Database Reference
In-Depth Information
Domain U i
Domain 0
Ring 3
Ring 3
Ring 3
Ring 3
Ring 3
Process
Process
Process
Process
Process
Host OS—Ring 0
Event channels
Shared memory
NIC back-end
NIC front-end
Disk back-end
Disk front-end
Shared memory
Guest OS—Ring 1
NIC device
driver
Disk device
driver
Hypervisor—Ring 0
Hardware
FIGURE 16.24 Xen's approach to I/O virtualization assuming a system with four rings
(e.g., Intel-32). Xen co-locates the hypervisor with a host OS on the physical platform to bor-
row the host OS's device drivers and avoid coding them within the hypervisor. This makes
the hypervisor thinner and accordingly more reliable. Besides, it makes it easier on the hyper-
visor developers.
not clear what vCPU scheduler Amazon EC2 applies, but Xen's default scheduler is
the Credit Scheduler (see Section 16.6.5). It is suspected though that Amazon EC2
has a modified version of the Credit Scheduler [66].
Memory and I/O resources are virtualized in Xen in a way similar to what is
described in Sections 16.7.2 and 1.8.3, respectively. First, Xen uses a two-level page
mapping method. Second, the hardware page tables are allocated and managed by
guest OSs, with a minimal involvement from the hypervisor [9]. Specifically, guest
OSs can read directly from hardware page tables, but writes are intercepted and vali-
dated by the Xen hypervisor to ensure safety and isolation. For performance reasons,
however, guest OSs can batch write requests to amortize the overhead of passing
by the hypervisor per every write request. Finally, with Xen, existing hardware I/O
devices are not emulated as is typically done in fully virtualized environments. In
contrast, I/O requests are always transferred from user instances to domain 0 and
vice versa, using a shared memory communication paradigm as demonstrated in
Figure 16.24. At domain 0, device drivers of the host OS are borrowed to handle the
I/O requests.
16.9.2 a mazon eC2 P roPerties
Amazon EC2 is characterized with multiple effective properties, some that are
stemmed naturally from its underlying virtualization technology and others that are
employed specifically for the AWS cloud computing platform to meet certain per-
formance, scalability, flexibility, security, and reliability criteria. We next concisely
discuss some of these properties.
 
Search WWH ::




Custom Search