” 辅导CS 449编程、 写作programming程序Malloc Lab: Writing a Dynamic Storage Allocator1 IntroductionIn this lab you will be writing a dynamic storage allocator for C programs, i.e., your own version of themalloc and free routines. You are encouraged to explore the design space creatively and implement anallocator that is correct, efficient and fast. The learning objectives are as follows: Implement a memory allocator using an explicit free list. Examine how algorithm choice impacts tradeoffs between utilization and throughput. Read, understand, and modify a substantial C program. Improve your C programming skills including gaining more experience with structs, pointers, anddebugging.This is a classic implementation problem with many interesting algorithms and opportunities to put severalof the skills you have learned in this course to good use. It is quite involved. Start early!2 LogisticsThis is individual work. In general we encourage students to discuss high-level ideas from the labs andhomeworks, Not solutions and implementation details. Please refer to the course policy for a reminder onwhat is appropriate behavior.In particular, we remind you that referring to solutions from previous quarters or from a similarcourse at another university or on the web is cheating. We will run similarity-detection software oversubmitted student programs, including programs from past quarters and online repositories.3 Hand Out InstructionsYour lab materials are contained in an archive file called malloclab-handout.zip, which you candownload to your Linux machine as follows.1$ wget httpss://bit.ly/2UvVz3i -O malloclab-handout.zipStart by copying malloclab-handout.zip to a protected directory in which you plan to do your work.Then give the command: unzip malloclab-handout.zip. This will cause a number of files to beunpacked into the directory. The only file you will be modifying and handing in is mm.c. The mdriver.cprogram is a Driver program that allows you to evaluate the performance of your solution. Use the commandmake to generate the driver code and run it with the command ./mdriver -V. (The -V flag displayshelpful summary information.)When you have completed the lab, you will hand in only one file (mm.c), which contains your solution.4 Programming TaskThe mm-reference.c file we have discussed in class (See: httpss://bit.ly/2AC3NA1) implementsa 64-bit struct-based simple memory allocator using implicit free lists. To optimize for memoryutilization, it employs splitting during block placement and coalescing of adjacent free blocks using boundary/footertags. However, since it manages a fixed size heap, it simply cannot satisfy arbitrary memoryallocation requests. This is the first thing you need to work on as an improvement over the given implementation.This implementation is also very inefficient in time since it traverses the entire heap to search for afree block. Using the reference code as a starting place, your main programming task will be to implementa more efficient allocator based on explicit free lists. To that end, you may start with a single doubly-linkedfree block list using a LIFO insertion policy (as discussed in class).We provide a code skeleton (mm.c) to assist you in implementing your dynamic storage allocator. It willconsist of the following functions (and several helper functions), which are defined in mm.c:int mm_init(void);void *mm_malloc(size_t size);void mm_free(void *ptr);The mm.c file we have given you partially implements an allocator using an explicit free list. Your jobis to complete this implementation by filling out mm malloc and mm free. The three main memorymanagement functions should work as follows: mm init: Before calling mm malloc or mm free, the application program (i.e., the trace-drivendriver program that you will use to evaluate your implementation) calls mm init to perform anynecessary initialization, such as allocating the initial heap area. The return value is -1 if there was aproblem in performing the initialization, 0 otherwise. mm malloc: The mm malloc routine returns a pointer to an allocated block payload of at leastsize bytes. (size t is a type for describing sizes; its an unsigned integer that can represent a sizespanning all of memory, so on x86 64 it is a 64-bit unsigned value.) The entire allocated blockshould lie within the heap region and should not overlap with any other allocated block.2 mm free: The mm free routine frees the block pointed to by ptr. It returns nothing. This routine isguaranteed to work only when the passed pointer (ptr) was returned by an earlier call to mm mallocand has not yet been freed. These semantics match the semantics of the corresponding malloc and freeroutines in libc. Refer to the malloc manpage ( httpss://linux.die.net/man/3/malloc)for complete documentation.You will Notice that most part of the mm init is already implemented for you. That is used before callingmm malloc or mm free. We will compare your implementation to the version of malloc