MySQL allocates buffers and caches to improve performance of database operations. The default configuration is designed to permit a MySQL server to start on a virtual machine that has approximately 512MB of RAM. You can improve MySQL performance by increasing the values of certain cache and buffer-related system variables. You can also modify the default configuration to run MySQL on systems with limited memory.
The following list describes some of the ways that MySQL uses memory. Where applicable, relevant system variables are referenced. Some items are storage engine or feature specific.
The
InnoDB
buffer pool is a memory area that holds cachedInnoDB
data for tables, indexes, and other auxiliary buffers. For efficiency of high-volume read operations, the buffer pool is divided into pages that can potentially hold multiple rows. For efficiency of cache management, the buffer pool is implemented as a linked list of pages; data that is rarely used is aged out of the cache, using a variation of the LRU algorithm. For more information, see Section 17.5.1, “Buffer Pool”.The size of the buffer pool is important for system performance:
InnoDB
allocates memory for the entire buffer pool at server startup, usingmalloc()
operations. Theinnodb_buffer_pool_size
system variable defines the buffer pool size. Typically, a recommendedinnodb_buffer_pool_size
value is 50 to 75 percent of system memory.innodb_buffer_pool_size
can be configured dynamically, while the server is running. For more information, see Section 17.8.3.1, “Configuring InnoDB Buffer Pool Size”.On systems with a large amount of memory, you can improve concurrency by dividing the buffer pool into multiple buffer pool instances. The
innodb_buffer_pool_instances
system variable defines the number of buffer pool instances.A buffer pool that is too small may cause excessive churning as pages are flushed from the buffer pool only to be required again a short time later.
A buffer pool that is too large may cause swapping due to competition for memory.
The storage engine interface enables the optimizer to provide information about the size of the record buffer to be used for scans that the optimizer estimates are likely to read multiple rows. The buffer size can vary based on the size of the estimate.
InnoDB
uses this variable-size buffering capability to take advantage of row prefetching, and to reduce the overhead of latching and B-tree navigation.All threads share the
MyISAM
key buffer. Thekey_buffer_size
system variable determines its size.For each
MyISAM
table the server opens, the index file is opened once; the data file is opened once for each concurrently running thread that accesses the table. For each concurrent thread, a table structure, column structures for each column, and a buffer of size3 *
are allocated (whereN
N
is the maximum row length, not countingBLOB
columns). ABLOB
column requires five to eight bytes plus the length of theBLOB
data. TheMyISAM
storage engine maintains one extra row buffer for internal use.The
myisam_use_mmap
system variable can be set to 1 to enable memory-mapping for allMyISAM
tables.If an internal in-memory temporary table becomes too large (as determined using the
tmp_table_size
andmax_heap_table_size
system variables), MySQL automatically converts the table from in-memory to on-disk format. As of MySQL 8.0.16, on-disk temporary tables always use theInnoDB
storage engine. (Previously, the storage engine employed for this purpose was determined by theinternal_tmp_disk_storage_engine
system variable, which is no longer supported.) You can increase the permissible temporary table size as described in Section 10.4.4, “Internal Temporary Table Use in MySQL”.For
MEMORY
tables explicitly created withCREATE TABLE
, only themax_heap_table_size
system variable determines how large a table can grow, and there is no conversion to on-disk format.The MySQL Performance Schema is a feature for monitoring MySQL server execution at a low level. The Performance Schema dynamically allocates memory incrementally, scaling its memory use to actual server load, instead of allocating required memory during server startup. Once memory is allocated, it is not freed until the server is restarted. For more information, see Section 29.17, “The Performance Schema Memory-Allocation Model”.
Each thread that the server uses to manage client connections requires some thread-specific space. The following list indicates these and which system variables control their size:
A stack (
thread_stack
)A connection buffer (
net_buffer_length
)A result buffer (
net_buffer_length
)
The connection buffer and result buffer each begin with a size equal to
net_buffer_length
bytes, but are dynamically enlarged up tomax_allowed_packet
bytes as needed. The result buffer shrinks tonet_buffer_length
bytes after each SQL statement. While a statement is running, a copy of the current statement string is also allocated.Each connection thread uses memory for computing statement digests. The server allocates
max_digest_length
bytes per session. See Section 29.10, “Performance Schema Statement Digests and Sampling”.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.
Each request that performs a sequential scan of a table allocates a read buffer. The
read_buffer_size
system variable determines the buffer size.When reading rows in an arbitrary sequence (for example, following a sort), a random-read buffer may be allocated to avoid disk seeks. The
read_rnd_buffer_size
system variable determines the buffer size.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
BLOB
columns are stored on disk.Most requests that perform a sort allocate a sort buffer and zero to two temporary files depending on the result set size. See Section B.3.3.5, “Where MySQL Stores Temporary Files”.
Almost all parsing and calculating is done in thread-local and reusable memory pools. No memory overhead is needed for small items, thus avoiding the normal slow memory allocation and freeing. Memory is allocated only for unexpectedly large strings.
For each table having
BLOB
columns, a buffer is enlarged dynamically to read in largerBLOB
values. If you scan a table, the buffer grows as large as the largestBLOB
value.MySQL requires memory and descriptors for the table cache. Handler structures for all in-use tables are saved in the table cache and managed as “First In, First Out” (FIFO). The
table_open_cache
system variable defines the initial table cache size; see Section 10.4.3.1, “How MySQL Opens and Closes Tables”.MySQL also requires memory for the table definition cache. The
table_definition_cache
system variable defines the number of table definitions that can be stored in the table definition cache. If you use a large number of tables, you can create a large table definition cache to speed up the opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the table cache.A
FLUSH TABLES
statement 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 TABLES
does not return until all tables have been closed.The server caches information in memory as a result of
GRANT
,CREATE USER
,CREATE SERVER
, andINSTALL PLUGIN
statements. This memory is not released by the correspondingREVOKE
,DROP USER
,DROP SERVER
, andUNINSTALL PLUGIN
statements, so for a server that executes many instances of the statements that cause caching, there is an increase in cached memory use unless it is freed withFLUSH PRIVILEGES
.In a replication topology, the following settings affect memory usage, and can be adjusted as required:
The
max_allowed_packet
system variable on a replication source limits the maximum message size that the source sends to its replicas for processing. This setting defaults to 64M.The system variable
replica_pending_jobs_size_max
(from MySQL 8.0.26) orslave_pending_jobs_size_max
(before MySQL 8.0.26) on a multithreaded replica sets the maximum amount of memory that is made available for holding messages awaiting processing. This setting defaults to 128M. The memory is only allocated when needed, but it might be used if your replication topology handles large transactions sometimes. It is a soft limit, and larger transactions can be processed.The
rpl_read_size
system variable on a replication source or replica controls the minimum amount of data in bytes that is read from the binary log files and relay log files. The default is 8192 bytes. A buffer the size of this value is allocated for each thread that reads from the binary log and relay log files, including dump threads on sources and coordinator threads on replicas.The
binlog_transaction_dependency_history_size
system variable limits the number of row hashes held as an in-memory history.The
max_binlog_cache_size
system variable specifies the upper limit of memory usage by an individual transaction.The
max_binlog_stmt_cache_size
system variable specifies the upper limit of memory usage by the statement cache.
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. To verify this, check available swap with
swap -s
. We test mysqld
with several memory-leakage detectors (both commercial and
Open Source), so there should be no memory leaks.