Operating Systems Questions

(OS)



(Spring 2012 - Joseph & Culler):


Time and Factors in Distributed Systems

A key factor that differentiates distributed systems from systems
running on a single computer is the lack of global time.  Each of
the components in a distributed system typically maintains its local
clock and timestamps of various sorts are exchanged in messages. 
An obvious example is the Network Time Protocol (NTP). Unfortunately,
the communication of clocks uses essentially the same basic mechanisms,
and suffers the same vagaries, as all other information transfer within 
the distributed system, and this places certain fundamental limits on
distributed system operation.


1.	Briefly outline how clocks and/or timestamps are used at various
	levels of distributed systems, such as the transport layer, the
	filesystem, or applications (e.g., web applications).

2.	There has been quite a bit of progress in reducing the end-to-end
	latency of communication networks and increasing the transfer
	bandwidth. What impact does this have on the uses that you have
	outlined?

3.	What additional benefits would derive from improving the
	predictability of communication behavior, say by improving the
	worst-case (as opposed to the best-case) latency?  Why is this
	kind of guarantee hard?

4.  	What measures can be taking in the distributed algorithm, i.e.,
	the protocol to overcome faults, foibles, and limitations in
	currently deployed systems and networks?

5.	Overall, would you prefer improvements in maximum performance or
	predictability?

Hint: Sometimes formalizing performance characteristics can simplify
discussion of system design trade-offs.


Disconnected Operation in File Systems

In this question, we.ll explore the challenges of associated with
intermittent connectivity in the Coda client-server distributed file system.


1.	Explain the advantages and design choices for Coda versus using
	a version control system, such as SVN or git

2.	Briefly outline how Coda uses caching to improve performance
	and scalability.

3.	Suppose multiple disconnected Coda clients update the same file.
	Explain what Coda does when the clients reconnect with the server.

4.	Describe how you would change Coda to support a Dropbox or box.net
	model of cloud-based DFS


Scheduling, Interupt Handling, and Scaling in Multicore Systems

In this problem we will explore system design trade-offs and the
interplay of OS and architecture in the specific context of multicore systems.

1.	In a Multicore system, what measures can be taken in the operating
	system to maximize the effectiveness of caches? When do such measures work particularly well?
	And when do they introduce problems?  What measures can be taken to overcome these problems? 
	Are there any specific solutions that you are familiar with?

2.	If the thread scheduler is protected by a lock, what is the maximum
	rate that threads can be scheduled onto cores?  What can be done to
	improve the scalability of the scheduler?  Are there any specific
	approaches that you can name?

3.	When an interrupt occurs, which core services the interrupt?
	What measures can be taken to improve the scalability of interrupt
	processing?  Of these, which require hardware support and which
	can be done entirely in software? 

4.	Network I/O tends to be the most demanding of interrupt servicing?
	In particular, we want to be able to scale the TCP/IP bandwidth as we
	increase the processing capabilities.  Besides just a faster link
	rate (1 gig and 10 gig ethernet will stay for a while) what measures
	can be taken to scale up the transmit bandwidth?  What additional
	measures are required to scale up the receive bandwidth?


(Spring 2005 - Brewer & Culler):
"The exam contained three questions centered on topics of error handling,
driver isolation and migration.  However, each question provided a chance
to demonstrate broad knowledge of aspects of operating systems."

(Fall 2004 - Smith & Wagner):
"The exam consisted of 3 questions.  The first concerned the interface 
between the operating system and the computer architecture. The second 
involved distributed system design, tradeoffs and optimizations.  The 
third concerned virtual machine design and operation."

(Fall 2003 - Brewer & Joseph):
"The exam contained three questions centered on 
(1) disk read/write caching and filesystem interactions,
(2) segments and paging issues, and 
(3) issues in migrating a centralized server to a decentralized, fault-tolerant environment.  

In each case the question started from basic knowledge and extended toward open-ended design."

(Fall 2002 - Katz & Joseph):
"The exam consisted of the following topics: 
1) How do you build a transaction for disk writes? (only single page 
   writes are atomic)
2) How do you design a system for airplane collision prevention? (distributed 
   systems problem with some reliability challenges)
3) How would you build a new streaming protocol? (application- and OS-level networking)  

(Fall 2001 - Smith & Wagner):
"The exam consisted of questions regarding: memory management, fault
tolerance, virsuses, and network-attached storage."

(Fall 2000 - Smith & Brewer):
"The exam consisted of three questions regarding: (1) paging,
(2) virtual machines, and (3) RPC."

(Spring 2000 - Katz & Joseph):
"The exam consisted of three questions centered on (1) reliability,
availability, and consistency, (2) time management, ordering,
synchronization, and communication in distributed systems, and (3)
redirection in operating systems and networking.  In each case, the
question started from basic knowledge and extended towards open-ended
design."

(Fall 1999 - Culler & Brewer):
"Q1: Microkernel architectures were traditionally designed with
distributed computation and high-performance servers in mind.  These
days, however, microkernels are almost never used in these situations.
Microkernels are making a comeback in the development of small devices.
Describe the properties of microkernels that might contribute to these
phenomenon.

Q2: Logs, what are they good for?  And transaction?  (Fishing for: ACID
applications, batched writes, auditing, etc.)

Q3: Suppose you were interested in accessing network services (e.g.,
Amazon.com, information about a room you just entered, etc.) while
protecting your anonymity.  How would you do this?  How would you
support persistent sessions?  How would you prevent different services
from correlating your actions to deduce additional information about
you?"

(Spring 1999 - Brewer & Joseph):
"The exam consisted of three questions centered on (1) the impact of
technology trends on the design of distributed computing systems, (2)
the benefits and drawbacks to microkernels, and (3) the construction of
an RPC protocol that ensures exactly once execution.  In each case, the
question started from basic knowledge and extended towards open-ended
design."

(Fall 1998 - Culler & Joseph):
"The exam consisted of three questions centered on (1) the impact of
technology trends on operating system design, (2) time management,
ordering, synchronization, and communication in distributed systems, and
(3) security in financial systems with smart user devices.  In each
case, the question started from basic knowledge and extended towards
open-ended design."


August 2000