Operating System Support for Virtual Machines

1. 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: cklee@cse.psu.edu) Jan 23, 2006

2. Outline • Introduction • Review of Virtual Machines • UMLinux – an evaluated Type II VMMs • Host OS Support for Type II VMMs • Performance Results • Conclusions

3. 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.

4. Introduction • Two types of VMMs • Type I • Type II

5. 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.

6. 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.

7. 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,

8. 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.

9. 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

10. 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.

11. 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.

12. 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

13. 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.

14. 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.

15. 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.

16. 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.

17. Performance Results • Results • The effect are cumulated among all proposed schemes. Significant improvement by reducing context switches Standalone must be the best

18. Performance Results • Results (Cont’d) Significant improvement by reducing context switches

19. Performance Results • Results CPU intensive tasks

20. Performance Results • Conclusion from the result: • The improvement made according • to the proposal is shown • effective. • Results (Cont’d)

21. 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.