|
Advanced Topics in Computer Systems |
9/5/01 |
|
Anthony Joseph & Joe Hellerstein |
|
Entrance exams back at break. Basic standard was 65 or
above. Between 50 and 64 you have to come in for an interview. Below 50, please
review undergraduate OS material again.
Office hours today: 10:30 to 12:00 for interviews
I’m away at a PC meeting for the rest of the week.
Office hours next week: TBA
OS History
Material
should mostly be a review.
The
UNIX Time-Sharing System – Classic system and paper: described almost entirely
in 10 pages. Key idea: elegant combination of a few concepts that fit together
well.
Multics – The First Seven Years – More of a classic retrospective paper (lots of lessons learned). Key idea: well-thought out grand set of goals that were mostly successfully delivered.
What is an Operating System?
· Traffic Cop
· Coordinator
· Provides standardized services
· Fairness – central point of control
· Implement once
· Everyday users: office applications
· CS researchers
· Programmers: edit, compile, debug cycle
· Scientists
· Process control
· Back room business servers
· Top-down (Multics)
o Clear goals drive design and primitives
· Bottom-up (UNIX)
o Primitives define functionality
o No predefined goals yields general purpose solution
o Most important job of UNIX is “to provide a filesystem”
· Multics started 1965 (ran to 10/00)
o First relational DB 6/76
· UNIX started 1969 –
o Born when Multics was being deployed
· Multics: classic text book style
o MIT Project MAC (now LCS)
§ Design is key strength
o Bell Telephone Labs (now Lucent Labs)
§ Online update capabilities, robustness, scalability
o GE/Honeywell
§ Defense contractor
· Unix: two people
o Ken Thompson, Dennis Ritchie (Bell Labs)
o Dissatisfied with available facilities (“stealth project”)
· Multics
o Fall 1964 to spring 1967
o 3K page manual BEFORE code (PL/1)
o Lessons learned: devil is in the details
§ Abstraction choices
§ Complex algorithm for simple functions
§ Overly complex (and limiting) specifications
o Need to evolve: design, build, evaluate, design, build, evaluate…
o Ready for system programmers in late 1968
o Users 1969 (five years later!)
o $6M/year for five years (same scale as mainframe OS development of once per decade)
o 150 person-years plus 50 person-years of improvements
· Unix
o Two person years
§ Built as ASM program
§ Some second system effect
§ Build/design mixed at same time (goal is low level: good primitives)
· Easy access as norm
· Reliable
· Dynamic, online growth of resources
· Reliable filesystem
· Security and secure selective information sharing
· Hierarchical information storage and administration
· Fair share to large and small
· Support for different programming environments and user interfaces
· Capability to evolve with technology and legacy support
· Efficient resource utilization
· Alternatives?
o Operator console
o Batch
· Online, time sharing as primary mode of access
o Primary UNIX goal
· 24 x 7 x 365 operation
o Must be self-maintaining (Unix primary goal)
o Users: universities
· Why?
o Multics users: military, governmental agencies, NSA, Ford, process controllers, …
· Reality
o Multics: 1 – 2 crashes/day early on
o Unix: Crash every other day early on
· Very complex
· Why provide?
o System growth!
o Multics:
§ 5 – 10 new modules/day (75% done online)
§ Even applications can upgrade online
§ Phone switches can do this, but not routers!
o Enables rapid prototyping
§ Can change HW/SW
§ Multics: 5 – 10 times per day without effect
§ Don’t try this with Windows XP
· Why?
o Multics: Replace punch cards as backup
o Unix: Compare treatment of metadata with user data
§ Logical consistency, synchronous for metadata only
· Multics:
o Protection rings: Appear in later Intel CPUs, not Unix
o Windows NT/2K has rings
o Achieved B2 in 8/85
· Unix:
o All or nothing: super user model
· Why have multiple levels?
o Do you trust the operator?
o How to handle backups?
· CPU
o Multics: multiprocessor support from start
· Memory/disk
o Multics:
§ Transparent multi-level memory model
§ True memory-mapped world view
o Unix:
§ Simple 3 segment view
§ Partial memory mapped model later
· Filesystem
o Multics/Unix: Directories instead of flat files in user volumes (e.g., VM/370)
§ Why?
o Unix: Directories as special files (like indexes as relations)
· Multics/Unix: Lots of languages
· Multics: Virtual environments (e.g., BASIC for class)
· Unix: Shell environments
o Same as any program
o Enables trivial recursion: read, evaluate (exec)
o But, how to bootstrap?
§ init process creates login process (“shell”) per channel
· 35 years for Multics
o From drum memory to VLSI
· 31+ years for Unix (primary goal is evolution)
· Unix:
o Easy to run any program (primary goal)
o Frugal: 42 Kbytes (primary goal)
o Old, unused PDP-7
· Multics:
o Cadillac: 1 MWord = 4 MBytes
o Custom hardware
§ Paging device: 4 MWords (36 bit) = 16 MBytes in 1964!
· Compare with 1 MByte of RAM = $1 million
· Multics:
o “Failed” because it was a computer utility
§ Ahead of its time – no Internet for access
o Follow on to Compatible Time Sharing System (CTSS)
§ Too complicated, too costly hardware
§ “Second System Effect” term coined by Fred Brooks
· System one was successful (CTSS)
· System two aims to correct all flaws from #1
· Almost always fails: too bloated, slow, etc.
· Unix:
o Low cost hardware
o University adoption
o Simple, elegant, but slow early on
§ Example: file system