MySQL 5.7 Release Notes
When InnoDB was developed, the memory
allocators supplied with operating systems and run-time libraries
were often lacking in performance and scalability. At that time,
there were no memory allocator libraries tuned for multi-core
CPUs. Therefore, InnoDB implemented its own
memory allocator in the mem subsystem. This
allocator is guarded by a single mutex, which may become a
bottleneck.
InnoDB also implements a wrapper interface
around the system allocator (malloc and
free) that is likewise guarded by a single
mutex.
Today, as multi-core systems have become more widely available,
and as operating systems have matured, significant improvements
have been made in the memory allocators provided with operating
systems. These new memory allocators perform better and are more
scalable than they were in the past. Most workloads, especially
those where memory is frequently allocated and released (such as
multi-table joins), benefit from using a more highly tuned memory
allocator as opposed to the internal,
InnoDB-specific memory allocator.
You can control whether InnoDB uses its own
memory allocator or an allocator of the operating system, by
setting the value of the system configuration parameter
innodb_use_sys_malloc in the MySQL option file
(my.cnf or my.ini). If set
to ON or 1 (the default),
InnoDB uses the malloc and
free functions of the underlying system rather
than manage memory pools itself. This parameter is not dynamic,
and takes effect only when the system is started. To continue to
use the InnoDB memory allocator, set
innodb_use_sys_malloc to 0.
When the InnoDB memory allocator is disabled,
InnoDB ignores the value of the parameter
innodb_additional_mem_pool_size. The
InnoDB memory allocator uses an additional
memory pool for satisfying allocation requests without having to
fall back to the system memory allocator. When the
InnoDB memory allocator is disabled, all such
allocation requests are fulfilled by the system memory allocator.
On Unix-like systems that use dynamic linking, replacing the
memory allocator may be as easy as making the environment variable
LD_PRELOAD or
LD_LIBRARY_PATH point to the dynamic library
that implements the allocator. On other systems, some relinking
may be necessary. Please refer to the documentation of the memory
allocator library of your choice.
Since InnoDB cannot track all memory use when
the system memory allocator is used
(innodb_use_sys_malloc is
ON), the section “BUFFER POOL AND
MEMORY” in the output of the SHOW ENGINE INNODB
STATUS command only includes the buffer pool statistics
in the “Total memory allocated”. Any memory allocated
using the mem subsystem or using
ut_malloc is excluded.
Note
innodb_use_sys_malloc and
innodb_additional_mem_pool_size were
deprecated in MySQL 5.6 and removed in MySQL
5.7.
For more information about the performance implications of
InnoDB memory usage, see
Section 8.10, “Buffering and Caching”.