What does the passivation flag do?
Passivation is a mode of storing entries in the cache store only when they are evicted from memory. The benefit of this approach is to prevent a lot of expensive writes to the cache store if an entry is hot (frequently used) and hence not evicted from memory. The reverse process, known as activation, occurs when a thread attempts to access an entry which is not in memory but is in the store (i.e., a passivated entry). Activation involves loading the entry into memory, and then removing it from the cache store. With passivation enabled, the cache uses the cache store as an overflow tank, akin to swapping memory pages to disk in virtual memory implementations in operating systems. If passivation is disabled, the cache store behaves as a write-through (or write-behind if asynchronous) cache, where all entries in memory are also maintained in the cache store. The effect of this is that the cache store will always contain a superset of what is in memory.
