CS 213 Introduction to Computer Systems Thinking Inside the Box

1
CS 213Introduction to Computer SystemsThinking
Inside the Box

• Instructor Brian M. Dennis
• bmd_at_cs.northwestern.edu
• Teaching Assistant Bin Lin
• binlin_at_cs.northwestern.edu

2
Today's Topics

• Dynamic memory management schemes
• Intro to UNIX I/O

3
Malloc Package

• include ltstdlib.hgt
• void malloc(size_t size)
• If successful
• Returns a pointer to a memory block of at least
  size bytes, (typically) aligned to 8-byte
  boundary.
• If size 0, returns NULL
• If unsuccessful returns NULL (0) and sets errno.
• void free(void p)
• Returns the block pointed at by p to pool of
  available memory
• p must come from a previous call to malloc or
  realloc.
• void realloc(void p, size_t size)
• Changes size of block p and returns pointer to
  new block.
• Contents of new block unchanged up to min of old
  and new size.

4
Assumptions

• Assumptions
• Memory is word addressed (each word can hold a
  pointer, probably 4 bytes)

Free word
Allocated block (4 words)
Free block (3 words)
Allocated word
5
Constraints

• Applications
• Can issue arbitrary sequence of allocation and
  free requests
• Free requests must correspond to an allocated
  block
• Allocators
• Cant control number or size of allocated blocks
• Must respond immediately to all allocation
  requests
• i.e., cant reorder or buffer requests
• Must allocate blocks from free memory
• i.e., can only place allocated blocks in free
  memory
• Must align blocks so they satisfy all alignment
  requirements
• 8 byte alignment for GNU malloc (libc malloc) on
  Linux boxes
• Can only manipulate and modify free memory
• Cant move the allocated blocks once they are
  allocated
• i.e., compaction is not allowed

6
Goals of Good malloc/free

• Primary goals
• Good time performance for malloc and free
• Ideally should take constant time (not always
  possible)
• Should certainly not take linear time in the
  number of blocks
• Good space utilization
• User allocated structures should be large
  fraction of the heap.
• Want to minimize fragmentation.
• Some other goals
• Good locality properties
• Structures allocated close in time should be
  close in space
• Similar objects should be allocated close in
  space
• Robust
• Obeys constraints!!
• Can check that free(p1) is on a valid allocated
  object p1
• Can check that memory references are to allocated
  space

7
Performance Goals Throughput

• Given some sequence of malloc and free requests
• R0, R1, ..., Rk, ... , Rn-1
• Want to maximize throughput and peak memory
  utilization.
• These goals are often conflicting
• Throughput
• Number of completed requests per unit time
• Example
• 5,000 malloc calls and 5,000 free calls in 10
  seconds
• Throughput is 1,000 operations/second.

8
Performance Goals Peak Memory Utilization

• Given some sequence of malloc and free requests
• R0, R1, ..., Rk, ... , Rn-1
• Def Aggregate payload Pk
• malloc(p) results in a block with a payload of p
  bytes..
• After request Rk has completed, the aggregate
  payload Pk is the sum of currently allocated
  payloads.
• Def Current heap size is denoted by Hk
• Assume that Hk is monotonically nondecreasing
• Def Peak memory utilization
• After k requests, peak memory utilization is
• Uk ( maxiltk Pi ) / Hk

9
Internal Fragmentation

• Poor memory utilization caused by fragmentation.
• Comes in two forms internal and external
  fragmentation
• Internal fragmentation
• For some block, internal fragmentation is the
  difference between the block size and the payload
  size.
• Caused by overhead of maintaining heap data
  structures, padding for alignment purposes, or
  explicit policy decisions (e.g., not to split the
  block).
• Depends only on the pattern of previous requests,
  and thus is easy to measure.

block
Internal fragmentation
payload
Internal fragmentation
10
External Fragmentation
p1 malloc(4)
Occurs when there is enough aggregate heap
memory, but no single free block is large enough
p2 malloc(5)
p3 malloc(6)
free(p2)
p4 malloc(7)
oops!
External fragmentation depends on the pattern of
future requests, and thus is difficult to
measure.
11
Implementation Issues

• How do we keep track of the free blocks?
• How do we pick a block to use for allocation --
  many might fit?
• What do we do with the extra space when
  allocating a structure that is smaller than the
  free block it is placed in?
• How do we know how much memory to free just given
  a pointer?
• How do we reinsert freed block?

p0
free(p0)
p1 malloc(1)
12
Knowing How Much to Free

• Standard method (influences rest of decisions)
• Keep the length of a block in the word preceding
  the block.
• This word is often called the header field or
  header
• Requires an extra word for every allocated block

p0 malloc(4)
p0
5
free(p0)
Block size
data
13
Keeping Track of Free Blocks

• Method 1 Implicit list using lengths -- links
  all blocks
• Method 2 Explicit list among the free blocks
  using pointers within the free blocks
• Method 3 Segregated free list
• Different free lists for different size classes
• Method 4 Blocks sorted by size
• Can use a balanced tree (e.g. Red-Black tree)
  with pointers within each free block, and the
  length used as a key

5
4
2
6
5
4
2
6
14
Method 1 Implicit List

• Need to identify whether each block is free or
  allocated
• Can use extra bit
• Bit can be put in the same word as the size if
  block sizes are always multiples of two (mask out
  low order bit when reading size).

