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					CS 414 Review
            Goals for Today
• Review half the book
  – Make sure intuition is clear
  – Ask questions

• For more detailed information
  – Use past slides, “redo” homework and prelims

    Operating System: Definition
   An Operating System (OS) provides a virtual machine
    on top of the real hardware, whose interface is more
        convenient than the raw hardware interface.
                                   OS interface

              Operating System

                                   Physical machine interface

Easy to use, simpler to code, more reliable, more secure, …
      You can say: “I want to write XYZ into file ABC”      3
                    What is in an OS?
                           Quake                Sql Server
                      System Utils     Shells        Windowing & graphics
  OS Interface

                      Naming           Windowing & Gfx
     System           Networking       Virtual Memory        Access Control
                      Generic I/O      File System      Process Management

                      Device Drivers         Memory Management
Physical m/c Intf
                    Interrupts, Cache, Physical Memory, TLB, Hardware Devices

                           Logical OS Structure                                 4
Crossing Protection Boundaries
• User calls OS procedure for “privileged” operations
• Calling a kernel mode service from user mode program:
   – Using System Calls
   – System Calls switches execution to kernel mode

                                          User Mode
                                          Mode bit = 1       Resume process
 User process            System Call

                           Trap            Kernel Mode         Return
                        Mode bit = 0       Mode bit = 0      Mode bit = 1

                Save Caller‟s state    Execute system call   Restore state
             What is a process?

• The unit of execution
• The unit of scheduling
• Thread of execution + address space
• Is a program in execution
   – Sequential, instruction-at-a-time execution of a program.

The same as “job” or “task” or “sequential process”

 Process State Transitions

New                                                  Exit
             Ready                   Running


      Processes hop across states as a result of:
      • Actions they perform, e.g. system calls
      • Actions performed by OS, e.g. rescheduling
      • External actions, e.g. I/O
                 Context Switch
• For a running process
   – All registers are loaded in CPU and modified
      • E.g. Program Counter, Stack Pointer, General Purpose Registers
• When process relinquishes the CPU, the OS
   – Saves register values to the PCB of that process
• To execute another process, the OS
   – Loads register values from PCB of that process
 Context Switch
    Process of switching CPU from one process to another
    Very machine dependent for types of registers

        Threads and Processes
• Most operating systems therefore support two entities:
   – the process,
      • which defines the address space and general process attributes
   – the thread,
      • which defines a sequential execution stream within a process
• A thread is bound to a single process.
   – For each process, however, there may be many threads.
• Threads are the unit of scheduling
• Processes are containers in which threads execute

• Process migrates among several queues
   – Device queue, job queue, ready queue
• Scheduler selects a process to run from these queues
• Long-term scheduler:
   – load a job in memory
   – Runs infrequently
• Short-term scheduler:
   – Select ready process to run on CPU
   – Should be fast
• Middle-term scheduler
   – Reduce multiprogramming or memory consumption

     CPU Scheduling Algorithms
•   FCFS
•   LIFO
•   SJF
•   SRTF
•   Priority Scheduling
•   Round Robin
•   Multi-level Queue
•   Multi-level Feedback Queue

        CPU Scheduling Metrics
•   CPU utilization: percentage of time the CPU is not idle
•   Throughput: completed processes per time unit
•   Turnaround time: submission to completion
•   Waiting time: time spent on the ready queue
•   Response time: response latency

                 Race conditions
• Definition: timing dependent error involving shared state
   – Whether it happens depends on how threads scheduled
• Hard to detect:
   – All possible schedules have to be safe
       • Number of possible schedule permutations is huge
       • Some bad schedules? Some that will work sometimes?
   – they are intermittent
       • Timing dependent = small changes can hide bug
 The Fundamental Issue: Atomicity
• Our atomic operation is not done atomically by machine
   – E.g. incrementing a variable by one (i++) is three machine
     instructions (load, increment, store).
   – Process can be interrupted between any machine instruction
• Atomic Unit: instruction sequence guaranteed to execute
   – Also called “critical section” (CS)

 When 2 processes want to execute their Critical Section,
   – One process finishes its CS before other is allowed to enter

          Critical Section Problem
 • Problem: Design a protocol for processes to cooperate,
   such that only one process is in its critical section
     – How to make multiple instructions seem like one?

