Question: I have a data reduction issue that is proving to be very difficult to solve. Essentially, I have a program that calculates incremental values (floating point) for pairs of keys from a set of about 60 million keys total. The program will generate values for about 53 trillion pairs 'relatively' quickly (simply iterating through the values would take about three days 🤣). Not every pair of keys will occur, and many pairs will come up many times. There is no reasonable way to have the pairs come up in a particular order. What I need is a way to find the sum of the values generated for each pair of keys. For data that would fit in memory, this is a very simple problem. In python it would look something like: from collections import Counter res = Counter() for key1,key2,val in data_generator(): res[(key1,key2)] += val  The problem, of course, is that a mapping like that won't fit in memory. So I'm looking for a way to do this efficiently with a mix of on-disk and in-memory processing. So far I've tried:  A postgresql table with upserts (ON CONFLICT UPDATE). This turned out to be far, far to slow. A hybrid of in-memory dictionaries in python that write to a RocksDB or LMDB key value store when they get too big. Though these DBs are much faster than postgresql for this kind of task, the time to complete is still on the order of months.  At this point, I'm hoping someone has a better approach that I could try. Is there a way to break this problem up into smaller parts? Is there a standard MapReduce approach to this kind of problem? Any tips or pointers would be greatly appreciated. Thanks! Edit: The computer I'm using has 64GB of RAM, 96 cores (most of my work is very parallelizable), and terabytes of HDD (and some SSD) storage. It's hard to estimate the total number of key pairs that will be in the reduced result, but it will certainly be at least in the hundreds of billions.

Answer: As Frank Yellin observes, there's a one-round MapReduce algorithm. The mapper produces key-value pairs with key key1,key2 and value val. The MapReduce framework groups these pairs by key (the shuffle). The reducer sums the values. In order to control the memory usage, MapReduce writes the intermediate data to disk. Traditionally there are n files, and all of the pairs with key key1,key2 go to file hash((key1,key2)) mod n. There is a tension here: n should be large enough that each file can be handled by an in-memory map, but if n is too large, then the file system falls over. Back of the envelope math suggests that n might be between 1e4 and 1e5 for you. Hopefully the OS will use RAM to buffer the file writes for you, but make sure that you're maxing out your disk throughput or else you may have to implement buffering yourself. (There also might be a suitable framework, but you don't have to write much code for a single machine.) I agree with user3386109 that you're going to need a Really Big Disk. If you can regenerate the input multiple times, you can trade time for space by making k passes that each save only a 1/k fraction of the files. I'm concerned that the running time of this MapReduce will be too large relative to the mean time between failures. MapReduce is traditionally distributed for fault tolerance as much as parallelism. If there's anything you can tell us about how the input arises, and what you're planning to do with the output, we might be able to give you better advice.