Was it perhaps a multi-threaded task? Because that would almost definitely crawl. In general, unmapping expensive, much more expensive than mapping memory, bec

Was it perhaps a multi-threaded task? Because that would almost definitely crawl. In general, unmapping expensive, much more expensive than mapping memory, because you need to do a TLB-shootdown/flush/whatever to make sure a cached version of the old mapping is not used. A read/write does a copy, so no need to mess with mappings and TLBs, hence it can scale very well.