`total_memory_tracker.setOrRaiseHardLimit(...);` in `ConfigReloader` callback (see `Server.cpp`).
{
        Int64 addition = new_size - old_size;
        if (addition > 0)
            alloc(addition);
        else
            free(-addition);
    }
    /** This function should be called after memory deallocation.
      */
    void free(Int64 size);
    Int64 get() const