InnoDB: Management of the Process Global Area (PGA)
In InnoDB, the Process Global Area (PGA) manages per-connection memory and data structures associated with each client connection, including query execution buffers, sort buffers, and join buffers.
- Connection Context: When a client establishes a connection to the InnoDB storage engine, a dedicated PGA is created for that connection. Each connection has its own memory space within the PGA, which is used to hold temporary data and execution structures during query processing.
- Memory Allocation: InnoDB dynamically manages the PGA memory allocation for each connection based on the query’s requirements. The Memory Allocator component is responsible for efficient distribution and tracking of memory blocks using a combination of techniques, including fixed-size memory allocation and variable-size memory pools, to optimize memory usage and minimize fragmentation.
- Query Execution Buffers: The PGA allocates memory for various query execution buffers, such as read buffers, write buffers, and doublewrite buffers. These buffers hold data during different stages of query execution, including fetching data from disk, modifying data, and writing data back to disk. Buffer size can be configured using parameters such as innodb_read_io_threads, innodb_write_io_threads, and innodb_doublewrite_buffer_size.
- Sort and Join Buffers: InnoDB allocates memory within the PGA for sort and join operations. Sort buffers are used for sorting data during query execution, while join buffers are used for handling join operations between tables. The size of these buffers can be configured using parameters such as sort_buffer_size, join_buffer_size, and tmp_table_size.
- Temporary Tables: When temporary tables are created during query execution, InnoDB allocates memory within the PGA to hold the temporary table data. The size of the memory allocated for temporary tables can be influenced by parameters such as tmp_table_size and max_heap_table_size. Large temporary tables exceeding the configured limit may be stored on disk instead of residing entirely within the PGA.
- Memory Deallocation: When a connection is terminated or a query completes, InnoDB releases the memory allocated within the PGA for that connection. The Memory Allocator component tracks allocated memory blocks and ensures their proper deallocation to prevent memory leaks and fragmentation. InnoDB also implements adaptive memory management techniques, such as flushing unused buffers and releasing memory when the system is under memory pressure.
InnoDB’s optimization of PGA management ensures efficient memory utilization and enhances the performance of query processing within the MySQL server.