辅导CSCI 2400、讲解C/C++语言、Dynamic Storage辅导、C/C++编程调试

12/7/2018 Assignment

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Home / My courses / Fall 2018 / CSCI2400-Fa18 / 12 November - 18 November / Malloc Lab

CSCI 2400 - Grunwald - Computer Systems

Malloc Lab

CS2400 - Malloc Lab: Writing a

Dynamic Storage Allocator

(short version)

Introduction

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

version of the malloc , free and realloc routines. You are encouraged to explore the

design space creatively and implement an allocator that is correct, ecient

and fast.

Logistics

You may work in a group of up to two people. Any clarications

and revisions to the

assignment will be posted on the course Moodle.

Hand Out Instructions

Download the malloclab-handout.tar le

from the Moodle assignment page.

Start by copying malloclab-handout.tar ?to a directory in which you plan to do your work.

Then give the command:

tar xvf malloclab-handout.tar

This will cause a number of les

to be unpacked into the directory. The only le

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

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generate the driver code and run it with the command? ./mdriver -V . (The-V ag

displays helpful summary information.)

Looking at the le

mm.c you'll notice a C structure team into which you should insert the

requested identifying information about the one or two individuals comprising your

programming team.Do this right away so you don't forget.

When you have completed the lab, you will hand in only one le

( mm.c ), which contains

your solution.

How to Work on the Lab

Your dynamic storage allocator will consist of the following four functions, which are

declared in mm.h and dened

in mm.c .

int mm_init(void);

void *mm_malloc(size_t size);

void mm_free(void *ptr);

void *mm_realloc(void *ptr, size_t size);

The mm.c le

we have given you implements the simplest but still functionally correct

malloc package that we could think of. Using this as a starting place, modify these

functions (and possibly dene

other private static functions), so that they obey the

following semantics:

mm_init: Before calling mm_malloc mm_realloc 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 initializations, such as allocating the initial heap

area. The return value should be -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. The entire allocated block should lie within the heap region and

should not overlap with any other allocated chunk. We will comparing 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 8 bytes, your

malloc implementation should do likewise and always return 8-byte aligned pointers.

mm_free: The mm_free routine frees the block pointed to by ptr . It returns nothing.

This routine is only guaranteed to work when the passed pointer ( ptr ) was returned by

an earlier call to mm_malloc or mm_realloc and has not yet been freed.

mm_realloc: The mm_realloc routine returns a pointer to an allocated region of at

least size bytes with the following constraints.

if ptr is NULL, the call is equivalent to mm_malloc(size) ;

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if size is equal to zero, the call is equivalent to mm_free(ptr) ;

if ptr is not NULL, it must have been returned by an earlier call

to mm_malloc or mm_realloc . The call to mm_realloc changes the size of the memory

block pointed to by ptr (the {\em old block}) to size bytes and returns the address

of the new block. Notice that the address of the new block might be the same as the

old block, or it might be dierent,

depending on your implementation, the amount of

internal fragmentation in the old block, and the size of the realloc request. The

contents of the new block are the same as those of the old ptr block, up to the

minimum of the old and new sizes. Everything else is uninitialized. For example, if the

old block is 8 bytes and the new block is 12 bytes, then the rst

8 bytes of the new

block are identical to the rst

8 bytes of the old block and the last 4 bytes are

uninitialized. Similarly, if the old block is 8 bytes and the new block is 4 bytes, then the

contents of the new block are identical to the rst

4 bytes of the old block.

These semantics match the the semantics of the corresponding libc malloc realloc ,

and free routines. Type man malloc to the shell for complete documentation or just

google malloc .

Heap Consistency Checker

Dynamic memory allocators are notoriously tricky beasts to program correctly and

eciently.

They are dicult

to program correctly because they involve a lot of untyped

pointer manipulation. You will nd

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 int mm_check(void) in mm.c . It will check

any invariants or consistency conditions you consider prudent. It returns a nonzero value

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 mm_check fails.

This consistency checker is for your own debugging during development. When you

submit mm.c , make sure to remove any calls to mm_check as they will slow down your

throughput.

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Support Routines

The memlib.c package simulates the memory system for your dynamic memory allocator.

