[ncmncm]

Except, it's not magic. It costs processing cycles, and latency, and invalidations of cache rows you were using, and memory bus traffic. Lots of all of them.

Most of the costs are not counted as runtime for your process, so benchmarks invariably appear to show GC as costing less overhead than it does. Among the costs, as with all the myriad varieties of caching done in modern systems, is that GC makes it hard to know the costs of design choices you make. Many of the costs are in making the caches less effective.