The problem of concurrent memory allocation is to find the right balance between temporal and spatial performance and scalability across a large range of workloads. Our contributions to address this problem are: uniform treatment of small and big objects through the idea of virtual spans, efficiently and effectively reclaiming unused memory through fast and scalable global data structures, and constant-time (modulo synchronization) allocation and deallocation operations that trade off memory reuse and spatial locality without being subject to false sharing. We have implemented an allocator based on these ideas that generally performs and scales in our experiments better than other allocators while using less memory and is still competitive otherwise.