Implementing LRU page replacement algorithm - c

Question

Edited to include short description of what is expected from the code.
#include <sys/file.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#define MAX_PAGE 0xFF+1
/* page table entry you may need to add your own fields to it*/
typedef struct
{
unsigned short frame;/*location*/
unsigned int valid:1;
unsigned int in_mem:1;
unsigned int dirty:1;
unsigned int last_frame;
} pt_entry;
/* list entry for physical frames*/
struct list_item
{
unsigned short frame;
struct list_item *next;
struct list_item *prev;
int page_num;
};
typedef struct list_item *list;
void start_simulation(FILE *);
void resolve(int);
unsigned short find_frame(void);
unsigned short find_victim(void);
void display_stats(void);
void to_resident_set(list);
void free_mem(list);
void invalidate(unsigned short);
/*============================ header ends here ============================== *
/*#include "lru.h"*/
pt_entry pte[MAX_PAGE]; /* page table */
int mem_size; /* physical memory size in page frames */
list free_list_head; /* free list */
list res_set_head; /* resident set */
int total_fault = 0; /* total number of page faults */
int total_ref = 0; /* total number of memory references */
/* main program:
** read in paramters, and open the input file start the simulation */
int main(int argc, char *argv[])
{
FILE *stream;
if (argc != 3)
{
printf("The format is: pager file_name memory_size.\n");
exit(1);
}
printf("File used %s, resident set size %d\n", argv[1], atoi(argv[2]));
if ((stream = fopen(argv[1], "r")) == NULL)
{
perror("File open failed");
exit(1);
}
mem_size = atoi(argv[2]);
start_simulation(stream);
fclose(stream);
}
/*initialise the page table
** initialise the resident set, and the free list
** in the simulation loop
**16-bit memory addresses representing the program trace are read from the input
**file one by one the virtual address is resolved ie. physical frame for the
**virtual page identified
**the loop exits when it encounters the end of file
** free memory allocated for lists
** display statistics
*/
void start_simulation(FILE * stream)
{
char *addr_buf;
int address;
int i, n;
list new_entry, current;
/* initialise the page table */
for(i=0; i<MAX_PAGE;i++)
{
pte[i].frame = -1;
pte[i].valid = 0;
pte[i].dirty = 0;
pte[i].in_mem = 0;
}
/* initialise the resident set - empty*/
res_set_head = (list)malloc(sizeof(struct list_item));
res_set_head->next = res_set_head;
res_set_head->prev = res_set_head;
/* initialise free list - all physical pages*/
free_list_head = (list)malloc(sizeof(struct list_item));
free_list_head->next = free_list_head;
free_list_head->prev = free_list_head;
current = free_list_head;
for(i=0; i<mem_size;i++)
{
new_entry = (list)malloc(sizeof(struct list_item));
current->next = new_entry;
new_entry->prev = current;
new_entry->next = free_list_head;
new_entry->frame = i;
current = new_entry;
free_list_head->prev = current;
}
/* main simulation loop */
while( (n = fscanf(stream, "%x", &address)) != -1)
{
resolve(address);
total_ref++;
}
free_mem(free_list_head);
free_mem(res_set_head);
display_stats();
return;
}
/* resolve address reference
** if page table entry valid - do nothing
** if page table entry invalid - find a physical frame for this page
**and update pte for the page
*/
void resolve(int address)
{
unsigned short frame_alloc;
int virt_page;
static int disp_counter = 0;
virt_page = address >> 8;
if (pte[virt_page].valid == 1)
{
/*Was trying to implement */
//pte[virt_page].frame = pte[0];
}
else
{
frame_alloc = find_frame();
pte[virt_page].valid = 1;
pte[virt_page].frame = frame_alloc;
total_fault++;
}
}
/* find_frame:
** if free list is empty find a victim frame
** else detach the last frame of the free list and attach it
** to the resident set
** return frame number
*/
unsigned short find_frame()
{
unsigned short frame;
list current, new_tail;
if (free_list_head == free_list_head->prev) /* free list empty */
frame = find_victim();
else
{
new_tail = free_list_head->prev->prev;
new_tail->next = free_list_head;
current = free_list_head->prev;
free_list_head->prev = new_tail;
to_resident_set(current);
frame = current->frame;
}
return frame;
}
/* to_resident_set:
** attach a list entry at the end of resident set
*/
void to_resident_set(list current)
{
list tail;
tail = res_set_head->prev;
tail->next = current;
current->next = res_set_head;
current->prev = tail;
res_set_head->prev = current;
}
/* find_victim:
** As you can see I simply take the first page frame from the resident set list.
** This implements the FIFO replacement strategy. Your task is to replace it with
** a more efficient strategy.
*/
unsigned short find_victim()
{
int i;
unsigned short frame=0;
list current;
for(i=0;i<MAX_PAGE;i++)
{
if (pte[i].frame == frame && pte[i].valid == 1)
{
frame = res_set_head->next->frame;
invalidate(frame);
current = res_set_head->next;
res_set_head->next = current->next;
res_set_head->next->prev = res_set_head;
to_resident_set(current);
break;
}
}
return frame;
}
/* invalidate:
** invalidate the page table entry for the victim page */
void invalidate(unsigned short frame)
{
int i;
for(i=0;i<MAX_PAGE;i++)
{
if (pte[i].frame == frame && pte[i].valid == 1)
{
pte[i].valid = 0;
pte[i].frame = -1;
break;
}
}
}
/* display_stats:
** This is very basic, you may want to make it more sophisticated,
** for example save the data from multiple runs into a file for
** comparison etc
*/
void display_stats()
{
printf("\nProcess issued %d memory references\n", total_ref);
printf("Process triggered %d page faults\n", total_fault);
printf("Pafe fault rate is %d percent\n",((total_fault*100)/total_ref));
}
/* free memory allocated to the list */
void free_mem(list head)
{
list current,tail;
tail = head->prev;
current = head;
while (current->prev != tail)
{
current = current->next;
free(current->prev);
}
}

Answer 1

The most obvious problem lies in the input to your algorithm.
The restpage array is a global array and will thus be initialised to contain only the value 0. You then use these array elements as the page-numbers you are requesting, which means that your algorithm processes only requests for page 0 if mem_size < 100.
And if mem_size >= 100, you are overrunning the array bounds and land squarely in the land of undefined behaviour.
There are two fixes you need to make:
Just as you are checking for a valid file in the command-line arguments, you must also check that mem_size is not too large
Write an additional loop to give each element in restpage a random value, to ensure not all page requests are for the same page.

Answer 2

You have dimensioned restpage to [100] but mem_size seems freely configurable, is this the intent?
mem_size = atoi(argv[2]);
fclose(stream);
..
for(i=0;i<mem_size;i++)
{
totalabsence+=find_victim(&pt,restpage[i]);
}
EDIT:
I see one bug in your new code, in your find_victim you don't initialize the local variable 'frame'
EDITx2:
When you read from the file you may just want to put one hex address on each line
and use instead fgets() to read the file line by line (or load the whole file and
go through it line by line).