CS 414 Review

1. CS 414 Review

2. 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

3. Operating System: Definition 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. Applications OS interface Operating System Physical machine interface Hardware Advantages Easy to use, simpler to code, more reliable, more secure, … You can say: “I want to write XYZ into file ABC”

4. What is in an OS? Quake Sql Server Applications Windowing & graphics System Utils Shells OS Interface Naming Windowing & Gfx Operating System Services 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

5. 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 Mode bit = 0 Kernel Mode Mode bit = 0 Return Mode bit = 1 Save Caller’s state Execute system call Restore state

6. 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”

7. Process State Transitions interrupt New Exit admitted done Ready dispatch Running I/O or event completion I/O or event wait Waiting • 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

8. 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

9. 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

10. Schedulers • 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

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

12. 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

13. 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

14. 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 indivisibly • 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

15. 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? CS1 Process 1 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.

16. Solution Structure Shared vars: Initialization: Process: . . . . . . Entry Section Critical Section Exit Section Added to solve the CS problem

17. 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, …

18. Semaphores • 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) { while(S ≤ 0) ; S--; } V(S) { S++; }

19. Semaphore Types • Counting Semaphores: • Any integer • Used for synchronization • Binary Semaphores • Value 0 or 1 • Used for mutual exclusion (mutex) Process i P(S); Critical Section V(S); Shared: semaphore S Init: S = 1;

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

21. Monitors • 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

22. 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

23. 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

24. Deadlocks Definition: 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

25. 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

26. 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

27. 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

28. 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

29. Basic Algorithm • If request[i] > need[i] then error (asked for too much) • If request[i] > available[i] then wait (can’t supply it now) • 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.

30. gcc Memory Management Issues • Protection: Errors in process should not affect others • Transparency: Should run despite memory size/location Translation box (MMU) legal addr? Illegal? Physical address Load Store Physical memory virtual address CPU fault data How to do this mapping?

31. 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) • Issues: handling multiple segments, moving in memory OS static a.out 0x3000 0x6000 jump 0x2000 jump 0x2000 jump 0x5000 0x4000 0x1000

32. 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? • When process runs, base register = 0x3000, bounds register = 0x2000. Jump addr = 0x2000 + 0x3000 = 0x5000 OS a.out 0x6000 0x3000 MMU a.out Base: 0x3000 Limit: 0x2000 jump 0x2000 jump 0x2000 0x4000 0x1000

33. Segmentation • 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 gcc 0x1000 0x3000 0x5000 0x6000 0x2000 0x8000 0x6000 Text seg r/o Base&Limit? Stack seg r/w How to do the mapping?

34. fault Virtual addr no mem yes Seg table ? 3 128 + 0x1000 Prot base len Seg#offset 128 seg r 0x1000 512 Mapping Segments • Segment Table • An entry for each segment • Is a tuple <base, limit, protection> • Each memory reference indicates segment and offset

35. Fragmentation • “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 External fragmentation gcc Word ?? emacs Unused (“internal fragmentation”) allocated stack doom

36. Paging • Divide memory into fixed size pieces • Called “frames” or “pages” • Pros: easy, no external fragmentation Pages typical: 4k-8k gcc emacs internal frag

37. 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 ((1<<12)|128) 3 128 (12bits) 0x1000 VPN page offset 128 page table seg Prot VPN PPN ? PPN “invalid” r 3 1

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

39. disk page table Physical memory 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

40. 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)

41. 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

42. 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

43. Thrashing • 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

44. 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

45. 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.

46. 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

47. 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

48. 20 20 10 20 10 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 • Main challenge: avoid fragmentation a b malloc(20)?

49. 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

50. 30 20 10 30 30 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