Download Operating System Support for Virtual Machines

CSE598C Virtual Machines and Their Applications
Operating System Support
for Virtual Machines
Coauthored by Samuel T. King, George W. Dunlap and Peter M. Chen
In Proceedings of the 2003 USENIX Technical Conference
Presented by Ken C.K. Lee
(email: [email protected])
Jan 23, 2006
CSE598C Virtual Machines and Their Applications
Outline
•
•
•
•
•
•
Introduction
Review of Virtual Machines
UMLinux – an evaluated Type II VMMs
Host OS Support for Type II VMMs
Performance Results
Conclusions
CSE598C Virtual Machines and Their Applications
Introduction
• About Virtual Machine Monitor (VMM)
– A layer of software emulating hardware of a complete
computer system.
– Provide an abstraction – virtual machine (VM).
– Could provide a VM identical to underlying hardware platform
running VMM or totally different hardware platform.
• Uses of VMMs
– To create illusion of multiple machines on a single physical
machines.
– To provide software environment for OS debugging.
– To provide means of isolation that untrusted applications run
separately.
CSE598C Virtual Machines and Their Applications
Introduction
• Two types of VMMs
– Type I
– Type II
CSE598C Virtual Machines and Their Applications
Virtual Machines
• The classification of VMMs can be based on whether
the VM created by a VMM emulates the same
underlying hardware.
– VMs emulating the underlying hardware (homogeneous)
• Some performance problems due to enumeration overheads,
additional complexity in term of frequent task switches and
memory mapping.
– VMs emulating different hardware (heterogeneous)
• Various degree of compatibility:
– Denali supports only some instructions.
– Microkernel provides high-level services that are not provided by
hardware.
– Java VM is completely hardware independent.
CSE598C Virtual Machines and Their Applications
Virtual Machines
• Another classification based on Type I/II VMMs
• This paper focuses on homogeneous Type II VMMs:
– Pros:
• Run as a process that system developers/administrators can
have an easier control on it.
• As a debugging platform
– Cons:
• Undesirable performance due to lack of sufficiently powerful
interfaces provided by underlying operating systems.
• That’s work to be presented in this paper.
CSE598C Virtual Machines and Their Applications
UMLinux
• What is UMLinux?
– UMLinux is a Type II VMM , a case Type II VMM studied in
this paper
– It runs upon Linux and the guest operating systems and
guest applications run as a single process.
– Note: The interfaces provided by UMLinux is similar but not
identical to underlying hardware, so modifications on both
guest OS and VMM are needed.
– It makes use of functionality supplied by underlying OS, e.g.
•
•
•
•
•
•
process as CPU,
Host memory mapping and protection as virtual MMU
Memory files as file systems etc.
files and devices as virtual devices,
TUN/TAP devices as virtual network,
host signal as virtual interrupts,
CSE598C Virtual Machines and Their Applications
UMLinux
• UMLinux system structure
– A VMM process and a guest-machine process
– VMM process
• Redirects operating signal and system calls
• Restricts the set of system calls allowed by guest OS
• VMM uses “ptrace” to mediate access between guest
machine process and host OS.
* ptrace is a system call to observe and control another process, and examine
and change its core image and registers. It is primarily used to implement
breakpoint debugging and system call tracing.
CSE598C Virtual Machines and Their Applications
UMLinux
• UMLinux operations
– Example:
System call intercepted by VMM
process via ptrace
guest SIGUSR1 handler calls mmap
to access guest kernel data;
intercepted by VMM process
CSE598C Virtual Machines and Their Applications
Host OS support for Type II VMMs
• Three bottlenecks in running a Type II VMM
– Inordinate number of context switches between processes.
– A large number of memory protection operations.
– A large number of memory mapping operations.
– This paper proposed possible modifications to VMM and in
general, the modifications involves only a few number of lines
of code.
CSE598C Virtual Machines and Their Applications
Host OS support for Type II VMMs
• Dealing with extra host
context (process) switches
– Causes:
• Using ptrace between VMM and
host OS to intercept all requests.
– Solution:
• Moving VMM process’s
functionality into host kernel.
CSE598C Virtual Machines and Their Applications
Host OS support for Type II VMMs
• Dealing with a large number of
memory protection operations
– Causes:
• When the guest machine process
switches between guest kernel mode
to guest user mode, the access
mode of guest kernel’s portion
address space must be changed
appropriately.
• The access mode alternation is
invoked by making host system calls
– mmap, munmap and mprotect
that incur significant overhead.
Memory map of UMLinux
CSE598C Virtual Machines and Their Applications
Host OS support for Type II VMMs
• Dealing with a large number of memory
protection operations (Cont’d)
– 2 Solutions:
• Solution 1: By adjusting the bound on the
user code and data segments rather than
granting entire address space.
• Drawbacks:
– Limited use of actual and available memory
address space.
CSE598C Virtual Machines and Their Applications
Host OS support for Type II VMMs
• Dealing with a large number of memory
protection operations (Cont’d)
– 2 Solutions:
• Solution 2: By using the page table’s
supervisor-only bit to distinguish between
guest kernel mode and guest user mode.
CSE598C Virtual Machines and Their Applications
Host OS support for Type II VMMs
• Dealing with a large number of memory mapping
operations
– Causes:
• Switching of multiple guest applications.
• Changing the current memory mapping between guest virtual
pages and the page in virtual machine’s physical memory file.
• System calls – mmap and munmap are invoked.
– Solutions:
• UMLinux defers the system call til it is needed but it does not
reduce mmap and munmap calls.
• Modification of OS (i.e. switchguest) to support multiple several
address space definitions.
• The system call switchguest is pretty fast since it need to
change the pointer rather than manipulating the actual memory.
CSE598C Virtual Machines and Their Applications
Performance Results
• Objectives:
– Measuring the 3 proposals for identified bottlenecks.
• Experiment setup:
– Performance metrics:
• A null system call
• Switching between two concurrent guest application processes
(64KB each)
• (Time of) Transferring 10MB of data using TCP across 100Mb/s
Ethernet switch.
– Three benchmarks:
• POV-Ray
• Kernel-build
• SPECweb99
– AMD Athlon 1800+ CPU, 256MB Memory and Linux kernel
2.4.18 ported to UMLinux.
CSE598C Virtual Machines and Their Applications
Performance Results
• Results
– The effect are cumulated among all proposed schemes.
Significant improvement by
reducing context switches
Standalone must
be the best
CSE598C Virtual Machines and Their Applications
Performance Results
• Results (Cont’d)
Significant improvement by
reducing context switches
CSE598C Virtual Machines and Their Applications
Performance Results
• Results
CPU intensive tasks
CSE598C Virtual Machines and Their Applications
Performance Results
• Results (Cont’d)
•Conclusion from the result:
The improvement made according
to the proposal is shown
effective.
CSE598C Virtual Machines and Their Applications
Summary
• Three performance bottlenecks of Type II VMM (i.e.,
UMLinux) are identified, namely,
– A number of context switches between host processes
– A number of memory protection operations
– A number of memory operations
• Corresponding to these bottlenecks, improvements
are made in terms of structural change of VMM and
exploring alternatives to some expensive system calls.
• Performance results prove the claims of the proposal.