Version 2.9.1
Per-process shared caches

WiredTiger supports sharing a single cache among multiple databases within a process.

An application configures a shared cache by specifying a shared_cache name to the wiredtiger_open function. Applications can optionally set a minimum amount of cache any connection in the pool will be assigned and the granularity at which the cache pool is redistributed among connections - called the chunk size.

The shared cache implementation assigns a certain amount of cache to each participating database. Each database manages its allocated cache as it would when not using a shared cache - thus databases using a shared cache can have different eviction policies. There is a thread that monitors the cache usage of each database and redistributes the cache among participants according to where it is most likely to improve performance. The cache is redistributed in chunks which are of a configurable size. Once a database has had a chunk of cache added or removed it will be given time to start effectively using that cache before it is considered for further adjustment. If a small chunk size is configured it will take longer for the shared cache to adjust to changes in participants. Reallocation of resources happens periodically and whenever a database joins the shared cache.

The reallocation of resources is determined by comparing the amount of eviction activity in a particular database to that of the other participating databases.

When a database is opened it will be allocated the amount of cache configured as the shared cache minimum, regardless of whether the cache pool is currently fully utilized. Other databases will have their assigned cache size reduced so the total cache size used will return within the bounds - there may be a period when the actual usage exceeds the configured maximum. This is especially likely if many databases join the shared cache in a short period. When a database is closed any resources it is using are distributed among the other databases.

WiredTiger shared cache tuning options can be configured when first opening a database via wiredtiger_open or changed after open using the WT_CONNECTION::reconfigure method.