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Access method tuning
There are a few different issues to consider when tuning the performance of Berkeley DB access method applications.
- access method
- An application's choice of a database access method can significantly affect performance. Applications using fixed-length records and integer keys are likely to get better performance from the Queue access method. Applications using variable-length records are likely to get better performance from the Btree access method, as it tends to be faster for most applications than either the Hash or Recno access methods. Because the access method APIs are largely identical between the Berkeley DB access methods, it is easy for applications to benchmark the different access methods against each other. See Selecting an access method for more information.
- cache size
- The Berkeley DB database cache defaults to a fairly small size, and most applications concerned with performance will want to set it explicitly. Using a too-small cache will result in horrible performance. The first step in tuning the cache size is to use the db_stat utility (or the statistics returned by the DB->stat function) to measure the effectiveness of the cache. The goal is to maximize the cache's hit rate. Typically, increasing the size of the cache until the hit rate reaches 100% or levels off will yield the best performance. However, if your working set is sufficiently large, you will be limited by the system's available physical memory. Depending on the virtual memory and file system buffering policies of your system, and the requirements of other applications, the maximum cache size will be some amount smaller than the size of physical memory. If you find that db_stat shows that increasing the cache size improves your hit rate, but performance is not improving (or is getting worse), then it's likely you've hit other system limitations. At this point, you should review the system's swapping/paging activity and limit the size of the cache to the maximum size possible without triggering paging activity. Finally, always remember to make your measurements under conditions as close as possible to the conditions your deployed application will run under, and to test your final choices under worst-case conditions.
- shared memory
- By default, Berkeley DB creates its database environment shared regions in filesystem backed memory. Some systems do not distinguish between regular filesystem pages and memory-mapped pages backed by the filesystem, when selecting dirty pages to be flushed back to disk. For this reason, dirtying pages in the Berkeley DB cache may cause intense filesystem activity, typically when the filesystem sync thread or process is run. In some cases, this can dramatically affect application throughput. The workaround to this problem is to create the shared regions in system shared memory (DB_SYSTEM_MEM) or in application private memory (DB_PRIVATE).
- large key/data items
- Storing large key/data items in a database can alter the performance
characteristics of Btree, Hash and Recno databases. The first parameter
to consider is the database page size. When a key/data item is too
large to be placed on a database page, it is stored on "overflow" pages
that are maintained outside of the normal database structure (typically,
items that are larger than one-quarter of the page size are deemed to
be too large). Accessing these overflow pages requires at least one
additional page reference over a normal access, so it is usually better
to increase the page size than to create a database with a large number
of overflow pages. Use the db_stat utility (or the statistics
returned by the DB->stat function) to review the number of overflow
pages in the database.
The second issue is using large key/data items instead of duplicate data items. While this can offer performance gains to some applications (because it is possible to retrieve several data items in a single get call), once the key/data items are large enough to be pushed off-page, they will slow the application down. Using duplicate data items is usually the better choice in the long run.
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