memory-management-early-systems in BS INFORMATION TECHNOLOGY
- 1. Memory Management, Early Systems • Single-User Contiguous Scheme • Fixed Partitions • Dynamic Partitions • Deallocation • Relocatable Dynamic Partitions • Conclusion Single User Configurations Fixed Partitions Dynamic Partitions Relocatable Dynamic Partitions 1
- 2. 2 Single-User Contiguous Scheme • Each program loaded in its entirety into memory and allocated as much contiguous memory space as needed. • If program was too large -- it couldn’t be executed. • Minimal amount of work done by Memory Manager. • Hardware needed : 1) register to store base address; 2) accumulator to track size of program as it is loaded into memory.
- 3. 3 Algorithm to Load a Job in a Single-user System 1. Store first memory location of program into base register 2. Set program counter equal to address of first memory location 3. Load instructions of program 4. Increment program counter by number of bytes in instructions 5. Has the last instruction been reached? If yes, then stop loading program If no, then continue with step 6 6. Is program counter greater than memory size? If yes, then stop loading. If no, then continue with step 7 7. Load instruction in memory 8. Go to step 3.
- 4. 4 Fixed (Static) Partitions • Attempt at multiprogramming using fixed partitions – one partition for each job – size of partition designated by reconfiguring the system – partitions can’t be too small or too large. • Critical to protect job’s memory space. • Entire program stored contiguously in memory during entire execution. • Internal fragmentation is a problem.
- 5. Algorithm to Load a Job in a Fixed Partition 5 1. Determine job’s requested memory size 2. If job_size > size of largest partition then reject job Print appropriate message Go to step 1 to handle next job Else continue with step 3 3. Set counter to 1 4. Do while counter <= number of partitions in memory If job_size > mem_partition_size (counter) then counter = counter + 1 Else If mem_partition_status (counter) = “free” then load job into mem_partition(counter) change mem_partition_status(counter) to “busy” go to step 1 Else counter = counter + 1 End do 5. No partition available at this time, put job in waiting queue 6. Go to step 1
- 6. 6 Simplified Fixed Partition Memory Table (Table 2.1) Partition size Memory address Access Partition status 100K 200K Job 1 Busy 25K 300K Job 4 Busy 25K 325K Free 50K 350K Job 2 Busy
- 7. Original State 100K Partition 1 25K Partition 2 Partition 3 25K Partition 4 50K Job List : J1 J2 J3 J4 30K 50K 30K 25K Job 1 (30K) Job 4 (25K) Job 2 (50K) 7 After Job Entry Partition 1 Partition 2 Partition 3 Partition 4 Table 2.1 : Main memory use during fixed partition allocation of Table 2.1. Job 3 must wait.
- 8. 8 Dynamic Partitions • Available memory kept in contiguous blocks and jobs given only as much memory as they request when loaded. • Improves memory use over fixed partitions. • Performance deteriorates as new jobs enter the system – fragments of free memory are created between blocks of allocated memory (external fragmentation).
- 9. Dynamic Partitioning of Main Memory & Fragmentation (Figure 2.2) 9
- 10. 10 Dynamic Partition Allocation Schemes • First-fit: Allocate the first partition that is big enough. – Keep free/busy lists organized by memory location (low- order to high-order). – Faster in making the allocation. • Best-fit: Allocate the smallest partition that is big enough – Keep free/busy lists ordered by size (smallest to largest). – Produces the smallest leftover partition. – Makes best use of memory.
- 11. First-Fit Allocation Example (Table 2.2) Job List J1 J2 J3 J4 10K 20K 30K* 10K Memory location Memory Job block size number Job size Status Internal fragmentation 10240 30K J1 10K Busy 20K 40960 15K J4 10K Busy 5K 56320 50K J2 20K Busy 30K 107520 20K Free Total Available: 115K Total Used: 40K 11
- 12. Best-Fit Allocation Example (Table 2.3) Job List J1 J2 J3 J4 10K 20K 30K 10K Memory location Memory Job block size number Job size Status Internal fragmentation 40960 15K J1 10K Busy 5K 107520 20K J2 20K Busy None 10240 30K J3 30K Busy None 56230 50K J4 10K Busy 40K Total Available: 115K Total Used: 70K 12
- 13. 13 First-Fit Algorithm 1. Set counter to 1 2. Do while counter <= number of blocks in memory If job_size > memory_size(counter) then counter = counter + 1 else load job into memory_size(counter) adjust free/busy memory lists go to step 4 End do 3. Put job in waiting queue 4. Go fetch next job
- 14. First-Fit Memory Request (Table 2.4) Before request After request Beginning Memory Beginning Memory address block size address block size 4075 105 4075 105 5225 5 5225 5 6785 600 *6985 400 7560 20 7560 20 7600 205 7600 205 10250 4050 10250 4050 15125 230 15125 230 24500 1000 24500 1000 14
- 15. 15 Best-Fit Algorithm If initial_mem_waste > mem_waste Then subscript = counter initial_mem_waste = mem_waste counter = counter + 1 End do 6. If subscript = 0 Then put job in waiting queue Else load job into mem_size(subscript) adjust free/busy memory lists 7. Go fetch next job 1. Initialize mem_block(0) = 99999 2. Compute initial_mem_waste = memory_block(0) – job_size 3. Initialize subscript = 0 4. Set counter to 1 5. Do while counter <= number of blocks in memory If job_size > mem_size(counter) Then counter = counter + 1 Else mem_waste = mem_size(counter) – job_size
- 16. Best-Fit Memory Request (Table 2.5) Before request After request Beginning Memory Beginning Memory address block size address block size 4075 105 4075 105 5225 5 5225 5 6785 600 6785 600 7560 20 7560 20 7600 205 *7800 5 10250 4050 10250 4050 15125 230 15125 230 24500 1000 24500 1000 16
- 17. 17 Best-Fit vs. First-Fit • Increases memory use • Memory allocation takes more time • Reduces internal fragmentation First-Fit Best-Fit • More complex algorithm • Searches entire table before allocating memory • Results in a smaller “free” space (sliver)
- 18. 18 Release of Memory Space : Deallocation • Deallocation for fixed partitions is simple – Memory Manager resets status of memory block to “free”. • Deallocation for dynamic partitions tries to combine free areas of memory whenever possible – Is the block adjacent to another free block? – Is the block between 2 free blocks? – Is the block isolated from other free blocks?
