Constant MALLOC_CONF_PERCPU_ARENA 

const MALLOC_CONF_PERCPU_ARENA: &str;

Assigns arenas by CPU where jemalloc can tell which CPU a thread runs on.

Jemalloc needs sched_getcpu, or its own Apple and Windows paths. Without them a release build falls back to its default arena count and prints a notice, and a debug build aborts during allocator initialization.