Comment by uecker

Virtual machines do not change memory behind your back without your permission.. The issue with jemalloc is very specific problem with a specific Linux API, i.e. MADV_FREE that has the problematic behavior, i.e. it reallocates pages when written-to but not already when accessed. When using this API, jemalloc is not conforming implementation of malloc. We can not weaken semantics of language semantics everytime someone implements something broken. Why MADV_FREE behaves like this is unclear to me, it was criticized the moment it was introduced into the kernel. But the main problem is using it for a memory allocator in C.