TC 670 Review Session

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TC 670Review Session
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All Topics

• Processes and threads, scheduling policies
• Inter-process communication, concurrency
• Memory management
• Secondary storage management
• File Systems
• Device management
• Multimedia operating systems
• Multimedia network protocols
• Computer security
• Network security
• Wireless sensor networks

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File Systems

• The concept of a file system is simple
• Storage abstraction the implementation of the
  abstraction for secondary storage
• abstraction files
• Organizational structure logical organization of
  files into directories
• the directory hierarchy
• Access interface a set of operations that can be
  used by users to manipulate the info
• You can retrieve what you store
• Access control sharing/protection of data
  between users, processes, and machines
• access control, consistency,

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File Access Methods

• Some file systems provide different access
  methods that specify ways the application will
  access data
• sequential access
• read bytes one at a time, in order
• direct access
• random access given a block/byte
• record access
• file is array of fixed- or variable-sized records
• indexed access
• FS contains an index to a particular field of
  each record in a file
• apps can find a file based on value in that
  record (similar to DB)
• Why do we care about distinguishing sequential
  from direct access?
• what might the FS do differently in these cases?

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Device Management
keyboard
graphics
printer
CPU
mouse
tape
Memory
modem
network
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I/O Software System
User Level Software
Device Independent Software
Device Drivers
Interrupt Handlers
Hardware
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Multimedia Operating Systems

• Real-time requirements!
• A real-time process is a process which delivers
  the results of the processing in given time-span.
• The system must enforce externally-defined time
  constraints.
• Speed and efficiency are not the main
  characteristics of a real-time system.
• The playback of a video sequence is only
  acceptable when it is presented neither too
  quickly nor too slowly.
• Timing and logical dependencies among different
  related tasks, processed at the same time, also
  must be considered.
• Audio data sometimes must be synchronized with
  video data.

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Multimedia Process Scheduling

• Rate Monotonic Scheduling
• Earliest Deadline First Scheduling

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Multimedia Storage Management

• Disk scheduling
• File placement

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Admission Controls
Video files
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Multimedia network Protocols

• RTP/RTCP
• RTSP
• RSVP
• Can you describe a typical streaming process
  using these protocols?

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Computer System Security

• Basics of Cryptography
• Digital signature
• Authentication

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Computer System Security

• Attacks from inside the system
• Trojan Horses
• Trap Doors
• Login Spoofing
• Buffer Overflow
• Attacks from outside the system
• Virus

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Computer System Security

• Protection mechanisms
• Protection Domains
• Access control lists

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Network Security

• Routing Attacks
• DDoS and Traceback
• Intrusion Detection Systems
• Firewalls
• IP Security
• Web Security
• Examples
• What are the tradeoffs between different
  architectures of firewalls?
• What are the tradeoffs between different modes
  (transport/tunnel) of IP security?

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Wireless Sensor Networks

• Physical constraints and unique requirements
• Scheduling (interrupt, tasks, etc).
• Message passing
• Topology
• Routing algorithms
• Security in WSN.

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Sensor Node Hardware
Transceiver
Embedded Processor
Memory
Sensors
Battery
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TinyOS Solutions

• Support concurrency event-driven architecture
• Modularity application scheduler graph of
  components
• Compiled into one executable
• Efficiency Get done quickly and sleep
• Event function calls
• Less context switch FIFO/non-preemptable
  scheduling
• No kernel/application boundary

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TinyOS Two-level Scheduling

• Tasks do computation
• Un-preemtable FIFO scheduling
• Bounded number of pending tasks
• Events handle interrupts
• Interrupts trigger lowest-level events
• Events can signal events, call commands, or post
  tasks
• Two priorities
• Event/command
• Tasks

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Active Message

• Every message contains the name of an event
  handler
• Sender
• Declaring buffer space in a frame
• Naming a handler
• Requesting transmission exit
• Done completion signal
• Receiver
• The event handler is fired automatically in a
  target node
• No blocked or waiting threads on sender or
  receiver
• Behaves like any other events
• Single buffering

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Adaptive Topology Problem Description

• Simple Formulation (Geometric Disk Covering)
• Given a distribution of N nodes in a plane.
• Place a minimum number of disks of radius r
  (centered on the nodes) to cover them.
• Disk represents the radio connectivity (simple
  circle model).
• The problem is NP-hard.

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Routing

• Problem Given a topology, how to route data?
• Different from Internet (wired) routing.

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Midterm
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Problem 1 Processes
0xFFFFFFFF
stack (dynamic allocated mem)
3 points
i
SP
address space
heap (dynamic allocated mem)
static data (data segment)
4 points
hello
code (text segment)
ilt100
3 points
PC
0x00000000

• main()
• int i
• for(i 0 i lt 100 i) printf(hello)

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Problem 2 CPU Scheduling

• First part easy (5 points)
• Second part 2 possibilities
• New job is put in the ready queue first
• New job immediately runs