     Process 1                      CS1

     Process 2                            CS2

                                Time 

Processes progress with non-zero speed, no assumption on clock speed

                Used extensively in operating systems:
            Queues, shared variables, interrupt handlers, etc.    15
        Solution Structure
Shared vars:



Entry Section
                     Added to solve the CS problem
Critical Section

Exit Section
         Solution Requirements
• Mutual Exclusion
   – Only one process can be in the critical section at any time

• Progress
   – Decision on who enters CS cannot be indefinitely postponed
       • No deadlock

• Bounded Waiting
   – Bound on #times others can enter CS, while I am waiting
       • No livelock

• Also efficient (no extra resources), fair, simple, …
• Non-negative integer with atomic increment and decrement
• Integer „S‟ that (besides init) can only be modified by:
   – P(S) or S.wait(): decrement or block if already 0
   – V(S) or S.signal(): increment and wake up process if any
• These operations are atomic

      semaphore S;

      P(S) {                             V(S) {
        while(S ≤ 0)                       S++;
           ;                             }
               Semaphore Types
• Counting Semaphores:
    – Any integer
    – Used for synchronization
• Binary Semaphores
    – Value 0 or 1
    – Used for mutual exclusion (mutex)

                                     Process i

Shared: semaphore S                  P(S);

Init: S = 1;                         Critical Section

                                     V(S);              19
   Mutexes and Synchronization
semaphore S;
                             Init: S = 1;
P(S) {
  while(S ≤ 0)
     ;           Process i                  Process j
                 P(S);                      P(S);
V(S) {
  S++;           Code XYZ                   Code ABC
                 V(S);                      V(S);

• Hoare 1974
• Abstract Data Type for handling/defining shared resources
• Comprises:
   – Shared Private Data
      • The resource
      • Cannot be accessed from outside
   – Procedures that operate on the data
      • Gateway to the resource
      • Can only act on data local to the monitor
   – Synchronization primitives
      • Among threads that access the procedures
Synchronization Using Monitors
• Defines Condition Variables:
   – condition x;
   – Provides a mechanism to wait for events
       • Resources available, any writers
• 3 atomic operations on Condition Variables
   – x.wait(): release monitor lock, sleep until woken up
        condition variables have waiting queues too
   – x.notify(): wake one process waiting on condition (if there is one)
       • No history associated with signal
   – x.broadcast(): wake all processes waiting on condition
       • Useful for resource manager
• Condition variables are not Boolean
   – If(x) then { } does not make sense
              Types of Monitors
What happens on notify():
• Hoare: signaler immediately gives lock to waiter (theory)
   – Condition definitely holds when waiter returns
   – Easy to reason about the program
• Mesa: signaler keeps lock and processor (practice)
   – Condition might not hold when waiter returns
   – Fewer context switches, easy to support broadcast
• Brinch Hansen: signaler must immediately exit monitor
   – So, notify should be last statement of monitor procedure

Deadlock exists among a set of processes if
– Every process is waiting for an event
– This event can be caused only by another process in the set
    • Event is the acquire of release of another resource

                            One-lane bridge

   Four Conditions for Deadlock
• Coffman et. al. 1971
• Necessary conditions for deadlock to exist:
   – Mutual Exclusion
      • At least one resource must be held is in non-sharable mode
   – Hold and wait
      • There exists a process holding a resource, and waiting for another
   – No preemption
      • Resources cannot be preempted
   – Circular wait
      • There exists a set of processes {P1, P2, … PN}, such that
          – P1 is waiting for P2, P2 for P3, …. and PN for P1

    All four conditions must hold for deadlock to occur

        Dealing with Deadlocks
• Proactive Approaches:
   – Deadlock Prevention
      • Negate one of 4 necessary conditions
      • Prevent deadlock from occurring
   – Deadlock Avoidance
      • Carefully allocate resources based on future knowledge
      • Deadlocks are prevented
• Reactive Approach:
   – Deadlock detection and recovery
      • Let deadlock happen, then detect and recover from it
• Ignore the problem
   – Pretend deadlocks will never occur
   – Ostrich approach

                       Safe State
• A state is said to be safe, if it has a process sequence
             {P1, P2,…, Pn}, such that for each Pi,
  the resources that Pi can still request can be satisfied by
  the currently available resources plus the resources held
  by all Pj, where j < i

• State is safe because OS can definitely avoid deadlock
   – by blocking any new requests until safe order is executed

• This avoids circular wait condition
   – Process waits until safe state is guaranteed

