What's the point of using NSPurgeabledata in NSCache?

Question

I've read some article about using suggestion of NSCache, for many it mentioned a recommendation is that to use NSPurgeabledata in an NSCache. However I just can't catch the point, while the NSCache already be able to evict its content when memory is tight or it reached its count/cost limit, why we still need to use NSPurgeabledata here? Isn't that just potentially slower than using the data object we already have? What kind of advantage can we take here?

Answer 1

The count limit and the total-cost limit are not strictly enforced. That is, when the cache goes over one of its limits, some of its objects might get evicted immediately, later, or never, all depending on the implementation details of the cache. So the advantages of using NSPurgeabledata here is :-

By using purgeable memory, you allow the system to quickly recover memory if it needs to, thereby increasing performance. Memory that is marked as purgeable is not paged to disk when it is reclaimed by the virtual memory system because paging is a time-consuming process. Instead, the data is discarded, and if needed later, it will have to be recomputed. It works like a locking mechanism or we can say that it works like synchronization. If data is accessing by one thread then no other thread can access the same one, until unless the first one get completed.