代写CS 449编程、代做programming程序、C++语言编程调试

日期：2020-12-08 11:39

Malloc Lab: Writing a Dynamic Storage Allocator

1 Introduction

In this lab you will be writing a dynamic storage allocator for C programs, i.e., your own version of the

malloc and free routines. You are encouraged to explore the design space creatively and implement an

allocator 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, and

debugging.

This is a classic implementation problem with many interesting algorithms and opportunities to put several

of the skills you have learned in this course to good use. It is quite involved. Start early!

2 Logistics

This is individual work. In general we encourage students to discuss high-level ideas from the labs and

homeworks, not solutions and implementation details. Please refer to the course policy for a reminder on

what is appropriate behavior.

In particular, we remind you that referring to solutions from previous quarters or from a similar

course at another university or on the web is cheating. We will run similarity-detection software over

submitted student programs, including programs from past quarters and online repositories.

3 Hand Out Instructions

Your lab materials are contained in an archive file called malloclab-handout.zip, which you can

download to your Linux machine as follows.

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$ wget https://bit.ly/2UvVz3i -O malloclab-handout.zip

Start 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 be

unpacked into the directory. The only file you will be modifying and handing in is mm.c. The mdriver.c

program is a driver program that allows you to evaluate the performance of your solution. Use the command

make to generate the driver code and run it with the command ./mdriver -V. (The -V flag displays

helpful summary information.)

When you have completed the lab, you will hand in only one file (mm.c), which contains your solution.

4 Programming Task

The mm-reference.c file we have discussed in class (See: https://bit.ly/2AC3NA1) implements

a 64-bit struct-based simple memory allocator using implicit free lists. To optimize for memory

utilization, it employs splitting during block placement and coalescing of adjacent free blocks using boundary/footer

tags. However, since it manages a fixed size heap, it simply cannot satisfy arbitrary memory

allocation 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 a

free block. Using the reference code as a starting place, your main programming task will be to implement

a more efficient allocator based on explicit free lists. To that end, you may start with a single doubly-linked

free 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 will

consist 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 job

is to complete this implementation by filling out mm malloc and mm free. The three main memory

management functions should work as follows:

• mm init: Before calling mm malloc or mm free, the application program (i.e., the trace-driven

driver program that you will use to evaluate your implementation) calls mm init to perform any

necessary initialization, such as allocating the initial heap area. The return value is -1 if there was a

problem in performing the initialization, 0 otherwise.

• mm malloc: The mm malloc routine returns a pointer to an allocated block payload of at least

size bytes. (size t is a type for describing sizes; it’s an unsigned integer that can represent a size

spanning all of memory, so on x86 64 it is a 64-bit unsigned value.) The entire allocated block

should lie within the heap region and should not overlap with any other allocated block.

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• mm free: The mm free routine frees the block pointed to by ptr. It returns nothing. This routine is

guaranteed to work only when the passed pointer (ptr) was returned by an earlier call to mm malloc

and has not yet been freed. These semantics match the semantics of the corresponding malloc and free

routines in libc. Refer to the malloc manpage (https://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 calling

mm malloc or mm free. We will compare your implementation to the version of malloc supplied in

the standard C library (libc). Since the libc malloc always returns payload pointers that are aligned to 16

bytes, your malloc implementation should do likewise and always return 16-byte aligned pointers. These

semantics 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, the

mm_malloc and mm_free functions should work as quickly as possible, and the total amount of memory

used by your allocator should stay as close as possible to the amount of memory needed to hold the payload

of mm_malloc calls not yet balanced by mm_free calls. You are encouraged to explore the design space

creatively and implement an allocator that is correct, space-efficient, and fast.

5 Provided Code

We define a block t struct to be used as a node in an explicit free list, and the following functions for

manipulating 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 (as

discussed 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 important

parts of the allocator:

• block t *extend heap(size t size): enlarges the heap by size bytes (if enough memory

is available on the machine to do so), and recreates end header. It returns a pointer to the result

of coalescing the newly-created block with previous free block, if applicable, or NULL in case of

failure. Don’t forget to include the header at the end of the Heap (epilogue header) so as to keep

the Heap structure consistent.

• block t *find fit(size t asize): returns a block of at least the requested size (asize) if

one 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 (just

allocated by malloc) can be split into one to satisfy allocation and one to keep free. Don’t forget to

update the free list accordingly.

