Berkeley DB Reference Guide: Access Methods

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 application private memory (DB_PRIVATE), or, in cases where this behavior is configurable, to turn off the operating system's flushing of memory-mapped pages.

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 method) 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.

A common question when tuning Berkeley DB applications is scalability. For example, people will ask why, when adding additional threads or processes to an application, the overall database throughput decreases, even when all of the operations are read-only queries.

First, while read-only operations are logically concurrent, they still have to acquire mutexes on internal Berkeley DB data structures. For example, when searching a linked list and looking for a database page, the linked list has to be locked against other threads of control attempting to add or remove pages from the linked list. The more threads of control you add, the more contention there will be for those shared data structure resources.

Second, once contention starts happening, applications will also start to see threads of control convoy behind locks (especially on architectures supporting only test-and-set spin mutexes, rather than blocking mutexes). On test-and-set architectures, threads of control waiting for locks must attempt to acquire the mutex, sleep, check the mutex again, and so on. Each failed check of the mutex and subsequent sleep wastes CPU and decreases the overall throughput of the system.

Third, every time a thread acquires a shared mutex, it has to shoot down other references to that memory in every other CPU on the system. Many modern snoopy cache architectures have slow shoot down characteristics.

Fourth, schedulers don't care what application-specific mutexes a thread of control might hold when de-schedule a thread. If a thread of control is descheduled while holding a shared data structure mutex, other threads of control will be blocked until the scheduler decides to run the blocking thread of control again. The more threads of control that are running, the smaller their quanta of CPU time, and the more likely they will be descheduled while holding a Berkeley DB mutex.

The results of adding new threads of control to an application, on the application's throughput, is application and hardware specific and almost entirely dependent on the application's data access pattern and hardware. In general, using operating systems that support blocking mutexes will often make a tremendous difference, and limiting threads of control to to some small multiple of the number of CPUs is usually the right choice to make.

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