8.12.4.1 How MySQL Uses Memory

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 cached InnoDB 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 14.5.1, “Buffer Pool”.

  The size of the buffer pool is important for system performance:

  • InnoDB allocates memory for the entire buffer pool at server startup, using malloc() operations. The innodb_buffer_pool_size system variable defines the buffer pool size. Typically, a recommended innodb_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 14.8.3.1, “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.

• All threads share the MyISAM key buffer. The key_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 size 3 * N are allocated (where N is the maximum row length, not counting BLOB columns). A BLOB column requires five to eight bytes plus the length of the BLOB data. The MyISAM 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 all MyISAM tables.

• If an internal in-memory temporary table becomes too large (as determined using the tmp_table_size and max_heap_table_size system variables), MySQL automatically converts the table from in-memory to on-disk format. On-disk temporary tables use the storage engine defined by the internal_tmp_disk_storage_engine system variable. You can increase the permissible temporary table size as described in Section 8.4.4, “Internal Temporary Table Use in MySQL”.

  For MEMORY tables explicitly created with CREATE TABLE, only the max_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 25.17, “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 to max_allowed_packet bytes as needed. The result buffer shrinks to net_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 25.10, “Performance Schema Statement Digests”.

• 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 larger BLOB values. If you scan a table, the buffer grows as large as the largest BLOB 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 8.4.3.1, “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 (from .frm files) 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, and INSTALL PLUGIN statements. This memory is not released by the corresponding REVOKE, DROP USER, DROP SERVER, and UNINSTALL PLUGIN statements, so for a server that executes many instances of the statements that cause caching, cached memory use is very likely to increase unless it is freed with FLUSH PRIVILEGES.

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.