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• block t *coalesce block(block t *block): coalesces current block with previous and

next blocks (into a single new large block) if either or both adjacent blocks are unallocated; otherwise

the block is not modified. It returns a pointer to the coalesced block. After coalescing, the immediate

contiguous 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 more

confident about how those important functions work, you may consider completing and testing a fully working

implicit list implementation while abstracting the explicit free list structure and taking advantage of the

shared reference code (mm-reference.c).

Most importantly, by completing the function to increase the heap space (expand_heap) your memory allocator

can 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 throughput

score would be miserable. Also, to simplify your first working implementation, you can abstract coalescing

and 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 can

make 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 the

code. You are welcome to create your own functions as well, though the ones already included in mm.c are

the 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 the

heap 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 System

The 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 system

calls are already covered).

• void *mem sbrk(int incr): Expands the heap by incr bytes, where incr is a positive

non-zero integer and returns a generic pointer to the first byte of the newly allocated heap area. The

semantics are identical to the Unix sbrk function, except that mem sbrk accepts only a positive

non-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 system’s page size in bytes (4K on Linux systems).

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7 The Trace-driven Driver Program

The 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 with

the 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 (subdirectory

called traces). Each trace file contains a sequence of allocate and free directions that instruct

the driver to call your mm malloc and mm free routines in some sequence. The driver and the trace

files are the same ones we will use when we grade your handin mm.c file. Trace files are structured in the

following manner:

20000 # suggested heap size (unused)

2 # number of ids -- in this case, 0-1

4 # number of alloc + free operations

1 # weight for this tracefile (unused)

a 0 2040 # alloc block "0" with payload size 2040

a 1 2040 # alloc block "1" with payload size 2040

f 1 # free block "1"

f 0 # free block "0"

The mdriver program accepts the following command line arguments:

• -t <tracedir>: Look for the default trace files in directory tracedir instead of the default

directory 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 student’s 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, but

it is not acceptable to copy or look at any code of malloc implementations found online or in other

sources, except for the allocators described in the textbook, in K&R, 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 of

parameters, etc.).

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• You should not invoke any memory-management related library calls or system calls. This excludes

the use of malloc, calloc, free, realloc, sbrk, brk or any variants of these calls in your

code. (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 such

as integers, floats, and pointers in mm.c, but try to keep these to a minimum. (It is possible to complete

the 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 driver

will enforce this requirement for you.

9 Evaluation

You 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 that

your solution passes. (11 tests).

• Performance (67 points). Performance represents another portion of your grade. For the most part, a

correct implementation will yield reasonable performance. Two performance metrics will be used to

evaluate 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 by

your allocator. The optimal ratio equals to 1, although in practice we will not be able to achieve

that ratio. You should find good policies to minimize fragmentation in order to make this ratio

as 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 throughput

P = wU + (1 − w) min 

1,

T

Tlibc



where U is your space utilization, T is your throughput, and Tlibc is the estimated throughput of

libc malloc on your system on the default traces. The performance index favors space utilization

over 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 just

over 0.8 and 0.9. Thus if you have a performance index greater or equal than 0.8 (mdriver prints this

as “80/100”) then you will get the full points for Performance. For extra credit, you will get percent

points when P is above 0.90. For example, if you have P = 1.0, you will have 10% extra credit.

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Observing that both memory and CPU cycles are expensive system resources, we adopt this formula to

encourage balanced optimization of both memory utilization and throughput. Ideally, the performance

index will reach P = w + (1 − w) = 1 or 100%. Since each metric will contribute at most w and

1 − w to the performance index, respectively, you should not go to extremes to optimize either the

memory utilization or the throughput only. To receive a good score, you must achieve a balance

between utilization and throughput.

10 Heap Consistency Checker

This is an optional, but recommended, addition that will help you check to see if your allocator is doing what

it should (or figure out what it’s doing wrong if not). Dynamic memory allocators are notoriously tricky

beasts to program correctly and efficiently. They are difficult to program correctly because they involve a

lot of pointer manipulation. Thus, you will find it very helpful to write a heap checker that scans the heap

and 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 invariants

or 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 encouraged

to print out error messages when check heap fails.

This consistency checker is for your own debugging during development. When you submit mm.c, make

sure to remove any calls to check heap as they will slow down your throughput.