supplied inthe standard C library (libc). Since the libc malloc always returns payload pointers that are aligned to 16bytes, your malloc implementation should do likewise and always return 16-byte aligned pointers. Thesesemantics match the semantics of the corresponding malloc, and free routines in libc.Beyond correctness, your goal is to produce an allocator that performs well in time and space. That is, themm_malloc and mm_free functions should work as quickly as possible, and the total amount of memoryused by your allocator should stay as close as possible to the amount of memory needed to hold the payloadof mm_malloc calls not yet balanced by mm_free calls. You are encouraged to explore the design spacecreatively and implement an allocator that is correct, space-efficient, and fast.5 Provided CodeWe define a block t struct to be used as a node in an explicit free list, and the following functions formanipulating free lists (to be implemented by you): void insert block(block t *block): inserts the given block in the free list. void remove block(block t *block): removes the given block from the free list.To implement the free list, you can use a doubly-linked list and can try different insertion strategies (asdiscussed in lecture). We recommend starting with a LIFO insertion policy.In addition, we provide a code skeleton you need to complete for two helper functions implementing importantparts of the allocator: block t *extend heap(size t size): enlarges the heap by size bytes (if enough memoryis available on the machine to do so), and recreates end header. It returns a pointer to the resultof coalescing the newly-created block with previous free block, if applicable, or NULL in case offailure. Dont forget to include the header at the end of the Heap (epilogue header) so as to keepthe Heap structure consistent. block t *find fit(size t asize): returns a block of at least the requested size (asize) ifone exists (and NULL otherwise). To speed up your search, you should use the explicit free list. void split Block(block t *block, size t asize): checks if the given block (justallocated by malloc) can be split into one to satisfy allocation and one to keep free. Dont forget toupdate the free list accordingly.3 block t *coalesce block(block t *block): coalesces current block with previous andnext blocks (into a single new large block) if either or both adjacent blocks are unallocated; otherwisethe block is not modified. It returns a pointer to the coalesced block. After coalescing, the immediatecontiguous previous and next blocks must be allocated. It needs to update the free list accordingly.It is worth mentioning that before you actually start implementing the explict free list version, to be moreconfident about how those important functions work, you may consider completing and testing a fully workingimplicit list implementation while abstracting the explicit free list structure and taking advantage of theshared reference code (mm-reference.c).Most importantly, by completing the function to increase the heap space (expand_heap) your memory allocatorcan satisfy arbritary memory allocation requests. Next, you can implement your search (find_fit)to identify free blocks, initially, using the implicit free list; this would still be correct, but your throughputscore would be miserable. Also, to simplify your first working implementation, you can abstract coalescingand splitting blocks (this would still be correct, but your memory utilization would be miserable). At first,abstracting some of those functions may help you break the problem down into smaller parts that you canmake a start on solving, and it will feel more manageable and less overwhelming.Finally, we provide you with a number of helper functions to extract common pieces of code (bit manipulation,constants, tedious casts/pointer manipulation) that might be prone to error. Each is documented in thecode. You are welcome to create your own functions as well, though the ones already included in mm.c arethe only ones we used in our sample solution, so it is possible without more.For debugging purposes, you may want to print the contents of the heap, for example, as you manipulate theheap structure after a malloc or free call. This can be accomplished with the provided examine heap()function. See Sections 10 and 11 for additional info on debugging.6 Memory SystemThe memlib.c package simulates the memory system for your dynamic memory allocator. In your allocator,you can call the following functions (if you use the provided code, most uses of the memory systemcalls are already covered). void *mem sbrk(int incr): Expands the heap by incr bytes, where incr is a positivenon-zero integer and returns a generic pointer to the first byte of the newly allocated heap area. Thesemantics are identical to the Unix sbrk function, except that mem sbrk accepts only a positivenon-zero integer argument. void *Mem heap lo(void): Returns a generic pointer to the first byte in the