              Banker‟s Algorithm
• Decides whether to grant a resource request.
• Data structures:

  n: integer              # of processes
  m: integer              # of resources
  available[1..m]         available[i] is # of avail resources of type i
  max[1..n,1..m]          max demand of each Pi for each Ri
  allocation[1..n,1..m]   current allocation of resource Rj to Pi
  need[1..n,1..m]         max # resource Rj that Pi may still request

  let request[i] be vector of # of resource Rj Process Pi wants

                    Basic Algorithm
1.   If request[i] > need[i] then
        error (asked for too much)
2. If request[i] > available[i] then
        wait (can’t supply it now)
3. Resources are available to satisfy the request
     Let’s assume that we satisfy the request. Then we would have:
         available = available - request[i]
         allocation[i] = allocation [i] + request[i]
         need[i] = need [i] - request [i]
     Now, check if this would leave us in a safe state:
         if yes, grant the request,
         if no, then leave the state as is and cause process to wait.
   Memory Management Issues
 • Protection: Errors in process should not affect others
 • Transparency: Should run despite memory size/location

   gcc         Translation box (MMU)
Load Store                 legal addr? address Physical
              virtual      Illegal?             memory
   CPU        address

                      How to do this mapping?           30
  Scheme 1: Load-time Linking
• Link as usual, but keep list of references
• At load time: determine the new base address
   – Accordingly adjust all references (addition)

   static a.out
               0x3000                                 0x6000
   jump 0x2000                          jump 0x5000

• Issues: handling multiple segments, moving in memory
  Scheme 2: Execution-time Linking
 • Use hardware (base + limit reg) to solve the problem
    – Done for every memory access
    – Relocation: physical address = logical (virtual) address + base
    – Protection: is virtual address < limit?
              0x3000         MMU                                0x6000
                       Base: 0x3000
                                              jump 0x2000
jump 0x2000            Limit: 0x2000
    – When process runs, base register = 0x3000, bounds register =
      0x2000. Jump addr = 0x2000 + 0x3000 = 0x5000
• Processes have multiple base + limit registers
• Processes address space has multiple segments
   – Each segment has its own base + limit registers
   – Add protection bits to every segment Real memory
0x1000                                                  0x2000
         Text seg

0x5000 Stack seg       Base&Limit?
          r/w                                           0x6000

                 How to do the mapping?                      33
             Mapping Segments
 • Segment Table
   – An entry for each segment
   – Is a tuple <base, limit, protection>
• Each memory reference indicates segment and offset
Virtual addr                 no                      mem
                          ?     yes
3     128                            +      0x1000

Seg# offset                                          128
                   Seg table                seg
                 Prot base          len

                  r     0x1000 512
  • “The inability to use free memory”
  • External Fragmentation:
       – Variable sized pieces  many small holes over time
  • Internal Fragmentation:
       – Fixed sized pieces  internal waste if entire piece is not used
Word          ??               gcc                           fragmentation

  allocated                      stack               (“internal
   • Divide memory into fixed size pieces
      – Called “frames” or “pages”
   • Pros: easy, no external fragmentation

 typical: 4k-8k          gcc

                         emacs               internal frag

                    Mapping Pages
 • If 2m virtual address space, 2n page size
       (m - n) bits to denote page number, n for offset within page

    Translation done using a Page Table

 Virtual addr                                                          mem
3       128 (12bits)                   ((1<<12)|128)        0x1000

VPN      page offsetpage table                                          128
                    Prot VPN         PPN
              ?                               PPN

      “invalid”                                                        37
                     r      3         1
       Paging + Segmentation
• Paged segmentation
  – Handles very long segments
  – The segments are paged
• Segmented Paging
  – When the page table is very big
  – Segment the page table
  – Let‟s consider System 370 (24-bit address space)

 Seg #         page # (8 bits)         page offset (12 bits)
 (4 bits)

        What is virtual memory?
• Each process has illusion of large address space
   – 232 for 32-bit addressing
• However, physical memory is much smaller
• How do we give this illusion to multiple processes?
   – Virtual Memory: some addresses reside in disk
        page table

                                 Physical memory
                  Virtual Memory
• Load entire process in memory (swapping), run it, exit
   – Is slow (for big processes)
   – Wasteful (might not require everything)
• Solutions: partial residency
   – Paging: only bring in pages, not all pages of process
   – Demand paging: bring only pages that are required
• Where to fetch page from?
   – Have a contiguous space in disk: swap file (pagefile.sys)

