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Hints for Computer System
Butler W. Lampson
System Design: Introduction
• Designing a computer system is hard!
• Is it like designing an algorithm?
• Designers hesitate about unclear choices &
how they affect
– Flexibility
– Performance
– The freedom to make subsequent choices within the
constraints of previous ones because:
• OSs don’t partition into isolated subsystems very well
• OS subsystems may interact in unexpected ways
System Design: Introduction
• Other reasons it’s so hard:
– Moore’s law
• OSs don’t improve by a factor of 100 every decade
– Backward compatibility
– OSs are fundamentally different than standard apps.
• Extremely large programs
– UNIX: 1 Million lines of code
– Windows: 29 million lines of code
– OSs have to deal with potentially hostile users & at
same time allow users to share information
System Design: Introduction
• Even more reasons it’s so hard:
– OSs live for a very long time
• UNIX is over a quarter of a century old
• Must prepare for hardware/software changes
– OS designers do not have a good idea how their
systems will ultimately be used
• Email & web browsers came after UNIX
– OSs have to run on multiple hardware platforms
– OSs frequently have to be backward compatible
System Design: The Slogans
Functionality: The Interface
• Do one thing well (keep it simple)
– Don’t generalize
• Extensible OS designs
– Microkernels: general interface, few assumptions made about
– Exokernel: kernel provides resource protection, just exports hardware
– Get it right
– Make it fast
• Don’t hide power
– Virtualization: virtual hardware exposed is functionally identical to
the underlying hardware
– Exokernel: the high-performance of the hardware is just exported
to the user
Functionality: The Interface
• Use procedure arguments to provide flexibility in an
• Leave it to the client
– Monitors: locking & signaling mechanisms to very little, leaving all
the real work to the client
• Keep basic interfaces stable
• Keep a place to stand if you must change interfaces
(backward compatibility)
– Examples:
• Emulation library: microkernels emulate interfaces by trap redirection
• Elephant file system: Keep One
• Virtualization: VMM multiplexing at the granularity of an operating
Functionality: Implementation
• Plan to throw one away
• Keep secrets (encapsulation)
– Virtualization:
• Hide the messy stuff like NUMA
– Monitors
• Allow access only at certain entry points
• Use a good idea again
• Divide and Conquer
• Progression solved a problem in parts: cross-machine, crossdomain, better cross-domain (less kernel involvement)
Functionality: Completeness
• Separate normal and worst case
• The normal case must be fast
• The worst case must make progress
– RPC & its variations
• Cross-domain common case
• Cross-machine happens only sometimes
– Specialization
• Partial evaluation when possible
• Replugging when necessary
Speed: Interface
• Split resources
– VMM: Multiplexing resources
• Guest OSs aren’t even aware that they’re sharing
• Use static analysis
– Specialization (static): predicates known at compile
time allows for partial evaluation
• Dynamic Translation
– Specialization (dynamic): at run-time predicates that
hold for the remainder of execution or some bounded
time period (packet filter interpreter)
Speed: Implementation
• Cache Answers to expensive computations
• Use hints
– The Ethernet: exponential backoff
– Links in web pages
• When in doubt use brute force
• Compute in background
• Use batch processing
– Soft timers: uses trigger states to batch process handling events
to avoid trashing the cache more often than necessary
– RCU: memory reclamation done in batches
– Cleanup in log structured file systems: segment cleaning could be
scheduled at nighttime.
Speed: Completeness
• Shed load to control demand
• Adaptive load shedding
– Apache
• Bounded thread pool
• Safety First
– When allocating resources remember safety first
– Avoid temptation to try to obtain optimum performance
(instead try to avoid disaster)
• Progression of kernel structuring approaches for extensibility
– Microkernels, sandboxing, SPIN vs. Mach (some versions) &
• End-to-end
– We’ll see this next!
• Log updates
– Logs can be reliably written/read
– Logs can be cheaply forced out to disk, which can
survive a crash
• Log structured file systems
• RAID5 in Elephant
• Make actions atomic or restartable
– RCU: changes are seen as atomic to all processes
System Design: Conclusion
• Just remember:
– Designing an OS starts with determining what it
should do
– The interface should be simple, complete, & efficient
– There probably isn’t a “best” solution for even one part
of any system
– Just don’t choose a terrible solution
– Have a clear division of responsibility among the parts
System Design: Quotes
“Perfection is reached not when there is no
longer anything to add, but when there is no
longer anything to take away”
--A. Saint-Exupery
“The unavoidable price of reliability is simplicity.”
--C. Hoare
System Design: References
• Jon Walpole
• “Hints for Computer System Design” Lampson.
• Modern Operating Systems, Second Edition.
Tannenbaum, pp. 855-894.