11 Debugging your Code with GDB

Bugs can be especially difficult to track down in this lab, and you will probably spend a significant amount

of time debugging your code. Buggy code will not get any credit. The debugging program gdb can be a

valuable tool for tracking down bugs in your memory allocator. We hope by this point in the course that you

are familiar with many of the features of gdb. You will want to take full advantage of them. Part of being a

productive programmer is to make use of the tools available.

Below, we present a brief tutorial on using gdb with your program.

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11.1 Viewing Heap Contents

A typical gdb session to examine the header of a block on a call to free might go something like this. In the

following, all text in bold was typed by the user. The session has been edited to remove some uninteresting

parts of the printout.

$ gdb ./mdriver

(gdb) break mm_free

Breakpoint 1 at 0x402c43: file mm.c, line 277.

(gdb) run -f traces/short1-bal.rep

Breakpoint 1, mm_free (bp=0x7ffff6bcf840) at mm.c:277

(gdb) print /x *((unsigned long *) bp - 1)

$1 = 0x811

(gdb) quit

A few things about this session are worth noting:

• The gdb command “print /x *((unsigned long *) bp - 1)” first casts the argument

to free to be a pointer to an unsigned long. It then decrements this pointer to point to the block

header and then prints it in hex format.

• The printed value 0x811 indicates that the block is of size 0x810 (decimal 2, 064), and the lowerorder

bit is set to indicate that the block is allocated. Looking at the trace file, you will see that the

block to be freed has a payload of 2, 040 bytes. This required allocating a block of size 2, 064 to hold

the header, payload, and footer.

12 Submission

Your submission will be graded using Gradescope. Submit only your mm.c file. You may submit your

solution for testing as many times as you wish up until the due date. Only the last version you submit will

be graded. When testing your files locally, make sure to use the Thoth Linux machine. This will insure

that the score you get from mdriver is representative of the grade you will receive when you submit your

solution.

13 Strategic Advice (Useful for Extra Credit)

You must design algorithms and data structures for managing free blocks that achieve the right balance of

space utilization and speed. This involves a trade-off — it is easy to write a fast allocator by allocating blocks

and never freeing them, or a high-utilization allocator that carefully packs blocks as tightly as possible. You

must 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:

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• 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, because

it 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 requirements

To reduce external fragmentation, you will want to implement good block placement heuristics. To reduce

internal fragmentation, it helps to reduce the storage for your data structures as much as possible. As we

discussed 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. The

provided code implements first-fit search. Some allocators attempt best-fit search, but this is difficult to do

efficiently. You can find ways to introduce elements of best-fit search into a first-fit allocator, while keeping

the 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 results

only 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 data

structure overhead. There are multiple ways to do this, each with its own challenges. Possible approaches

and their associated challenges include:

• Eliminate footers in allocated blocks. But, you still need to be able to implement coalescing. See the

discussion 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 hold

the pointers for a doubly linked free list.

• Reduce headers below 8 bytes. But, you must support all possible block sizes, and so you must then

be 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 these

and be able to free a block when given only the starting address of its payload.

14 Some Notes on the Textbook

The code shown in the textbook (Section 9.9.12, and available from the CS:APP website) is a useful source

of inspiration for the lab, but it does not meet the required coding standards. It does not handle 64-bit

allocations, it makes extensive use of macros instead of functions, and it relies heavily on low-level pointer

arithmetic. Similarly, the code shown in K&R does not satisfy the coding requirements. You should use the

provided 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 in

case you find them helpful in preparing for this lab. You do not need to submit these, they are just good

practice. Read section 9.9 from the textbook for review. (Note “word” means 4 bytes for these problems)

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• 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. 879

15 Hints (Please read!)

• Read the handout instructions carefully (including these)!

• Understand the malloc implementation given in lecture. The lecture has a detailed example of a

simple allocator based on an implicit free list. Use this is a point of departure. Don’t start working on

your 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 data

structures should look like before and after major operations that would affect the heap organization.

• The heap footer’s tags need to be maintained as well. Keep in mind that the last “block” in the heap

is 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 debugging

and testing. We have included two such trace files (short1,2-bal.rep) that you can use for

initial debugging.

• Use the mdriver -v and -V options. The -v option will give you a detailed summary for each

trace 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 debugging

symbols, so gdb can follow the source code as it steps through the executable. The Makefile

provided should already be set up to do this. A debugger will help you isolate and identify out of

bounds memory references. You can specify any command line arguments for mdriver after the

run 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, it

can be some of the most difficult and sophisticated code you have written so far in your career. So

start early, and good luck!

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