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Memory Management =================== The problem will focus on memory. You will implement your own version of malloc() and free(), using a variety of allocation strategies. You will be implementing a memory manager for a block of memory. You will implement routines for allocating and deallocating memory, and keeping track of what memory is in use. You will implement one of the four strategies for selecting in which block to place a new requested memory block: 1) First-fit: select the first suitable block with smallest address. 2) Best-fit: select the smallest suitable block. 3) Worst-fit: select the largest suitable block. 4) Next-fit: select the first suitable block after the last block allocated (with wraparound from end to beginning). Here, "suitable" means "free, and large enough to fit the new data". Here are the functions you will need to implement: initmem(): Initialize memory structures. mymalloc(): Like malloc(), this allocates a new block of memory. myfree(): Like free(), this deallocates a block of memory. mem_holes(): How many free blocks are in memory? mem_allocated(): How much memory is currently allocated? mem_free(): How much memory is NOT allocated? mem_largest_free(): How large is the largest free block? mem_small_free(): How many small unallocated blocks are currently in memory? mem_is_alloc(): Is a particular byte allocated or not? A structure has been provided for use to implement these functions. It is a doubly-linked list of blocks in memory (both allocated and free blocks). Every malloc and free can create new blocks, or combine existing blocks. You may modify this structure, or even use a different one entirely. However, do not change function prototypes or files other than mymem.c. IMPORTANT NOTE: Regardless of how you implement memory management, make sure that there are no adjacent free blocks. Any such blocks should be merged into one large block. A few functions are given to help you monitor what happens when you call your functions. Most important is the try_mymem() function. If you run your code with "mem -try <args>", it will call this function, which you can use to demonstrate the effects of your memory operations. These functions have no effect on test code, so use them to your advantage. Running your code: After running "make", run 1) "mem" to see the available tests and strategies. 2) "mem -test <test> <strategy>" to test your code with provided tests. 3) "mem -try <args>" to run your code with your own tests (the try_mymem function). You can also use "make test" and "make stage1-test" for testing. "make stage1-test" only runs the tests relevant to stage 1. Running "mem -test -f0 ..." will allow tests to run even after previous tests have failed. Similarly, using "all" for a test or strategy name runs all of the tests or strategies. Note that if "all" is selected as the strategy, the 4 tests are shown as one. One of the tests, "stress", runs an assortment of randomized tests on each strategy. The results of the tests are placed in "tests.out" . You may want to view this file to see the relative performance of each strategy. Stage 1 ------- Implement all the above functions, for all the 4 strategy in a group Use "mem -test all first" to test your implementation Stage 2 ------- you should answer the 10 questions asked below together in a group. The strategy is passed to initmem(), and stored in the global variable "myStrategy". Some of your functions will need to check this variable to implement the correct strategy. You can test your code with "mem -test all worst", etc., or test all 4 together with "mem -test all all". The latter command does not test the strategies separately; your code passes the test only if all four strategies pass. Answer the following questions as part of your report ===================================================== 1) Why is it so important that adjacent free blocks not be left as such? What would happen if they were permitted? 2) Which function(s) need to be concerned about adjacent free blocks? 3) Name one advantage of each strategy. 4) Run the stress test on all strategies, and look at the results (tests.out). What is the significance of "Average largest free block"? Which strategy generally has the best performance in this metric? Why do you think this is? 5) In the stress test results (see Question 4), what is the significance of "Average number of small blocks"? Which strategy generally has the best performance in this metric? Why do you think this is? 6) Eventually, the many mallocs and frees produces many small blocks scattered across the memory pool. There may be enough space to allocate a new block, but not in one place. It is possible to compact the memory, so all the free blocks are moved to one large free block. How would you implement this in the system you have built? 7) If you did implement memory compaction, what changes would you need to make in how such a system is invoked (i.e. from a user's perspective)? 8) How would you use the system you have built to implement realloc? (Brief explanation; no code) 9) Which function(s) need to know which strategy is being used? Briefly explain why this/these and not others. 10) Give one advantage of implementing memory management using a linked list over a bit array, where every bit tells whether its corresponding byte is allocated.
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