                      Page Faults
• On a page fault:
   – OS finds a free frame, or evicts one from memory (which one?)
       • Want knowledge of the future?
   – Issues disk request to fetch data for page (what to fetch?)
       • Just the requested page, or more?
   – Block current process, context switch to new process (how?)
       • Process might be executing an instruction
   – When disk completes, set present bit to 1, and current process in
     ready queue

 Page Replacement Algorithms
• Random: Pick any page to eject at random
   – Used mainly for comparison
• FIFO: The page brought in earliest is evicted
   – Ignores usage
   – Suffers from “Belady‟s Anomaly”
       • Fault rate could increase on increasing number of pages
       • E.g. 0 1 2 3 0 1 4 0 1 2 3 4 with frame sizes 3 and 4
• OPT: Belady‟s algorithm
   – Select page not used for longest time
• LRU: Evict page that hasn‟t been used the longest
   – Past could be a good predictor of the future

• Processes in system require more memory than is there
   – Keep throwing out page that will be referenced soon
   – So, they keep accessing memory that is not there

• Why does it occur?
   – No good reuse, past != future
   – There is reuse, but process does not fit
   – Too many processes in the system

       Approach 1: Working Set
• Peter Denning, 1968
   – Defines the locality of a program

   pages referenced by process in last T seconds of execution
      considered to comprise its working set
   T: the working set parameter

• Uses:
   – Caching: size of cache is size of WS
   – Scheduling: schedule process only if WS in memory
   – Page replacement: replace non-WS pages

                    Working Sets

• The working set size is num pages in the working set
   – the number of pages touched in the interval (t, t-Δ).
• The working set size changes with program locality.
   – during periods of poor locality, you reference more pages.
   – Within that period of time, you will have a larger working set size.
• Don‟t run process unless working set is in memory.
Approach 2: Page Fault Frequency
• thrashing viewed as poor ratio of fetch to work
• PFF = page faults / instructions executed
• if PFF rises above threshold, process needs more memory
   – not enough memory on the system? Swap out.
• if PFF sinks below threshold, memory can be taken away

     Allocation and deallocation
• What happens when you call:
   – int *p = (int *)malloc(2500*sizeof(int));
       • Allocator slices a chunk of the heap and gives it to the program
   – free(p);
       • Deallocator will put back the allocated space to a free list
• Simplest implementation:
   – Allocation: increment pointer on every allocation
   – Deallocation: no-op
   – Problems: lots of fragmentation

                                        heap (free memory)

allocation             current free position                                47
                 Memory Allocator
  • What allocator has to do:
     – Maintain free list, and grant memory to requests
     – Ideal: no fragmentation and no wasted time
  • What allocator cannot do:
     – Control order of memory requests and frees
     – A bad placement cannot be revoked
                         20        10       20            10   20
  • Main challenge: avoid fragmentation

        What happens on free?
• Identify size of chunk returned by user
• Change sign on both signatures (make +ve)
• Combine free adjacent chunks into bigger chunk
   – Worst case when there is one free chunk before and after
   – Recalculate size of new free chunk
   – Update the signatures
• Don‟t really need to erase old signatures

                Design features
• Which free chunks should service request
   – Ideally avoid fragmentation… requires future knowledge
• Split free chunks to satisfy smaller requests
   – Avoids internal fragmentation
• Coalesce free blocks to form larger chunks
   – Avoids external fragmentation

        20 10         30                 30       30

Malloc & OS memory management
• Relocation
   – OS allows easy relocation (change page table)
   – Placement decisions permanent at user level
• Size and distribution
   – OS: small number of large objects
   – Malloc: huge number of small objects

• Prelims graded
  – Mean 76.3 (Median 76), Stddev 8.8, High 93 out of 104!
  – Mean 73.4 (Median 73.1), Stddev 8.4, High 89.4 out of 100!
     • Good job!

• Re-grade policy
  – Submit written re-grade request to Joy.
     • Entire prelim will be re-graded.
     • We were generous the first time…
  – If still unhappy, submit another re-grade request.
     • Joy will re-grade herself
  – If still unhappy, submit a third re-grade request.
     • I will re-grade. Final grade is law.
                     Grade distribution
                       Grade Distribution (out of 104 points)









                55    60      65       70           75    80    85   90

Hand back prelims