> In principle the downsides compared to compacting collectors is reduced cache locality, fragmentation, somewhat higher allocation overhead and higher memory f

> In principle the downsides compared to compacting collectors is reduced cache locality, fragmentation, somewhat higher allocation overhead and higher memory footprint. In actuality the downside of not copying is: - Your allocator has to be tuned to resemble first-fit in its allocation order. This is empirically enough to control memory usage. - You better have more virtual address space than physical address space. This means that the Robson bound for memory usage goes from M log M to M log P where M is memory size, P is page size, and log M or log P is the fragmentation wastage multiplier. People needed compaction really badly back when the state-of-the-art GCs ran on maxed-out 32-bit address spaces. - Mark-sweep collectors have to sweep, copying collectors don't have to. But this is a fixable problem. Sweeping can be made concurrent, parallel, incremental, or very cheap. We pick the very cheap, by using bump'n'pop. Initially we had some regressions from removing copying, but we fixed them by making sure that the first and third mitigation listed above were in place. WebKit rarely runs on a maxed-out 32-bit address space.