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7.4.6. The InnoDB Buffer Pool

InnoDB maintains a buffer pool for caching data and indexes in memory. InnoDB manages the pool as a list, using a least recently used (LRU) algorithm incorporating a midpoint insertion strategy. When room is needed to add a new block to the pool, InnoDB evicts the least recently used block and adds the new block to the middle of the list. The midpoint insertion strategy in effect causes the list to be treated as two sublists:

  • At the head, a sublist of “new” (or “young”) blocks that have been recently used.

  • At the tail, a sublist of “old” blocks that are less recently used.

As a result of the algorithm, the new sublist contains blocks that are heavily used by queries. The old sublist contains less-used blocks, and candidates for eviction are taken from this sublist.

The LRU algorithm operates as follows by default:

  • 3/8 of the buffer pool is devoted to the old sublist.

  • The midpoint of the list is the boundary where the tail of the new sublist meets the head of the old sublist.

  • When InnoDB reads a block into the buffer pool, it initially inserts it at the midpoint (the head of the old sublist). A block can be read in as a result of two types of read requests: Because it is required (for example, to satisfy query execution), or as part of read-ahead performed in anticipation that it will be required.

  • The first access to a block in the old sublist makes it “young”, causing it to move to the head of the buffer pool (the head of the new sublist). If the block was read in because it was required, the first access occurs immediately and the block is made young. If the block was read in due to read-ahead, the first access does not occur immediately (and might not occur at all before the block is evicted).

  • As long as no accesses occur for a block in the pool, it “ages” by moving toward the tail of the list. Blocks in both the new and old sublists age as other blocks are made new. Blocks in the old sublist also age as blocks are inserted at the midpoint. Eventually, a block that remains unused for long enough reaches the tail of the old sublist and is evicted.

In the default operation of the buffer pool, a block when read in is loaded at the midpoint and then moved immediately to the head of the new sublist as soon as an access occurs. In the case of a table scan (such as performed for a mysqldump operation), each block read by the scan ends up moving to the head of the new sublist because multiple rows are accessed from each block. This occurs even for a one-time scan, where the blocks are not otherwise used by other queries. Blocks may also be loaded by the read-ahead background thread and then moved to the head of the new sublist by a single access. These effects can be disadvantageous because they push blocks that are in heavy use by other queries out of the new sublist to the old sublist where they become subject to eviction.

InnoDB has several system variables that control the size of the buffer pool or enable LRU algorithm tuning:

  • innodb_buffer_pool_size

    Specifies the size of the buffer pool. If your buffer pool is small and you have sufficient memory, making the pool larger can improve performance by reducing the amount of disk I/O needed as queries access InnoDB tables.

  • innodb_old_blocks_pct

    Specifies the approximate percentage of the buffer pool that InnoDB uses for the old block sublist. The range of values is 5 to 95. The default value is 37 (that is, 3/8 of the pool).

  • innodb_old_blocks_time

    Specifies how long in milliseconds (ms) a block inserted into the old sublist must stay there after its first access before it can be moved to the new sublist. The default value is 0: A block inserted into the old sublist moves immediately to the new sublist the first time it is accessed, no matter how soon after insertion the access occurs. If the value is greater than 0, blocks remain in the old sublist until an access occurs at least that many ms after the first access. For example, a value of 1000 causes blocks to stay in the old sublist for 1 second after the first access before they become eligible to move to the new sublist.

innodb_old_blocks_pct and innodb_old_blocks_time are available as of MySQL 5.1.41, but only for InnoDB Plugin, not the built-in version of InnoDB.

By setting innodb_old_blocks_time greater than 0, you can prevent one-time table scans from flooding the new sublist with blocks used only for the scan. Rows in a block read in for a scan are accessed rapidly many times in succession, but the block is unused after that. If innodb_old_blocks_time is set to a value greater than the block scan time, the block is not moved to the new sublist during the table scan. Instead, it remains in the old sublist and ages to the tail of the list to be evicted quickly. This way, blocks used only for a one-time scan do not act to the detriment of heavily used blocks in the new sublist.

innodb_old_blocks_time can be set at runtime, so you can change it temporarily while performing operations such as table scans and dumps to prevent them from flooding the new sublist: 

SET GLOBAL innodb_old_blocks_time = 1000;
... perform queries that scan tables ...
SET GLOBAL innodb_old_blocks_time = 0;

This strategy does not apply if your intent is to fill the buffer pool with a table's content. For example, you might perform a table or index scan at server startup or during benchmarking or testing specifically to “warm up” the buffer pool. In this case, leaving innodb_old_blocks_time set to 0 accomplishes the goal of loading the scanned blocks into the new sublist.

The output from the InnoDB Standard Monitor contains several new fields in the BUFFER POOL AND MEMORY section that pertain to operation of the buffer pool LRU algorithm:

  • Old database pages: The number of pages in the old sublist of the buffer pool.

  • Pages made young, not young: The number of old pages that were moved to the head of the buffer pool (the new sublist), and the number of pages that have remained in the old sublist without being made new.

  • youngs/s non-youngs/s: The number of accesses to old pages that have resulted in making them young or not. This metric differs from that of the previous item in two ways. First, it relates only to old pages. Second, it is based on number of accesses to pages and not the number of pages. (There can be multiple accesses to a given page, all of which are counted.)

  • young-making rate: Hits that cause blocks to move to the head of the buffer pool.

  • not: Hits that do not cause blocks to move to the head of the buffer pool (due to the delay not being met).

The young-making rate and not rate will not normally add up to the overall buffer pool hit rate. Hits for blocks in the old sublist cause them to move to the new sublist, but hits to blocks in the new sublist cause them to move to the head of the list only if they are a certain distance from the head.

The preceding information from the Monitor can help you make LRU tuning decisions:

  • If you see very low youngs/s values when you do not have large scans going on, that indicates that you might need to either reduce the delay time, or increase the percentage of the buffer pool used for the old sublist. Increasing the percentage makes the old sublist larger, so blocks in that sublist take longer to move to the tail and be evicted. This increases the likelihood that they will be accessed again and be made young.

  • If you do not see a lot of non-youngs/s when you are doing large table scans (and lots of youngs/s), you will want to tune your delay value to be larger.

For more information about InnoDB Monitors, see Section 13.6.13.2, “SHOW ENGINE INNODB STATUS and the InnoDB Monitors”.

The MyISAM storage engine also uses an LRU algorithm, to manage its key cache. See Section 7.4.5, “The MyISAM Key Cache”.