#include <pthread.h> #include <semaphore.h> #include <stdlib.h> pthread_mutex_t sem_mut = PTHREAD_MUTEX_INITIALIZER; pthread_mutex_t cond_mut = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond = PTHREAD_COND_INITIALIZER; pthread_t tip ; pthread_t tip1 ; int rander ; void *sender(void *) ; void *receiver(void *) ; sem_t sema_dude ; main() { rander = rand() % 10 ; //returns random num between 0 - 9 printf("Rander is %d\n", rander) ; sleep(rander) ; sem_init(&sema_dude, 0, 0); pthread_create(&tip, NULL, sender, NULL) ; pthread_create(&tip1, NULL, receiver, NULL) ; pthread_join(tip, NULL); pthread_join(tip1, NULL) ; }

void *sender (void *param) {printf("Hello world!!\n") ; sem_wait(&sema_dude) ; printf("Im back\n") ; } void *receiver (void *param) {printf("Hello from ME!\n") ; sleep(2) ; sem_post(&sema_dude) ;

gcc try_sem.c -lpthread -lposix4 //on gandalf (g)cc try_sem.c –lpthread //most Linux systems

Random Number Generator #include <stdlib.h> int rand() //returns a random integer, not double int my_rand = rand() % 20 ; //returns a random int between 0 and 19

Page 240: sem_t sem mutex incorrect. sem_t mutex ; while (true) { sleep(….) ; rand = rand() ; …… } {sleep_time = rand() % 10 ; sleep(sleep_time) ; ……………..}

Background Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution. Logical address space can therefore be much larger than physical address space. Allows address spaces to be shared by several processes. Allows for more efficient process creation. Virtual memory can be implemented via: Demand paging Demand segmentation

Shared Library Using Virtual Memory

Demand Paging Bring a page into memory only when it is needed. Less I/O needed Less memory needed Faster response More users (higher level of multiprogramming) Page is needed  reference to it invalid reference  abort not-in-memory  bring to memory

Page Table When Some Pages Are Not in Main Memory

Page Fault If there is ever a reference to a page, first reference will trap to OS  page fault OS decides: Invalid reference  abort. Just not in memory. Get empty frame.

Page Fault Get empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction

Steps in Handling a Page Fault

What happens if there is no free frame? Page replacement – find some page in memory, but not really in use, swap it out. algorithm performance – want an algorithm which will result in minimum number of page faults. Same page may be brought into memory several times.

Page Replacement Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk. Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory.

Basic Page Replacement Find the location of the desired page on disk. Find a free frame: - If there is a free frame, use it. - If there is no free frame, use a page replacement algorithm to select a victim frame. Read the desired page into the (newly) free frame. Update the page and frame tables. Restart the process.

Page Replacement Algorithms Want lowest page-fault rate. Evaluate algorithm by running it on a particular string of memory references (reference string) and computing the number of page faults on that string. In examples, the reference string is: 7,0,1,2,0, 3,0,4,2,3, 0,3,2,1,2,0,1,7,0,1

Optimal Algorithm Replace page that will not be used for longest period of time. Used for measuring how well your algorithm performs.

Optimal Page Replacement

Optimal Page Replacement

Optimal Page Replacement

Optimal Page Replacement

Optimal Page Replacement

First-In-First-Out Throw out the page that has been in memory the longest. Good when talking about a set of pages for initialization. Bad when talking about heavily used variable.

FIFO Page Replacement

Least Recently Used (LRU) Algorithm Based on principal of “Locality of Reference”. A page that has been used in the near past is likely to be used in the near future. LRU: Determine the least recently used page in memory and evict it. Can be done but difficult to implement efficiently.

LRU Page Replacement

LRU Page Replacement

LRU Page Replacement

Second Chance Approximations Try to approximate LRU. Add a “reference” bit to page table. The hardware sets this bit to 1 when the page is accessed. The reference bit is maintained in the page table. Set of “second chance” algorithms that use the reference bit in page table entry to determine if it has been recently used. Example: Clock Page Replacement Algorithm.

Page Fault: Evict Current Page and increment pointer. Cur. Pos Evict

Second Page Fault: This guy looks good, evict him and increment pointer Cur. Pos Evict

Third Page Fault: Can’t evict this guy. He has been referenced Third Page Fault: Can’t evict this guy. He has been referenced. Reset reference flag to 0 and check next potential victim. Cur. Pos

Can’t evict this guy either Can’t evict this guy either. Reset Reference bit to 0 and move on to next potential victim. Cur. Pos

I can evict this page because it has not been referenced since last time I checked. Cur. Pos Victim

This page is referenced and reference bit is set to 1 This page is referenced and reference bit is set to 1. Which page will be evicted next assuming no other reference bits are set? 1 Cur. Pos Victim

Other Software Approximations to LRU Not Frequently Used (NFU). Associate a software counter with each page. On timer interrupt, OS scans all pages in memory. For each page, the R bit (Referenced bit)is added to the counter. Page with lowest count is evicted.

