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					         Summary of Free Space List (FSL) Management for Contiguous Memory Management

The FSL is just a linked list of “holes” (unused blocks) of main memory:
  FSL
We only need two pieces of information about a hole: its size and its base physical address in memory. Not that it really matters  we’re not
going to be coding this stuff  but here’s what the key declarations might look like in the C code of the operating system:
        typedef struct hole
        {
          unsigned int sizeOfHole;
          unsigned int baseOfHole;
          struct hole *nextHole; /* pointer to the next hole in the FSL                           */
        } HOLE;
        HOLE *FSL;

A process’s physical address space, you should remember, comes in four sections:

    1. Text section (a.k.a. code section), containing the instructions produced by the compiler
    2. Data section, containing global and static variables produced by the compiler
    3. Stack section, where local variables and function arguments are placed and removed dynamically (by your program during its execution)
       as blocks or functions are entered and exited dynamically
    4. Heap section, where storage dynamically requested by the program (e.g., malloc) is allocated

Whenever a new process applies for admission, the amount of memory it needs is equal to the size of the load module for the program that will
execute in the process plus the OS default for the stack section plus the OS default for the heap section. (The load module created by the compiler
and linker already contains the text and data sections.)

The long term scheduler asks the free space manager if there’s enough memory for the new process. The free space manager searches the free
space list. Assuming a big enough hole exists, it is removed from the free space list and its base address supplied to the loader. The free space
manager than creates a new, smaller hole whose size is what was left over after some of the old hole was allocated to the new process and whose
base address is the old hole’s base plus the size of the new process. E.g., if the old hole started at address 15000 and was of size 4K and the new
process needed 3K, the new process will be loaded at 15000 and the new hole of size 1K starts at 18000. The new hole must then be inserted into
the free space list

When a process is terminated, the free space manager reclaims its memory by creating a new hole where the process used to be, searching the FSL
to see if the new hole is adjacent to some existing hole and, if so, merging them into a bigger new hole, and then inserting the new (possibly now
bigger) hole into the free space list.
        Summary of Free Space List (FSL) Management for Contiguous Memory Management


    FSL
             FSL Management                                            Advantages                                       Disadvantages
Alternatives
 First-Fit    The FSL is an unordered list                            1. Fastest insertion into FSL  whenever a       Not as good as worst-fit at
                When a process needs memory for admission,               new hole is created, it can go right at       delaying external fragmentation
                 the list is searched from the beginning and the          the front of the list, no searching for the
                 first hole found that’s big enough is the one            right spot to insert it.
                 that gets used.                                       2. Less likely than best fit to create
                When a new hole is created (after a new                  stupidly small holes, so it delays
                 process’s required memory is taken from a                external fragmentation better than best-
                 bigger hole or after a process terminates and its        fit
                 memory is released), it doesn’t matter where it
                 goes in the list, we can put it (the new hole)
                 wherever is easiest (usually the front of the list)
  Best-Fit      The FSL is an ordered list, ordered by                Preserves big holes the longest of these         1.Creates smaller holes than the
                 ascending size, smallest hole at the front.           three alternatives, so a big job arriving          other alternatives so
                When admitting a new process and thus                 later, after a bunch of smaller jobs is            excessive fragmentation
                 searching for the best fit, the first hole that’s     more likely to be admitted.                        (unusable memory in many
                 big enough is the best fit (since the list is                                                            small holes) happens earliest.
                 ordered).                                                                                              2.Searching to find the right
                Inserting a new hole also requires searching for                                                         spot to admit a new process
                 the right spot to keep the list ordered.                                                                 or to insert a new hole usually
                                                                                                                          takes longer than first fit
                                                                                                                          since the list must be
                                                                                                                          searched from the front and
                                                                                                                          the front fills up with the
                                                                                                                          unusable small holes.
 Worst-Fit      The FSL is an ordered list, ordered by                1. Fastest at finding the right hole to          Gobbles up the biggest holes
                 descending size, largest hole at the front.              admit a new process, it’s memory is           very quickly so later arriving
                Admitting a new process uses the first hole on           always taken from the biggest hole            big jobs may not get admitted.
                 the list.                                                which is always at the front, so the list
                Inserting a new hole still requires searching the        doesn’t have to be searched at all to
                 list to find the right spot (to keep the list            admit a process.
                 ordered)                                              2. Least likely to create a useless small
                                                                          hole so it delays excessive external
                                                                          fragmentation the longest

				
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posted:12/15/2013
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