const page_allocator: type = if (builtin.target.isWasm())
    Allocator{
        .ptr = undefined,
        .vtable = &WasmPageAllocator.vtable,
    }
else if (builtin.target.os.tag == .plan9)
    Allocator{
        .ptr = undefined,
        .vtable = &SbrkAllocator(std.os.plan9.sbrk).vtable,
    }
else if (builtin.target.os.tag == .freestanding)
    root.os.heap.page_allocator
else
    Allocator{
        .ptr = undefined,
        .vtable = &PageAllocator.vtable,
    };

This allocator makes a syscall directly for every allocation and free. Thread-safe and lock-free.