heap. void *mem heap hi(void): Returns a generic pointer to the last byte in the heap. size t mem heapsize(void): Returns the current size of the heap in bytes. size t mem pagesize(void): Returns the systems page size in bytes (4K on Linux systems).47 The Trace-driven Driver ProgramThe driver program mdriver.c in the handout starter package tests your mm.c package for correctness,space utilization, and throughput. Use the command make to generate the driver program and run it withthe command ./mdriver -V (the -V flag displays helpful summary information as described below).The driver program is controlled by a set of trace files that are included in the handout distribution (subdirectorycalled traces). Each trace file contains a sequence of allocate and free directions that instructthe driver to call your mm malloc and mm free routines in some sequence. The driver and the tracefiles are the same ones we will use when we grade your handin mm.c file. Trace files are structured in thefollowing manner:20000 # suggested heap size (unused)2 # number of ids — in this case, 0-14 # number of alloc + free operations1 # weight for this tracefile (unused)a 0 2040 # alloc block 0 with payload size 2040a 1 2040 # alloc block 1 with payload size 2040f 1 # free block 1f 0 # free block 0The mdriver program accepts the following command line arguments: -t tracedir: Look for the default trace files in directory tracedir instead of the defaultdirectory Defined in config.h. -f tracefile: Use one particular tracefile for testing instead of the default set of trace-files. -h: Print a summary of the command line arguments. -l: Run and measure libc malloc in addition to the students malloc package. -v: Verbose output. Print a performance breakdown for each tracefile in a compact table. -V: More verbose output. Prints additional diagnostic information as each trace file is processed.Useful during debugging for determining which trace file is causing your malloc package to fail.8 Programming Rules It is okay to look at any high-level descriptions of algorithms found in the textbook or elsewhere, butit is not acceptable to copy or look at any code of malloc implementations found online or in othersources, except for the allocators described in the textbook, in KR, or as part of the provided code. You should not change any of the interfaces in mm.c (e.g. names of functions, number and type ofparameters, etc.).5 You should not invoke any memory-management related library calls or system calls. This excludesthe use of malloc, calloc, free, realloc, sbrk, brk or any variants of these calls in yourcode. (You may use all the functions in memlib.c, of course.) You are not allowed to define any global or static compound data structures such as arrays, structs,trees, or lists in your mm.c program. However, you are allowed to declare global scalar variables suchas integers, floats, and pointers in mm.c, but try to keep these to a minimum. (It is possible to completethe implementation of the explicit free list without adding any global variables.) For consistency with the libc malloc package, which returns blocks aligned on 16-byte boundaries,your allocator must always return pointers that are aligned to 16-byte boundaries. The driverwill enforce this requirement for you.9 EvaluationYou will receive zero points if you break any of the rules or your code is buggy and crashes the driver.Otherwise, your grade will be calculated as follows: Correctness (33 points). You will receive 3 points for each test performed by the driver program thatyour solution passes. (11 tests). Performance (67 points). Performance represents another portion of your grade. For the most part, acorrect implementation will yield reasonable performance. Two performance metrics will be used toevaluate your solution: Space utilization: The peak ratio between the aggregate amount of memory used by the driver(i.e., allocated via mm malloc but not yet freed via mm free) and the size of the heap used byyour allocator. The optimal ratio equals to 1, although in practice we will not be able to achievethat ratio. You should find good policies to minimize fragmentation in order to make this ratioas close as Possible to the optimal. Throughput: The average number of operations completed per second.The driver program summarizes the performance of your allocator by computing a performance index,P, which is a weighted sum of the space utilization and throughputP = wU + (1 w) min 1,TTlibcwhere U is your space utilization, T is your throughput, and Tlibc is the estimated throughput oflibc malloc on your system on the default traces. The performance index favors space utilizationover throughput, with a default of w = 0.6. You will receive 64 P points for Performance.A complete version of the explicit free list allocator will have a performance index P between justover 0.8 and 0.9. Thus if you have a performance index greater or equal than 0.8 (mdriver prints thisas 80/100) then you will get the