1 / 10

Dynamic Storage Allocation

Dynamic Storage Allocation. Algorithms for Allocation First-Fit Best-Fit Worst-Fit Algorithm for Delete Fifty-Percent Rule. Dynamic Storage-Allocation Problem. First-fit : Allocate the first hole that is big enough.

kolton
Download Presentation

Dynamic Storage Allocation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dynamic Storage Allocation • Algorithms for Allocation • First-Fit • Best-Fit • Worst-Fit • Algorithm for Delete • Fifty-Percent Rule Operating System Concepts

  2. Dynamic Storage-Allocation Problem • First-fit: Allocate the first hole that is big enough. • Best-fit: Allocate the smallest hole that is big enough; must search entire list, unless ordered by size. Produces the smallest leftover hole. • Worst-fit: Allocate the largest hole; must also search entire list. Produces the largest leftover hole. How to satisfy a request of size n from a list of free holes. First-fit and best-fit better than worst-fit in terms of speed and storage utilization. 50-percent rule: Given N allocated blocks another 1/2N will be lost due to fragmentation  1/3 of memory lost. Operating System Concepts

  3. Fragmentation • External Fragmentation – total memory space exists to satisfy a request, but it is not contiguous. • Internal Fragmentation – allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used. • Reduce external fragmentation by compaction • Shuffle memory contents to place all free memory together in one large block. • Compaction is possible only if relocation is dynamic, and is done at execution time. • I/O problem • Latch job in memory while it is involved in I/O. • Do I/O only into OS buffers. • 50-percent rule – Given N allocated blocks another 1/2N will be lost due to fragmentation  1/3 of memory lost. Operating System Concepts

  4. Best Fit vs. First Fit • Memory sizes 1300 and 1200 • Requests: 1000, 1100, 250 • Request First-Fit Best-Fit • 1300, 1200 1300, 1200 • 1000 300, 1200 1300, 200 • 1100 300, 100 200, 200 • 250 50, 100 stuck Operating System Concepts

  5. Best Fit vs. First Fit • Memory sizes 1300 and 1200 • Requests: 1100, 1050, 250 • Request First-Fit Best-Fit • 1300, 1200 1300, 1200 • 1100 200, 1200 1300, 100 • 1050 200, 150 250, 200 • 250 stuck 0, 200 Operating System Concepts

  6. First-Fit Method • Request a block of size N. Returns first available block of size N. If no block available, then returns NULL • AVAIL points to first available block • [Initialize] Q  AVAIL • [End of list?] P  Next(Q); If P = NULL return(NULL). • [Check size.] if Size(P)  N goto 4 else Q  P; goto 2. • [Reserve block.] K  Size(P) – N. if K = 0 Next(Q)  Next(P) else Size(P)  K; Return(P+K). Operating System Concepts

  7. First-Fit Method • Improvements • In step 4, if K < c (small constant) return block of size N + K. • Roving pointer. Start next search where previous search ended. This prevents accumulation of small blocks in the beginning of AVAIL. Operating System Concepts

  8. Free • Assume AVAIL is sorted by memory location (if P  NULL, Next(P) > P). • return block of size N beginning at location P0. • [Initialize.] Q  AVAIL. • [Advance P.] P  Next(Q). if P = NULL or P > P0 goto 3 else Q  P and repeat. • [Check upper bound.] if P0 + N = P and P  NULL N  N + Size(P); Next(P0)  Next(P) else Next(P0)  P. • [Check lower bound.] if Q + Size(Q) = P0 Size(Q)  Size(Q) + N; Link(Q)  Link(P0) else Link(Q)  Po; Size(P0)  N. Operating System Concepts

  9. Improvements to Free • Use doubly linked list and tags to free in constant time. Operating System Concepts

  10. Fifty Percent Rule • If allocation and deletion are used continually in such a way that the system tends to an equilibrium condition, where there are N reserved blocks in the system, on the average, each equally likely to be the next one deleted, and where the probability that allocating a new block does not change the number of blocks is p, then the average number of available blocks tends to approximately 1/2pN. • N = A + B + C (number of reserved blocks) • M = ½(2A + B + ),   {0,1,2} (number of available blocks) • Average change in M when a block is freed is (C-A)/N and when a block is allocated is (1-p) • C - A - N + Np = 0  2M = N + A - C +  B A B C A C B C B B B B Operating System Concepts

More Related