1 word
a 1 allocated block a 0 free block size
block size payload application data (allocated
blocks only)
size
a
payload
Format of allocated and free blocks
optional padding
15
Implicit List Finding a Free Block

• First fit
• Search list from beginning, choose first free
  block that fits
• Can take linear time in total number of blocks
  (allocated and free)
• In practice it can cause splinters at beginning
  of list
• Next fit
• Like first-fit, but search list from location of
  end of previous search
• Research suggests that fragmentation is worse
• Best fit
• Search the list, choose the free block with the
  closest size that fits
• Keeps fragments small --- usually helps
  fragmentation
• Will typically run slower than first-fit

void findblock(size_t len) p start while
((p lt end) \\ not passed end ((p
1) \\ already allocated (p lt
len))) \\ too small addblock(p, len)
16
Implicit List Allocating in Free Block

• Allocating in a free block - splitting
• Since allocated space might be smaller than free
  space, we might want to split the block

4
4
2
6
p
void addblock(ptr p, int len) int newsize
((len 1) gtgt 1) ltlt 1 // add 1 and round up
int oldsize p (0x1) // mask out low
bit p newsize 1 //
set new length if (newsize lt oldsize)
(pnewsize) oldsize - newsize // set length
in remaining
// part of block
addblock(p, 2)
2
4
2
4
4
17
Implicit List Freeing a Block

• Simplest implementation
• Only need to clear allocated flag
• void free_block(ptr p) p p -2
• But can lead to false fragmentation
• There is enough free space, but the allocator
  wont be able to find it

2
4
2
4
p
free(p)
2
4
4
2
4
malloc(5)
Oops!
18
Implicit List Coalescing

• Join (coelesce) with next and/or previous block
  if they are free
• Coalescing with next block
• But how do we coalesce with previous block, if
  need be?

void free_block(ptr p) p p -2
// clear allocated flag next p p
// find next block if ((next 1) 0)
p p next // add to this block if
// not allocated
2
4
2
4
p
free(p)
4
4
2
6
19
Implicit List Bidirectional Coalescing

• Boundary tags Knuth73
• Replicate size/allocated word at bottom of free
  blocks
• Allows us to traverse the list backwards, but
  requires extra space
• Important and general technique!

1 word
Header
size
a
a 1 allocated block a 0 free block size
total block size payload application
data (allocated blocks only)
payload and padding
Format of allocated and free blocks
size
a
Boundary tag (footer)
4
4
4
4
6
4
6
4
20
Constant Time Coalescing
Case 1
Case 2
Case 3
Case 4
allocated
allocated
free
free
block being freed
allocated
free
allocated
free
21
Constant Time Coalescing (Case 1)
m1
1
m1
1
m1
1
m1
1
n
1
n
0
n
1
n
0
m2
1
m2
1
m2
1
m2
1
22
Constant Time Coalescing (Case 2)
m1
1
m1
1
m1
1
m1
1
nm2
0
n
1
n
1
m2
0
nm2
0
m2
0
23
Constant Time Coalescing (Case 3)
m1
0
nm1
0
m1
0
n
1
n
1
nm1
0
m2
1
m2
1
m2
1
m2
1
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Constant Time Coalescing (Case 4)
m1
0
nm1m2
0
m1
0
n
1
n
1
m2
0
m2
0
nm1m2
0
25
Keeping Track of Free Blocks

• Method 1 Implicit list using lengths -- links
  all blocks
• Method 2 Explicit list among the free blocks
  using pointers within the free blocks
• Method 3 Segregated free list
• Different free lists for different size classes
• Method 4 Blocks sorted by size
• Can use a balanced tree (e.g. Red-Black tree)
  with pointers within each free block, and the
  length used as a key

5
4
2
6
5
4
2
6
26
Explicit Free Lists

• Use data space for link pointers
• Typically doubly linked
• Still need boundary tags for coalescing
• It is important to realize that links are not
  necessarily in the same order as the blocks

Forward links
A
B
4
4
4
4
6
6
4
4
4
4
C
Back links
27
Allocating From Explicit Free Lists
pred
succ
free block
Before
pred
succ
After (with splitting)
free block
28
Freeing With Explicit Free Lists

• Insertion policy Where in the free list do you
  put a newly freed block?
• LIFO (last-in-first-out) policy
• Insert freed block at the beginning of the free
  list
• Pro simple and constant time
• Con studies suggest fragmentation is worse than
  address ordered.
• Address-ordered policy
• Insert freed blocks so that free list blocks are
  always in address order
• i.e. addr(pred) lt addr(curr) lt addr(succ)
• Con requires search
• Pro studies suggest fragmentation is better
  than LIFO

