C++ design: How to cache most recent used

Question

We have a C++ application for which we try to improve performance. We identified that data retrieval takes a lot of time, and want to cache data. We can\'t store all data in mem

Answer 1

I agree with Neil, scanning 1000 elements takes no time at all.

But if you want to do it anyway, you could just use the additional list you propose and, in order to avoid scanning the whole list each time, instead of storing just the CacheEntry in your map, you could store the CacheEntry and a pointer to the element of your list that corresponds to this entry.

Answer 2

An alternative implementation that might make the 'aging' of the elements easier but at the cost of lower search performance would be to keep your CacheEntry elements in a std::list (or use a std::pair<long, CacheEntry>. The newest element gets added at the front of the list so they 'migrate' towards the end of the list as they age. When you check if an element is already present in the cache, you scan the list (which is admittedly an O(n) operation as opposed to being an O(log n) operation in a map). If you find it, you remove it from its current location and re-insert it at the start of the list. If the list length extends over 1000 elements, you remove the required number of elements from the end of the list to trim it back below 1000 elements.

Answer 3

Create a std:priority_queue<map<int, CacheEntry>::iterator>, with a comparer for the access stamp.. For an insert, first pop the last item off the queue, and erase it from the map. Than insert the new item into the map, and finally push it's iterator onto the queue.

Answer 4

I believe this is a good candidate for treaps. The priority would be the time (virtual or otherwise), in ascending order (older at the root) and the long as the key.

There's also the second chance algorithm, that's good for caches. Although you lose search ability, it won't be a big impact if you only have 1000 items.

The naïve method would be to have a map associated with a priority queue, wrapped in a class. You use the map to search and the queue to remove (first remove from the queue, grabbing the item, and then remove by key from the map).