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Free-Space Management

Free-Space Management. Junghyeon Joo(jhjoo@theory.snu.ac.kr) School of Computer Science and Engineering Seoul National University. Free-Space Management. Given free space, memory allocation and deallocation are requested

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Free-Space Management

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  1. Free-Space Management Junghyeon Joo(jhjoo@theory.snu.ac.kr) School of Computer Science and Engineering Seoul National University

  2. Free-Space Management • Given free space, memory allocation and deallocation are requested • OS allocates memory to a new process and deallocates memory from a terminated process • User program calls malloc() and free() • Fixed-sized memory unit • Easy to handle compared to variable-sized case • EX) paging • Variable-sized memory unit • EX) segmentation, malloc() / free()

  3. Assumption void * malloc (size_t size) void free (void * ptr) • Interface for allocation / deallocation • Allocation • Take the number of requested bytes as parameter • Return the start address of allocated memory • Deallocation • Take a pointer as parameter ( No other information ) • No internal fragmentation • Only focus on external fragmentation • No relocation • No compaction of free space as cure for fragmentation

  4. External Fragmentation used free free 0 10 20 30 Request for 15 bytes • The free space gets chopped into little pieces • Memory allocation request may fail, even though the total amount of free space exceeds the size of the request

  5. External Fragmentation used free free 0 10 20 30 Request for 15 bytes • The free space gets chopped into little pieces • Memory allocation request may fail, even though the total amount of free space exceeds the size of the request A request for 15 bytes will fail,even though there are 20 bytesfree

  6. Splitting used free free 0 10 20 30 head head NULL NULL addr:0 len:10 addr:0 len:10 addr:25 len:5 addr:20 len:10 Split used used free free 0 10 20 25 30 • Example Request for 5 bytes

  7. Coalescing used free free 0 10 20 30 head head NULL addr:0 len:10 addr:0 len:10 addr:20 len:10 addr:10 len:10 free free free 0 10 20 30 addr:20 len:10 NULL • Example free(10)

  8. Coalescing free free free 0 10 20 30 addr:20 len:10 NULL head head head addr:0 len:10 addr:0 len:10 addr:0 len:30 addr:20 len:10 addr:10 len:10 free 0 10 20 30 NULL • Merge neighboring free space Merge

  9. Tracking The Size Of Allocated Regions typedef struct __header_t { int size; int magic; } header_t; void free(void * ptr) { header_t *hptr = (void *) ptr – sizeof(header_t); int BlockSizeToFree = hprt->size + sizeof(header_t); assert(hprt->magic == 1234567) … • Recall ‘free(void * ptr)’ doesn’t take a size parameter • Stored in extra header block • When user calls free() • The magic number is used for a sanity check

  10. Tracking The Size Of Allocated Regions head size: 100 magic: 1234567 ptr 100B Memory Header size overhead also needs to be concerned in finding proper free space • Example

  11. sbrk() System Call char * sbrk (int incr) Before the call of malloc() sbrk(0) : 0x6000 malloc(4) malloc(4) malloc(4) malloc(4) sbrk(0) : 0x8000 sbrk(0) : 0x8000 sbrk(0) : 0x8000 sbrk(0) : 0x8000 • sbrk() • It specifies for the OS to give incr more bytes to the heap • It returns a pointer to the end of the heap before sbrk() wascalled • Thus the call of sbrk(0) returns the current end of the heap • malloc() calls sbrk() internally to grow the heap • But not every time… Each time sbrk() is called it requests sufficient memory

  12. Running Example on The Real OS [code] 1 main(){ 2int* buf, size, i = 1000; 3 for( size=4; size<20;size+=4{ 4 buf=(int*)malloc(size); 5 buf[0]=i++; 6printf(“Allocated %d bytes, buf : 0x%x, buf[0]:%d,buf[-1]:%d, buf[-2]:%d\n”,size, buf, buf[0], buf[-1], buf[-2]); 7 } [output] Allocated 4 bytes, buf : 0x8049780, buf[0]:1000,buf[-1]:17,buf[-2]:0 Allocated 8 bytes, buf : 0x8049780, buf[0]:1001,buf[-1]:17,buf[-2]:0 Allocated 12 bytes, buf : 0x8049780, buf[0]:1002,buf[-1]:17,buf[-2]:0 Allocated 16 bytes, buf : 0x8049780, buf[0]:1003,buf[-1]:25,buf[-2]:0 • Linux 2.2.10

  13. Memory Allocation Policy • Best Fit • Allocate to the smallest possible free space • It tries to reduce wasted space, but produces lots of small chunks • A full search of free space is required • Worst Fit • Allocate to the largest possible free space • It tries to leave big chunks free • A full search is required like Best Fit • First Fit • Allocate to the first possible free space • Search cost is reduced • Variation : search from the last search position rather than the beginning (Next Fit)

  14. Memory Allocation Policy A free list with three elements of sizes 10, 30 20 20 NULL 30 An allocation request of size 15 head head head head head head addr:0 len:10 addr:0 len:10 addr:0 len:10 addr:20 len:10 addr:20 len:10 addr:20 len:10 10 NULL 30 5 10 NULL 10 15 20 • Best Fit • Worst Fit & First Fit

  15. Other Approache 32KB 16KB 16KB 8KB 8KB • Segregated List • For a particular application that has a few popular-sized request • Keep a separate list just to manage request for that size • Other requests are handled by general policy • Buddy Allocation • Improve coalescing performance • Ex) search for a 7KB block

  16. Summary • Splitting & Coalescing • Header to store size of each allocated block • General Policy • Best Fit • Worst Fit • First Fit • Other Approache • Segregated List • Buddy Allocation

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