Making Buffer Cache Scan Resistant

Rather than using a strictly LRU algorithm, InnoDB uses a technique to minimize the amount of data that is brought into the buffer cache and never accessed again. The goal is to make sure that frequently accessed (“hot”) pages remain in the buffer cache, even as read-ahead and full table scans bring new blocks in that might or might not be accessed afterward.

Newly read blocks are inserted into the middle of the list representing the buffer cache. of the LRU list. All newly read pages are inserted at a location that by default is 3/8 from the tail of the LRU list. The pages are moved to the front of the list (the most-recently used end) when they are accessed in the buffer cache for the first time. Thus pages that are never accessed never make it to the front portion of the LRU list, and “age out” sooner than with a strict LRU approach. This arrangement divides the LRU list into two segments, where the pages downstream of the insertion point are considered “old” and are desirable victims for LRU eviction.

For an explanation of the inner workings of the InnoDB buffer pool and the specifics of its LRU replacement algorithm, see Section 7.9.1, “The InnoDB Buffer Pool”.

Starting with InnoDB 1.0.5, you can control the insertion point in the LRU list, and choose whether InnoDB applies the same optimization to blocks brought into the buffer pool by table or index scans.

The configuration parameter innodb_old_blocks_pct controls the percentage of “old” blocks in the LRU list. The default value of innodb_old_blocks_pct is 37, corresponding to the original fixed ratio of 3/8. The value range is 5 (new pages in the buffer pool age out very quickly) to 95 (only 5% of the buffer pool reserved for hot pages, making the algorithm close to the familiar LRU strategy).

The optimization that keeps the buffer cache from being churned by read-ahead can avoid similar problems due to table or index scans. In these scans, a data page is typically accessed a few times in quick succession and is never touched again. The configuration parameter innodb_old_blocks_time specifies the time window (in milliseconds) after the first access to a page during which it can be accessed without being moved to the front (most-recently used end) of the LRU list. The default value of innodb_old_blocks_time is 0, corresponding to the original behavior of moving a page to the most-recently used end of the buffer pool list when it is first accessed in the buffer pool. Increasing this value makes more and more blocks likely to age out faster from the buffer pool.

Both the new parameters innodb_old_blocks_pct and innodb_old_blocks_time are dynamic, global and can be specified in the MySQL option file (my.cnf or my.ini) or changed at runtime with the SET GLOBAL command. Changing the setting requires the SUPER privilege.

To help you gauge the effect of setting these parameters, some additional statistics are reported by SHOW ENGINE INNODB STATUS command. The BUFFER POOL AND MEMORY section now looks like:

Total memory allocated 1107296256; in additional pool allocated 0
Dictionary memory allocated 80360
Buffer pool size   65535
Free buffers       0
Database pages     63920
Old database pages 23600
Modified db pages  34969
Pending reads 32
Pending writes: LRU 0, flush list 0, single page 0
Pages made young 414946, not young 2930673
1274.75 youngs/s, 16521.90 non-youngs/s
Pages read 486005, created 3178, written 160585
2132.37 reads/s, 3.40 creates/s, 323.74 writes/s
Buffer pool hit rate 950 / 1000, young-making rate 30 / 1000 not 392 / 1000
Pages read ahead 1510.10/s, evicted without access 0.00/s
LRU len: 63920, unzip_LRU len: 0
I/O sum[43690]:cur[221], unzip sum[0]:cur[0]
  • Old database pages is the number of pages in the “old” segment of the LRU list.

  • Pages made young and not young is the total number of “old” pages that have been made young or not respectively.

  • youngs/s and non-young/s is the rate at which page accesses to the “old” pages have resulted in making such pages young or otherwise respectively since the last invocation of the command.

  • young-making rate and not provides the same rate but in terms of overall buffer cache accesses instead of accesses just to the “old” pages.

Because the effects of these parameters can vary widely based on your hardware configuration, your data, and the details of your workload, always benchmark to verify the effectiveness before changing these settings in any performance-critical or production environment.

In mixed workloads where most of the activity is OLTP type with periodic batch reporting queries which result in large scans, setting the value of innodb_old_blocks_time during the batch runs can help keep the working set of the normal workload in the buffer cache.

When scanning large tables that cannot fit entirely in the buffer pool, setting innodb_old_blocks_pct to a small value keeps the data that is only read once from consuming a significant portion of the buffer pool. For example, setting innodb_old_blocks_pct=5 restricts this data that is only read once to 5% of the buffer pool.

When scanning small tables that do fit into memory, there is less overhead for moving pages around within the buffer pool, so you can leave innodb_old_blocks_pct at its default value, or even higher, such as innodb_old_blocks_pct=50.

The effect of the innodb_old_blocks_time parameter is harder to predict than the innodb_old_blocks_pct parameter, is relatively small, and varies more with the workload. To arrive at an optimal value, conduct your own benchmarks if the performance improvement from adjusting innodb_old_blocks_pct is not sufficient.

For more information about the InnoDB buffer pool, see Section 7.9.1, “The InnoDB Buffer Pool”.

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