full points for Performance. For extra credit, you will get percentpoints when P is above 0.90. For example, if you have P = 1.0, you will have 10% extra credit.6Observing that both memory and CPU cycles are expensive system resources, we adopt this formula toencourage balanced optimization of both memory utilization and throughput. Ideally, the performanceindex will reach P = w + (1 w) = 1 or 100%. Since each metric will contribute at most w and1 w to the performance index, respectively, you should not go to extremes to optimize either thememory utilization or the throughput only. To receive a good score, you must achieve a balancebetween utilization and throughput.10 Heap Consistency CheckerThis is an optional, but recommended, addition that will help you check to see if your allocator is doing whatit should (or figure out what its doing wrong if not). Dynamic memory allocators are notoriously trickybeasts to program correctly and efficiently. They are difficult to program correctly because they involve alot of pointer manipulation. Thus, you will find it very helpful to write a heap checker that scans the heapand checks it for consistency.Some examples of what a heap checker might check are: Is every block in the free list marked as free? Are there any contiguous free blocks that somehow escaped coalescing? Is every free block actually in the free list? Do the pointers in the free list point to valid free blocks? Do any allocated blocks overlap? Do the pointers in a heap block point to valid heap addresses?Your heap checker will consist of the function bool check heap() in mm.c. It will check any invariantsor consistency conditions you consider prudent. It returns true if and only if your heap is consistent.You are not limited to the listed suggestions nor are you required to check all of them. You are encouragedto print out error messages when check heap fails.This consistency checker is for your own debugging during development. When you submit mm.c, makesure to remove any calls to check heap as they will slow down your throughput.11 Debugging your Code with GDBBugs can be especially difficult to track down in this lab, and you will probably spend a significant amountof time debugging your code. Buggy code will not get any credit. The debugging program gdb can be avaluable tool for tracking down bugs in your memory allocator. We hope by this point in the course that youare familiar with many of the features of gdb. You will want to take full advantage of them. Part of being aproductive programmer is to make use of the tools available.Below, we Present a brief tutorial on using gdb with your program.711.1 Viewing Heap ContentsA typical gdb session to examine the header of a block on a call to free might go something like this. In thefollowing, all text in bold was typed by the user. The session has been edited to remove some uninterestingparts of the printout.$ gdb ./mdriver(gdb) break mm_freeBreakpoint 1 at 0x402c43: file mm.c, line 277.(gdb) run -f traces/short1-bal.repBreakpoint 1, mm_free (bp=0x7ffff6bcf840) at mm.c:277(gdb) print /x *((unsigned long *) bp – 1)$1 = 0x811(gdb) quitA few things about this session are worth noting: The gdb command print /x *((unsigned long *) bp – 1) first casts the argumentto free to be a pointer to an unsigned long. It then decrements this pointer to point to the blockheader and then prints it in hex format. The printed value 0x811 indicates that the block is of size 0x810 (decimal 2, 064), and the lowerorderbit is set to indicate that the block is allocated. Looking at the trace file, you will see that theblock to be freed has a payload of 2, 040 bytes. This required allocating a block of size 2, 064 to holdthe header, payload, and footer.12 SubmissionYour submission will be graded using Gradescope. Submit only your mm.c file. You may submit yoursolution for testing as many times as you wish up until the due date. Only the last version you submit willbe graded. When testing your files locally, make sure to use the Thoth Linux machine. This will insurethat the score you get from mdriver is representative of the grade you will receive when you submit yoursolution.13 Strategic Advice (Useful for Extra Credit)You must design algorithms and data structures for managing free blocks that achieve the right balance ofspace utilization and speed. This involves a trade-off it is easy to write a fast allocator by allocating blocksand never freeing them, or a high-utilization allocator that carefully packs blocks as tightly as possible. Youmust seek to minimize wasted space while also making your program run fast.As described in the textbook and the lectures, utilization is reduced below 100% due to fragmentation,taking two different forms:8 External fragmentation: Unused space between allocated blocks or at the end of the heap Internal fragmentation: Space within an allocated block that cannot be used for storing data, becauseit is required for some of the managers data structures (e.g., headers, footers, and free-list pointers),or because extra bytes must be allocated to meet alignment or minimum block size requirementsTo reduce external fragmentation, you will want to implement good block placement heuristics. To reduceinternal fragmentation, it helps to reduce the storage for your data structures as much as possible. As wediscussed earlier, maximizing throughput requires making sure your allocator finds a suitable block quickly,e.g., by converting to an explicit-list allocator.To further Optimize your solution, you may want to experiment with different allocation policies. Theprovided code implements first-fit search. Some allocators attempt best-fit search, but this is difficult to doefficiently. You can find ways to introduce elements of best-fit search into a first-fit allocator, while keepingthe amount of search bounded.You may also want to use more elaborate data structures than is found in the provided code. Nevertheless,your code need not use any exotic data structures, such as search trees. You can achieve very good resultsonly using singly- and doubly-linked lists.It is worth mentioning that reducing external fragmentation may require achieving something closer to best-fit allocation, e.g., by using segregated lists, while reducing internal fragmentation may require reducing datastructure overhead. There are multiple ways to do this, each with its own challenges. Possible approachesand their associated challenges include: Eliminate footers in allocated blocks. But, you still need to be able to implement coalescing. See thediscussion about this optimization on page 852 of the textbook. Decrease the minimum block size. But, you must then manage free blocks that are too small to holdthe pointers for a doubly linked free list. Reduce headers below 8 bytes. But, you must support all possible block sizes, and so you must thenbe able to handle blocks with sizes that are too large to encode in the header. Set up special regions of memory for small, fixed-size blocks. But, you will need to manage theseand be able to free a block when given only the starting address of its payload.14 Some Notes on the TextbookThe code shown in the textbook (Section 9.9.12, and available from the CS:APP website) is a useful sourceof inspiration for the lab, but it does not meet the required coding standards. It does not handle 64-bitallocations, it makes extensive use of macros instead of functions, and it relies heavily on low-level pointerarithmetic. Similarly, the code shown in KR does not satisfy the coding requirements. You should use theprovided code mm.c and mm-reference.c as your starting point.Nevertheless, in the textbook, there are some homework-style practice problems for memory allocation incase you find them helpful in preparing for this lab. You do not need to submit these, they are just goodpractice. Read section 9.9 from the textbook for review. (Note word means 4 bytes for these problems)9 Practice Problem 9.6, p. 849 Practice Problem 9.7, p. 852 Practice Problem 9.10, p. 864 Homework Problem 9.15, p. 879 Homework Problem 9.16, p. 87915 Hints (Please read!) Read the handout instructions carefully (including these)! Understand the malloc implementation given in lecture. The lecture has a detailed example of asimple allocator based on an implicit free list. Use this is a point of departure. Dont start working onyour allocator until you understand everything about the simple implicit list allocator. Study the provided code and take notes while doing this. Draw some diagrams of what the datastructures should look like before and after major operations that would affect the heap organization. The heap footers tags need to be maintained as well. Keep in mind that the last block in the heapis a special marker we will call the heap footer that is allocated and has size 0. Use the mdriver -f option. During initial development, using tiny trace files will simplify debuggingand testing. We have included two such trace files (short1,2-bal.rep) that you can use forinitial debugging. Use the mdriver -v and -V options. The -v option will give you a detailed summary for eachtrace file. The -V will also indicate when each trace file is read, which will help you isolate errors. Compile with gcc -g and use a debugger like gdb. The -g flag tells gcc to include debuggingsymbols, so gdb can follow the source code as it steps through the executable. The Makefileprovided should already be set up to do this. A debugger will help you isolate and identify out ofbounds memory References. You can specify any command line arguments for mdriver after therun command in gdb e.g. run -f short1-bal.rep. Start early! It is possible to write an efficient malloc package with a few pages of code. However, itcan be some of the most difficult and sophisticated code you have written so far in your career. Sostart early, and Good luck!如有需要，请加QQ：99515681 或邮箱：99515681@qq.com
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