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Problem 3 Synchronization

• Solution
• Spin-locks is a low-level locking mechanism to
  synchronize processes. In spin-locks, the caller
  busy-waits or spins for a lock to be released
  (2 points). Spin-locks is not appropriate for
  uniprocessor systems, mainly because it is
  implemented in a less efficient way. One process
  will run at any time. However in a multiprocessor
  system, efficiency is less critical since often
  there are idle processors. On the other hand, the
  simplicity of spin-locks is very desirable (3
  points).
•  
• Monitor is software module that encapsulates
  thread data structure and procedures in an
  exclusive fashion. Monitor protects the data from
  unstructured access (2 points). With Hoare
  monitor, the waiter runs immediately and its
  condition is guarantee. Hence, it is efficient
  since the waiter does not have to re-check the
  condition. However, the signaler must do extra
  work to restore monitor invariant before
  signaling. With Mesa monitor, the signaler
  continues. Therefore it is possible that when the
  waiter runs again, the condition is changed again
  (so it needs to re-check the condition). Mesa
  monitor is easy for the signaler since it does
  not have to restore invariant. The signaler may
  simply run until out of the monitor session (3
  points).

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Problem 4 Deadlocks

• Solution
• Four conditions of deadlocks are (1) Mutual
  exclusion, (2) hold-and-wait, (3) no preemption,
  and (4) circular wait (5 points). In this
  example, condition (1) holds as two vehicles
  cannot take the same slot. Condition (2) holds as
  every vehicle holds a slot and is waiting for the
  slot ahead to be released. Condition (3) holds as
  no vehicle can be removed. Condition (4) hold as
  there is a circular wait (5 points). Note you may
  also argue that the circle can be broken and
  there is no deadlock.
• There are many examples of deadlocks in
  telecommunication applications. One example is
  teleconferencing you should pick a reasonable
  one (5 points).

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Problem 5 Memory hierachy

• Solution On average access to main memory takes
  110ns 0.1ns (3 points). On average access to
  disk takes 10.25ms 0.1ns (3 points). The
  expected time to get a word is 0.30.10.1
  0.5us (2 points). On average the CPU can access
  1sec/0.5us 2M words (2 points).

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Problem 6 Memory Addressing

• Please see the homework 2 (problem 1) for a
  simple draw of the addressing mechanisms (5
  points). The virtual address space has 32 bits,
  but really only 30 bits are useful. See the
  follows
•  
• For the virtual address 0x83A4C234, binary 1000
  0011 1010 0100 1100 0010 0011 0100, its segment
  is 2, its page is 0x3A4C, its offset is 0x234.
  Let us assume in the page table of segment 2,
  page 0x3A4C contains 0xDDDD. Then the physical
  address is 0xDDDD234 (5 points).
•  
• For the virtual address 0x94A4D234, binary 1001
  0100 1010 0100 1101 0010 0011 0100, it is invalid
  since two useless bits are set (5 points).
  Therefore it causes overflow. Note if you ignore
  this you may still get an answer.

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Problem 7 Replacement Algorithms

• Part one easy 5 points
• Part two new algorithms, table (5 points)
• The new algorithm is better. This example shows
  the new algorithm tries to keep the hot items
  (items 1 and 2) in the cache, and it pays off
  eventually. It does so by remembering the number
  of accessed to the items (using a count for each
  item). This algorithm is a more complicated than
  the LRU algorithm. You may also argue that the
  new algorithm may not work well for other access
  patterns. (5 points)

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Problem 8 File Access

• Solution First the root directory / is open,
  and checked for its sub-directory ssh. Then
  ssh is open, and checked for the file
  ssh_config. Finally, ssh_config is open (5
  points). There are at least 2 disk read
  operations, one to read / and one to read ssh
  (2 points). It is possible that the system needs
  more disk reads, for example if either the
  directory / or ssh is very large and needs
  several disk pages (3 points).

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Problem 9 File Organization
File pointer

• (1) There are 4 record pages and an index page.
  The index page contains many pairs of (phone,
  page). Each pair shows that the records with
  phone in a specific range are stored in that
  page. (5 points)
• (2) To search a record with key 617-353-8924, we
  first compare this with those in the index
  page. Then we know which record page potentially
  contains the record. Then we can read that record
  page to find the record. (4 points). Two disk I/O
  are required to get the record, one for the index
  and one for the record page. (1 point)
• (3) If there are 256000 records in the file,
  then a single index page will not work, instead
  we need more index pages (probably need up to
  dozens). One simple solution is to link these
  index pages together (5 points). But then if we
  have two many index pages, possibly we need to go
  through all of them to find a record! A better
  solution is to create a higher-level index page
  for the index pages. (It is ok if you do not
  answer in this way).

Index
64 records
64 records
64 records
64 records
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Problem 10 Disk Scheduling

• The Elevator algorithm is better in term of
  efficiency (it finishes all jobs in 23 units of
  time while FCFS needs 29 units. FCFS is better in
  terms of fairness in that it finishes the jobs in
  order, while with Elevator, some earlier jobs may
  wait for a longer time. (5 points)

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Format of Final

• Open book, 3 hours
• Everything is allowed
• 20 multiple-choice questions
• Choose one or two
• 2 points each
• Problem solving
• Solve 5 of 6 problems
• Total 60 points

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Topic Coverage
Multiple choices
Problem solving

• Processes and threads, scheduling policies
• Inter-process communication, concurrency
• Memory management
• Secondary storage management
• File Systems
• Device management
• Multimedia operating systems
• Multimedia network protocols
• Computer security
• Network security
• Wireless sensor networks

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Good luck!
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(No Transcript)