You can invoke the following functions in memlib.c :

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 rst

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 rst

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).

The Trace-driven Driver Program

The driver program mdriver.c in the malloclab-handout.tar distribution tests

your mm.c package for correctness, space utilization, and throughput. The driver program

is controlled by a set of trace les

that are included in the malloclabhandout.tar

distribution. Each trace le

contains a sequence of allocate, reallocate, and

free directions that instruct the driver to call your mm_malloc , mm_realloc ,

and mm_free routines in some sequence. The driver and the trace les

are the same ones

we will use when we grade your handin mm.c le.

The driver mdriver.c accepts the following command line arguments:

-t <tracedir> : Look for the default trace les

in directory tracedir instead of the

default directory dened

in config.h .

-f <tracefile> : Use one particular tracefile for testing instead of the default set of

traceles.

-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 tracele

in a compact

table.

-V : More verbose output. Prints additional diagnostic information as each trace le

is

processed. Useful during debugging for determining which trace le

is causing your

malloc package to fail.

Programming Rules

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You should not change any of the interfaces in mm.c .

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.

For consistency with the libc malloc package, which returns blocks aligned on 8-byte

boundaries,your allocator must always return pointers that are aligned to 8-byte boundaries.

The driver will enforce this requirement for you.

Evaluation

You'll be evaluated by having a functioning malloc .

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

where is your space utilization, is your throughput, and is the estimated

throughput of libc malloc on your system on the default traces. The value for is a

constant in the driver (600 Kops/s) that we established when we congured

the program.

Since we're using so many dierent

machines, you should take this as a nominal''

throughput for malloc on modern-day machines. The performance index favors space

utilization over throughput. default of w=0.6 .

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 or ( 100% ).

Since each metric will contribute at most w and 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.

There is a scoring program, called ./RUN-MM that will compile your program and run the

test cases. This will report an average score. Your grade on the "does it work" portion of the

machine problem is computed by the RUN-MM script using the average score.

The scoring function is based on using the https://coding.csel.io machines and specic

implementations as goals:

Score of 72, or ~65% is the implementation is the book, eshed

out and implemented.

This is a?rst

t''

implicit list allocator.

Score of 80, or ~90%, is a?next t''

implicit list allocator

Score of 84, or ~100%, is an?explicit'' list allocator (see 9.9.13)

Score of ~86, or ~105%, using?circular'' lists

Score of 95, or 110%, is a?tree based'' allocator

P = wU + (1 ? w)min(1, )

T

Tlibc

U T Tlibc

Tlibc

w = 0.6

P = w + (1 w) = 1

w 1 w

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You can earn up to 10% extra credit based on the score (i.e. a maximum score of 110).

Handin Instructions

You will handin a zip le

containing a valid mm.c, Makele

and other les

needed to

automatically build your program. You labs will be run by the TA's on the perf machine

and grades recorded using the grading script.

You must indicate the origin of any code your use or borrow from other people or sources. Failure

to properly attribute the origin of code you retrieve from any source is grounds for receiving a zero.

Hints

Use the mdriver -f option.During initial development, using tiny trace les

will

simplify debugging and testing. We have included two such trace les

(`short{1,2-

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 le.

The -V will also indicate when each trace le

is read, which

will help you isolate errors.

Compile with gcc -g and use a debugger.A debugger will help you isolate and

identify out of bounds memory references.

Understand every line of the malloc implementation in the textbook.The textbook

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. That's good for 70% on this

assignment.

Encapsulate your pointer arithmetic in C preprocessor macros or inline

functions.Pointer arithmetic in memory managers is confusing and error-prone

because of all the casting that is necessary. You can reduce the complexity signicantly

by writing macros or, better yet, inline functions for your pointer operations. See the

text for examples and look at the handout for provided samples.

Do your implementation in stages.The rst

9 traces contain requests

to malloc and free . The last 2 traces contain requests for realloc , malloc ,

and? free . We recommend that you start by getting your malloc and free routines

working correctly and eciently

on the rst

9 traces. Only then should you turn your

attention to the realloc implementation. For starters, we've built a realloc on top of

your existing malloc and free implementations. But to get really good performance,

you will need to build a stand-alone realloc .