Virtualizing Resources for the Cloud - Large Scale and Big Data: Processing and Management - page 516

Virtual address space A virtual page

68 K-76 K

60 K-68 K

52 K-60 K

44 K-52 K

A physical page Physical address space

32 K-40 K

16 K-24 K

16 K-24 K

8 K-16 K

8 K-16 K

0 K-8 K

0 K-8 K

FIGURE 16.17 Mapping a process virtual address space to physical address space. This is
captured in what is referred to as a page table. Each process has its own page table.

page miss, the OS is alerted to handle the miss. Subsequently, the OS fetches the
missed page from disk storage and updates the relevant entry in the page table.

16.7.2 t wo -l evel P age m aPPing
Contrary to OSs in traditional systems, with system virtualization, the hypervisor
allocates a contiguous addressable memory space for each created VM (not process).
This memory space per a VM is usually referred to as real memory . In return, each
guest OS running in a VM allocates a contiguous addressable memory space for
each process within its real memory. This memory space per a process is denoted
as virtual memory (same name as in traditional systems). Each guest OS maps the
virtual memories of its processes to the real memory of the underlying VM, while
the hypervisor maps the real memories of its VMs to the system physical memory.
Clearly, in contrast to traditional OSs, this entails two levels of mappings between
three types of addresses, virtual, real, and physical addresses. In fact, these virtual-
to-real and real-to-physical mappings define what is known as system memory
virtualization.
Like any general-purpose OS, a guest OS in a system VM would still own its set
of page tables. In addition, the hypervisor would own another set of page tables for
mapping real to physical addresses. The page tables in the hypervisor are usually
referred to as real map tables . Figure 16.18 demonstrates system memory virtualiza-
tion in a native system VM. It shows page tables maintained by guest VMs and real
map tables maintained by the hypervisor. Each entry in a page table maps a virtual
page of a program to a real page in the respective VM. Likewise, each entry in a real
map table maps a real page in a VM to a physical page in the physical memory. When
a guest OS attempts to establish a valid mapping entry in its page table, it traps to the
hypervisor. Subsequently, the hypervisor establishes a corresponding mapping in the
relevant VM's real map table.