issuing this statement:
You can also see some statistical and status indicators for a running server by issuing this statement:
System variable and status information also can be obtained using mysqladmin:
MySQL uses algorithms that are very scalable, so you can usually run with very little memory. However, normally you get better performance by giving MySQL more memory.
When tuning a MySQL server, the two most important variables to
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.
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:
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:
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:
mysqld_safe --key_buffer_size=512K --sort_buffer_size=100K \
Or even this:
mysqld_safe --key_buffer_size=512K --sort_buffer_size=16K \
--table_cache=32 --read_buffer_size=8K \
If you are performing
GROUP BY or
ORDER BY operations on tables that are much
larger than your available memory, you should increase the value
read_rnd_buffer_size to speed up the
reading of rows following sorting operations.
You can make use of the example option files included with your MySQL distribution; see Section 184.108.40.206, “Preconfigured Option Files”.
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:
mysqld --key_buffer_size=32M --verbose --help
The variable values are listed near the end of the output. Make
sure that the
--help options are last. Otherwise, the effect
of any options listed after them on the command line are not
reflected in the output.
For information on tuning the
engine, see Section 14.2.11, “
InnoDB Performance Tuning Tips”.
The task of the query optimizer is to find an optimal plan for executing an SQL query. Because the difference in performance between “good” and “bad” plans can be orders of magnitude (that is, seconds versus hours or even days), most query optimizers, including that of MySQL, perform a more or less exhaustive search for an optimal plan among all possible query evaluation plans. For join queries, the number of possible plans investigated by the MySQL optimizer grows exponentially with the number of tables referenced in a query. For small numbers of tables (typically less than 7–10) this is not a problem. However, when larger queries are submitted, the time spent in query optimization may easily become the major bottleneck in the server's performance.
MySQL 5.0.1 introduces a more flexible method for query optimization that allows the user to control how exhaustive the optimizer is in its search for an optimal query evaluation plan. The general idea is that the fewer plans that are investigated by the optimizer, the less time it spends in compiling a query. On the other hand, because the optimizer skips some plans, it may miss finding an optimal plan.
The behavior of the optimizer with respect to the number of plans it evaluates can be controlled via two system variables:
optimizer_prune_levelvariable tells the optimizer to skip certain plans based on estimates of the number of rows accessed for each table. Our experience shows that this kind of “educated guess” rarely misses optimal plans, and may dramatically reduce query compilation times. That is why this option is on (
optimizer_prune_level=1) by default. However, if you believe that the optimizer missed a better query plan, this option can be switched off (
optimizer_prune_level=0) with the risk that query compilation may take much longer. Note that, even with the use of this heuristic, the optimizer still explores a roughly exponential number of plans.
optimizer_search_depthvariable tells how far into the “future” of each incomplete plan the optimizer should look to evaluate whether it should be expanded further. Smaller values of
optimizer_search_depthmay result in orders of magnitude smaller query compilation times. For example, queries with 12, 13, or more tables may easily require hours and even days to compile if
optimizer_search_depthis close to the number of tables in the query. At the same time, if compiled with
optimizer_search_depthequal to 3 or 4, the optimizer may compile in less than a minute for the same query. If you are unsure of what a reasonable value is for
optimizer_search_depth, this variable can be set to 0 to tell the optimizer to determine the value automatically.
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 obtain the fastest executables when you link with
On Linux, it is best to compile the server with
-O3. You need about
200MB memory to compile
these options, because gcc or
pgcc needs a great deal of memory to make all
functions inline. You should also set
when configuring MySQL to avoid inclusion of the
libstdc++ library, which is not needed. Note
that with some versions of pgcc, the
resulting binary runs 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
By using a better compiler and compilation options, you can obtain a 10–30% speed increase in applications. This is particularly important if you compile the MySQL server yourself.
When we tested both the Cygnus CodeFusion and Fujitsu compilers, 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
Here is a list of some measurements that we have made:
If you use pgcc and compile everything with
-O6, the mysqld server is 1% faster than with gcc 2.95.2.
If you link dynamically (without
-static), the result is 13% slower on Linux. Note that you still can use a dynamically linked MySQL library for your client applications. It is the server that is most critical for performance.
For a connection from a client to a server running on the same host, if you connect using TCP/IP rather than a Unix socket file, performance is 7.5% slower. (On Unix, if you connect to the hostname
localhost, MySQL uses a socket file by default.)
For TCP/IP connections from a client to a server, connecting to a remote server on another host is 8–11% slower than connecting to a server on the same host, even for connections over 100Mb/s Ethernet.
When running our benchmark tests using secure connections (all data encrypted with internal SSL support) performance was 55% slower than with unencrypted connections.
If you compile with
--with-debug=full, most queries are 20% slower. Some queries may take substantially longer; for example, the MySQL benchmarks run 35% slower. If you use
=full), the speed decrease is only 15%. For a version of mysqld that has been compiled with
--with-debug=full, you can disable memory checking at runtime by starting it with the
--skip-safemallocoption. The execution speed should then be close to that obtained when configuring with
On a Sun UltraSPARC-IIe, a server compiled with Forte 5.0 is 4% faster than one compiled with gcc 3.2.
