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:
All threads share the
key buffer; its size is determined by the
Other buffers used by the server are allocated as needed.
See Section 8.12.2, “Tuning Server Parameters”.
Each thread that is used to manage client connections uses some thread-specific space. The following list indicates these and which variables control their size:
The connection buffer and result buffer each begin with a
size equal to
but are dynamically enlarged up to
as needed. The result buffer shrinks to
after each SQL statement. While a statement is running, a
copy of the current statement 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.
Before MySQL 5.1.4, only compressed
MyISAM tables are memory mapped. As of
MySQL 5.1.4, the
variable can be set to 1 to enable memory-mapping for all
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
be allocated 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
BLOB columns are stored on
If an internal in-memory temporary table becomes too
large, MySQL handles this automatically by changing the
table from in-memory to on-disk format, to be handled by
MyISAM storage engine. You can
increase the permissible temporary table size as described
in Section 8.4.4, “How MySQL Uses Internal Temporary Tables”.
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.5.4.4, “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, so the normal slow memory allocation and freeing is avoided. Memory is allocated only for unexpectedly large strings.
MyISAM table 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
N is the maximum row
length, not counting
requires five to eight bytes plus the length of the
BLOB data. The
MyISAM storage engine maintains one
extra row buffer for internal use.
Handler structures for all in-use tables are saved in a
cache and managed as a FIFO. The initial cache size is
taken from the value of the
variable. If a table has been used by two running threads
at the same time, the cache contains two entries for the
table. See Section 126.96.36.199, “How MySQL Opens and Closes Tables”.
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.
TABLES does not return until all tables have
The server caches information in memory as a result of
CREATE SERVER, and
INSTALL PLUGIN statements.
This memory is not released by the corresponding
DROP SERVER, and
statements, so for a server that executes many instances
of the statements that cause caching, there will be an
increase in memory use. This cached memory can be freed
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.