Hi, I'm wondering about ways to measure the optimum size for a cache, in terms of the "value" you gain from each GB of cache space. If you've got a 400GB cache and only 99% of your hits come from the first 350GB, there's probably no point looking for a larger cache. If only 80% come from the first 350GB, then a bigger cache might well be useful. I realise there are rules of thumb for cache size, it would be interesting to be able to analyse a particular squid installation. Squid obviously removes objects from its cache based on the chosen cache_replacement_policy. It appears from the comments in squid.conf that in the case of the LRU policy, this is implemented as a list, presumably a queue of pointers to objects in the cache. Objects which come to the head of the queue are presumably next for removal. I guess if an object in the cache gets used it goes back to the tail of the queue. I suppose this process must involve linearly traversing the queue to find the object and remove it, which is presumably why heap-based policies are available. I wonder if it would be feasible to calculate a "cache rank", which indicates the position an object was within the queue at the time of the hit. So, perhaps 0 means at the tail of the queue, 1 means at the head. If this could be reported alongside each hit in the access.log, one could draw stats on the amount of hits served by each portion of the queue and therefore determine the value of expanding or contracting your cache. In the case of simple LRU, if the queue must be traversed to find each element and requeue it (perhaps this isn't the case?), I suppose one could count the position in the queue and divide by the total length. With a heap, things are more complex. I guess you could give an indication of the depth in the heap but there would be so many objects on the lowest levels, I don't suppose this would be a great guide. Is there some better value available, such as the key used in the heap maybe? Or perhaps the whole idea is flawed somehow? Comments, criticisms, explanations, rebukes all welcome. Gavin
