IBM’s VM Operating System

1. IBM’s VM Operating System Keith Starling

2. Overview • Released by International Business Machines (IBM) in 1972 • Each login creates a virtual machine (guest) • Each guest can run another Operating System • Ideal for development and migration • Updated for 64-bit and Linux in 2000

3. Control Program • Responsible for device mapping • Real • Shared • Simulated • Dispatches guests to Central Processing Units (CPUs)

4. Modern VM Usage (International Business Machines, September 2004)

5. Processor Modes • Two processor modes • Supervisor • Problem • Set in the Program-Status Word, bit 15 • Determines which instructions can be run • Unprivileged • Semiprivileged • Privileged

6. Multiprocessing • Supported at multiple levels • Real multiprocessing • VM itself runs on multiple processors • Virtual multiprocessing • Each guest can have up to 64 virtual CPUs

7. Memory Model (Geiselhart et al., May 2003)

8. CPU Scheduling • Two time slices • Elapsed Time Slice • Dispatch (Minor) Time Slice • Four Classifications • Class 0 – Immediate Processor Access • Class 1 – Interactive Tasks • Class 2 – Non-Interactive Tasks • Class 3 – Resource-Intensive Tasks

9. CPU Scheduling (Cont.) • Three lists • Dormant • Eligible • Dispatch • Processes compete for CPU for their Elapsed Time Slice • Run on CPU for Dispatch Time Slice • Compete according to priority

10. CPU Scheduling (Cont.) • Determining Elapsed Time Slices • Class 1 Elapsed Time Slice starts at 1.2 sec • Increases or decreases as system runs • (50ms < t < 16 sec) • Class 0 Time Slice is 6 times Class 1 • Class 2 Time Slice is 8 times Class 1 • Class 3 Time Slice is 48 times Class 1 • Dispatch Time Slice determined at initialization

11. CPU Scheduling (Cont.) (Geiselhart et al., May 2003)

12. Summary • Robust Environment for OS development and clustering • Attachment to IBM’s z/Architecture allows hardware to perform or optimize some OS tasks • Unfortunately, attachment to mainframe hardware increases price

13. Bibliography • “IBM: About the z/VM Operating System.” URL: http://www.vm.ibm.com/overview/ • “IBM: VM History and Heritage References.” URL: http://www.vm.ibm.com/history/ • Geiselhart, G., Dupin, L., George, D., van der Heij, R., Langer, J., Norris, G., Robbins, D., Robinson, B., Sansoni, G., Thoss, S. (May 2003). “Linux on IBM eServer zSeries and S/390: Performance Measurement and Tuning, IBM Redbook.” URL: http://www.redbooks.ibm.com/redbooks/pdfs/sg246926.pdf • International Business Machines (May 2004). “z/Architecture Principles of Operation.” URL: http://publibz.boulder.ibm.com/epubs/pdf/dz9zr003.pdf • International Business Machines (September 2004). “Getting Started with Linux on zSeries: Version 5 Release 1.0.” URL: http://publibz.boulder.ibm.com/epubs/pdf/hcsx0b00.pdf • Starling Jr., Robert Keith. Personal Interview. November 19, 2005. • WRAL: Local Tech Wire (July 27, 2005). “Introducing the z9: IBM Unveils $1.2 Billion Successor to Its T-Rex Mainframe.” URL: http://www.localtechwire.com/article.cfm?u=11904