On a Sun UltraSPARC-IIe, a server compiled with Forte 5.0 is 4% faster in 32-bit mode than in 64-bit mode.
Compiling with gcc 2.95.2 for UltraSPARC with the
-mcpu=v8 -Wa,-xarch=v8plusaoptions gives 4% more performance.
On Solaris 2.5.1, MIT-pthreads is 8–12% slower than Solaris native threads on a single processor. With greater loads or more CPUs, the difference should be larger.
Compiling on Linux-x86 using gcc without frame pointers (
-fomit-frame-pointer -ffixed-ebp) makes mysqld 1–4% faster.
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 binaries that do 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 build a faster binary by compiling with pgcc. The standard MySQL Linux binary is linked statically to make it faster and more portable.
The following list indicates some of the ways that the mysqld server uses memory. Where applicable, the name of the system variable relevant to the memory use is given:
The key buffer (variable
key_buffer_size) is shared by all threads; other buffers used by the server are allocated as needed. See Section 7.5.2, “Tuning Server Parameters”.
Each connection uses some thread-specific space:
A stack (default 192KB, variable
A connection buffer (variable
A result buffer (variable
The connection buffer and result buffer are dynamically enlarged up to
max_allowed_packetwhen needed. While a query is running, a copy of the current query string is also allocated.
All threads share the same base memory.
When a thread is no longer needed, the memory allocated to it is released and returned to the system unless the thread goes back into the thread cache. In that case, the memory remains allocated.
MyISAMtables are memory mapped. This is because the 32-bit memory space of 4GB is not large enough for most big tables. When systems with a 64-bit address space become more common, we may add general support for memory mapping.
Each request that performs a sequential scan of a table allocates a read buffer (variable
When reading rows in an arbitrary sequence (for example, following a sort), a random-read buffer (variable
read_rnd_buffer_size) may be allocated in order to avoid disk seeks.
All joins are executed in a single pass, and most joins can be done without even using a temporary table. Most temporary tables are memory-based hash tables. Temporary tables with a large row length (calculated as the sum of all column lengths) or that contain
BLOBcolumns are stored on disk.
If an internal heap table exceeds the size of
tmp_table_size, MySQL handles this automatically by changing the in-memory heap table to a disk-based
MyISAMtable as necessary. You can also increase the temporary table size by setting the
tmp_table_sizeoption to mysqld, or by setting the SQL option
SQL_BIG_TABLESin the client program. See Section 13.5.3, “
Most requests that perform a sort allocate a sort buffer and zero to two temporary files depending on the result set size. See Section A.4.4, “Where MySQL Stores Temporary Files”.
Almost all parsing and calculating is done in a local memory store. No memory overhead is needed for small items, so the normal slow memory allocation and freeing is avoided. Memory is allocated only for unexpectedly large strings. This is done with
MyISAMtable that is opened, the index file is opened once; the data file is opened once for each concurrently running thread. For each concurrent thread, a table structure, column structures for each column, and a buffer of size
Nare allocated (where
Nis the maximum row length, not counting
BLOBcolumn requires five to eight bytes plus the length of the
MyISAMstorage engine maintains one extra row buffer for internal use.
For each table having
BLOBcolumns, a buffer is enlarged dynamically to read in larger
BLOBvalues. If you scan a table, a buffer as large as the largest
BLOBvalue is allocated.
Handler structures for all in-use tables are saved in a cache and managed as a FIFO. By default, the cache has 64 entries. If a table has been used by two running threads at the same time, the cache contains two entries for the table. See Section 7.4.8, “How MySQL Opens and Closes Tables”.
FLUSH TABLESstatement or mysqladmin flush-tables command closes all tables that are not in use at once and marks all in-use tables to be closed when the currently executing thread finishes. This effectively frees most in-use memory.
FLUSH TABLESdoes not return until all tables have been closed.
ps and other system status programs may
report that mysqld uses a lot of memory. This
may be caused by thread stacks on different memory addresses.
For example, the Solaris version of ps counts
the unused memory between stacks as used memory. You can verify
this by checking available swap with
We test mysqld with several memory-leakage
detectors (both commercial and Open Source), so there should be
no memory leaks.
When a new client connects to mysqld, mysqld spawns a new thread to handle the request. This thread first checks whether the hostname is in the hostname cache. If not, the thread attempts to resolve the hostname:
If the operating system supports the thread-safe
gethostbyname_r()calls, the thread uses them to perform hostname resolution.
If the operating system does not support the thread-safe calls, the thread locks a mutex and calls
gethostbyname()instead. In this case, no other thread can resolve hostnames that are not in the hostname cache until the first thread unlocks the mutex.
You can disable DNS hostname lookups by starting
mysqld with the
--skip-name-resolve option. However, in this
case, you can use only IP numbers in the MySQL grant tables.
If you have a very slow DNS and many hosts, you can get more
performance by either disabling DNS lookups with
--skip-name-resolve or by increasing the
HOST_CACHE_SIZE define (default value: 128)
and recompiling mysqld.
You can disable the hostname cache by starting the server with
--skip-host-cache option. To clear the
hostname cache, issue a
FLUSH HOSTS statement
or execute the mysqladmin flush-hosts
To disallow TCP/IP connections entirely, start
mysqld with the