- 19. 19 Algorithm to Deallocate Memory Blocks If job_location is adjacent to 1+ free blocks Then If job_location is between 2 free blocks Then merge all 3 blocks into 1 block mem_size(counter-1) = mem_size(counter-1) + job_size + mem_size(counter+1) Set status of mem_size(counter+1) to null entry Else merge both blocks into one mem_size(counter-1) = mem_size(counter-1) + job_size Else search for null entry in free memory list enter job_size and beginning_address in entry slot set its status to “free”
- 20. 20 Case 1: Joining 2 Free Blocks Before Deallocation After Deallocation Beginning address 4075 Memory block size 105 Status Free Beginning address 4075 Memory block size 105 Status Free 5225 5 Free 5225 5 Free 6785 600 Free 6785 600 Free 7560 20 Free 7560 20 Free (7600) (200) (Busy)1 *7600 205 Free *7800 5 Free 10250 4050 Free 10250 4050 Free 15125 230 Free 15125 230 Free 24500 1000 Free 24500 1000 Free
- 21. 21 Case 2: Joining 3 Free Blocks Before Deallocation After Deallocation Beginning address 4075 Memory block size 105 Status Free Beginning address 4075 Memory block size 105 Status Free 5225 5 Free 5225 5 Free 6785 600 Free 6785 600 Free 7560 20 Free 7560 245 Free (7600) (200) (Busy)1 * (null) *7800 5 Free 10250 4050 Free 10250 4050 Free 15125 230 Free 15125 230 Free 24500 1000 Free 24500 1000 Free
- 22. 22 Case 3: Deallocating an Isolated Block Busy List Before Busy List After Beginning address 7805 Memory block size 1000 Status Busy Beginning address 7805 Memory block size 1000 Status Busy *8805 445 Busy * (null entry) 9250 1000 Busy 9250 1000 Busy
- 23. 23 Relocatable Dynamic Partitions • Memory Manager relocates programs to gather all empty blocks and compact them to make 1 memory block. • Memory compaction (garbage collection, defragmentation) performed by OS to reclaim fragmented sections of memory space. • Memory Manager optimizes use of memory & improves throughput by compacting & relocating.
- 24. 24 Compaction Steps • Relocate every program in memory so they’re contiguous. • Adjust every address, and every reference to an address, within each program to account for program’s new location in memory. • Must leave alone all other values within the program (e.g., data values).
- 25. 25 Original Assembly Language Program (Figure 2.4) A: EXP 132, 144, 125, 110 ;the data values BEGIN: MOVEI 1,0 ;initialize register 1 MOVEI 2,0 ;initialize register 2 LOOP: ADD 2,A(1) ;add (A + reg 1) to reg 2 ADDI 1,1 ;add 1 to reg 1 CAIG 1,4-1 ;is reg 1 > 4-1? JUMPA LOOP ;if not, go to Loop MOVE 3,2 ;if so, move reg 2 to reg 3 IDIVI 3,4 ;divide reg 3 by 4, ;remainder to register 4 EXIT ;end END
- 26. 26 Assembly Language Program Loaded into Memory (Figure 2.4) 000000’ 000000 000132 A: EXP 132,144,125,110 000001’ 000000 000144 000002’ 000000 000125 000003’ 000000 000110 000004’ 201 01 0 00 000000 BEGIN: MOVEI 1,0 000005’ 201 02 0 00 000000 MOVEI 2,0 000006’ 270 02 0 01 000000’ LOOP: ADD 2,A(1) 000007’ 271 01 0 00 000001 ADDI 1,1 000008’ 307 01 0 00 000003 CAIG 1,4-1 000009’ 324 00 0 00 000006’ JUMPA LOOP 000010’ 200 03 0 00 000002 MOVE 3,2 000011’ 231 03 0 00 000004 IDIVI 3,4 000012’ 047 00 0 00 000012 EXIT 000000 END
- 27. 27 Program in Memory During Compaction & Relocation • Free list & busy list are updated – free list shows partition for new block of free memory – busy list shows new locations for all relocated jobs • Bounds register stores highest location in memory accessible by each program. • Relocation register contains value that must be added to each address referenced in program so it can access correct memory addresses after relocation.
- 28. Memory Before & After Compaction (Figure 2.5) 28
- 29. Contents of relocation register & close-up of Job 4 memory area (a) before relocation & (b) after relocation and compaction (Figure 2.6) 29
- 30. 30 More Overhead is a Problem with Compaction & Relocation • Timing of compaction (when, how often) is crucial. • Approaches to timing of compaction: 1. Compact when certain percentage of memory is busy (e.g., 75%). 2. Compact only when jobs are waiting. 3. Compact after certain amount of time.
- 31. 31 Key Terms • address • best-fit memory allocation • bounds register • compaction • deallocation • dynamic partitions • external fragmentation • first come first served • first-fit memory allocation • fixed partitions • internal fragmentation • K • multiprogramming • relocatable dynamic partitions • relocation • relocation register