Problem with NFU It never forgets. A page referenced often in earlier phases of the program may not be evicted long after it has been used. Would like to have an algorithm that “ages” the count. That is, the latest references should be the most important.

Aging Algorithm for Simulating LRU Now maintain an 8-bit counter for each page in memory. On each timer interrupt the OS scans the pages to determine which pages have been referenced since last time it checked. Shift right one bit of the counter. Place the R bit in the leftmost bit of the counter. Choose the page to evict that has the lowest count.

Example Assume all counters are currently 0. Consider the case when pages 0,2,4, and 5 are referenced between last interrupt.

Simulating LRU in Software The aging algorithm simulates LRU in software

Simulating LRU in Software Assume pages 0,1, and 4 are referenced since last interrupt. The aging algorithm simulates LRU in software

Simulating LRU in Software The aging algorithm simulates LRU in software

Simulating LRU in Software Assume 0,1,3,5 are referenced since last interrupt. The aging algorithm simulates LRU in software

Simulating LRU in Software The aging algorithm simulates LRU in software

Allocation of Frames Replacement Scope can be: Each process needs minimum number of pages. Two major allocation schemes. Equal allocation Proportional allocation. Replacement Scope can be: Local. Global.

Local Scope Number of pages per process is fixed based on some criteria. Can use equal allocation or proportional allocation. Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages. What are the drawbacks of equal allocation?

Proportional Allocation, Local Scope Allocate number of pages based on the size of the process. Problem?

What are the drawbacks of this approach? Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages. What are the drawbacks of this approach? Allocation may be too small causing significant page faulting. Allocation may too large reducing number of processes in memory and wasting memory that could be used by other processes.

Proportional Allocation Problem? Allocate pages based on the size of the process. Problem? Process needs will vary over its execution leading to the same problems as equal-size pages.

Global Replacement When a page fault occurs, new page frame allocated to the process. Page replacement based on previous approaches: e.g., LRU, FIFO, etc. No consideration of which process should (or can best afford) to lose a page. Can lead to high page-fault rates.

Thrashing If a process does not have “enough” pages, the page-fault rate is very high. This leads to: low CPU utilization. operating system thinks that it needs to increase the degree of multiprogramming. another process added to the system. Thrashing  a process is busy swapping pages in and out.

Thrashing

Locality In A Memory-Reference Pattern

Working-Set Model   working-set window  a fixed number of page references Example: 10,000 instruction WSSi (working set of Process Pi) = total number of pages referenced in the most recent  (varies in time) if  too small will not encompass entire locality. if  too large will encompass several localities. if  =   will encompass entire program. D =  WSSi  total demand frames if D > m (memory)  Thrashing Policy if D > m, then suspend one of the processes.

Working-set model

The page-fault rate will peak when transitioning between localities. Once the pages of the current working set have been loaded, page faults will decrease.

Page-Fault Frequency Scheme Establish “acceptable” page-fault rate. If actual rate too low, process loses frame. If actual rate too high, process gains frame.

Memory-Mapped Files File system calls are expensive (e.g., read(), write(), seek). An alternative is to map disk blocks into main memory. Will initially get mapped through page faults. Subsequent operations handled as routine memory accesses. Create shared memory by allowing multiple processes to map the same file concurrently.

Homework: 9.2, 9.3, 9.4, 9.6, 9.15.

Shared Memory Through Memory Mapping Same File

Other Considerations Prepaging: Predicting future page requests. In pure-demand paging, lots of page faults when process started and whenever it is swapped out (and later resumed). Example: Remember working set when process is swapped out– load entire working set when reloaded.

Page Size Trade-offs between larger/smaller page sizes. Advantage of smaller page sizes?

Page Size Trade-offs between larger/smaller page sizes. Advantage of smaller page sizes? Less internal fragmentation. Disadvantages of smaller page size?

Page Size Trade-offs between larger/smaller page sizes. Advantage of smaller page sizes? Less internal fragmentation. Disadvantages of smaller page size? Larger page tables More I/O operations More page faults

Advantages of larger page size?

Advantages of larger page size? Smaller page tables More efficient I/O Fewer page faults

Other Considerations TLB Reach - The amount of memory accessible from the TLB. TLB Reach = (TLB Size) X (Page Size) Ideally, the working set of each process is stored in the TLB.

Increasing the Size of the TLB Increase the Page Size. This may lead to an increase in fragmentation as not all applications require a large page size. Provide Multiple Page Sizes. This allows applications that require larger page sizes the opportunity to use them without an increase in fragmentation. Requires OS to manage the TLB in software. The current trend is to manage the TLB in software.

Windows NT Uses demand paging with clustering. Clustering brings in pages surrounding the faulting page. Processes are assigned working set minimum and working set maximum. Working set minimum is the minimum number of pages the process is guaranteed to have in memory.

Windows NT A process may be assigned as many pages up to its working set maximum. When the amount of free memory in the system falls below a threshold, automatic working set trimming is performed to restore the amount of free memory. Working set trimming removes pages from processes that have pages in excess of their working set minimum. Uses variation of the clock or FIFO algorithms.