29
Freeing With a LIFO Policy

• Case 1 a-a-a
• Insert self at beginning of free list
• Case 2 a-a-f
• Splice out next, coalesce self and next, and add
  to beginning of free list

pred (p)
succ (s)
self
a
a
p
s
before
self
a
f
p
s
after
f
a
30
Freeing With a LIFO Policy (cont)
p
s

• Case 3 f-a-a
• Splice out prev, coalesce with self, and add to
  beginning of free list
• Case 4 f-a-f
• Splice out prev and next, coalesce with self, and
  add to beginning of list

before
self
f
a
p
s
after
f
a
p1
s1
p2
s2
before
self
f
f
p1
s1
p2
s2
after
f
31
Explicit List Summary

• Comparison to implicit list
• Allocate is linear time in number of free blocks
  instead of total blocks -- much faster allocates
  when most of the memory is full
• Slightly more complicated allocate and free since
  needs to splice blocks in and out of the list
• Some extra space for the links (2 extra words
  needed for each block)
• Main use of linked lists is in conjunction with
  segregated free lists
• Keep multiple linked lists of different size
  classes, or possibly for different types of
  objects

32
Keeping Track of Free Blocks

• Method 1 Implicit list using lengths -- links
  all blocks
• Method 2 Explicit list among the free blocks
  using pointers within the free blocks
• Method 3 Segregated free list
• Different free lists for different size classes
• Method 4 Blocks sorted by size
• Can use a balanced tree (e.g. Red-Black tree)
  with pointers within each free block, and the
  length used as a key

5
4
2
6
5
4
2
6
33
Segregated Storage

• Each size class has its own collection of blocks

• Often have separate size class for every small
  size (2,3,4,)
• For larger sizes typically have a size class for
  each power of 2

34
Simple Segregated Storage

• Separate heap and free list for each size class
• No splitting
• To allocate a block of size n
• If free list for size n is not empty,
• allocate first block on list (note, list can be
  implicit or explicit)
• If free list is empty,
• get a new page
• create new free list from all blocks in page
• allocate first block on list
• Constant time
• To free a block
• Add to free list
• If page is empty, return the page for use by
  another size (optional)
• Tradeoffs
• Fast, but can fragment badly

35
Segregated Fits

• Array of free lists, each one for some size class
• To allocate a block of size n
• Search appropriate free list for block of size m
  gt n
• If an appropriate block is found
• Split block and place fragment on appropriate
  list (optional)
• If no block is found, try next larger class
• Repeat until block is found
• To free a block
• Coalesce and place on appropriate list (optional)
• Tradeoffs
• Faster search than sequential fits (i.e., log
  time for power of two size classes)
• Controls fragmentation of simple segregated
  storage
• Coalescing can increase search times
• Deferred coalescing can help
• Popular implementation strategy e.g. GNU malloc

36
Keeping Track of Free Blocks

• Method 1 Implicit list using lengths -- links
  all blocks
• Method 2 Explicit list among the free blocks
  using pointers within the free blocks
• Method 3 Segregated free list
• Different free lists for different size classes
• Method 4 Blocks sorted by size
• Can use a balanced tree (e.g. Red-Black tree)
  with pointers within each free block, and the
  length used as a key

5
4
2
6
5
4
2
6
37
Summary of Key Allocator Policies

• Placement policy (finding free blocks)
• First fit, next fit, best fit, etc.
• Trades off lower throughput for less
  fragmentation
• Interesting observation segregated free lists
  approximate a best fit placement policy without
  having the search entire free list.
• Splitting policy (increase utilization, decreases
  throughput)
• When do we go ahead and split free blocks?
• How much internal fragmentation are we willing to
  tolerate?
• Coalescing policy (increase utilization,
  decreases throughput)
• Immediate coalescing coalesce adjacent blocks
  each time free is called
• Deferred coalescing try to improve performance
  of free by deferring coalescing until needed.
  e.g.,
• Coalesce as you scan the free list for malloc.
• Coalesce when the amount of external
  fragmentation reaches some threshold.

38
Transition to I/O

• Last bit of a process to clean things up

39
Unix Files

• A Unix file is a sequence of m bytes
• B0, B1, .... , Bk , .... , Bm-1
• Named using a rooted hierarchy

• All I/O devices are represented as files
• /dev/sda2 (/usr disk partition)
• /dev/tty2 (terminal)
• Even the kernel is exposed as a file
• /dev/kmem (kernel memory image)
• /proc (kernel data structures)

40
Unix File Types

• Regular file
• Binary or text file.
• Unix does not know the difference!
• Directory file
• A file that contains the names and locations of
  other files.
• Character special and block special files
• Terminals (character special) and disks ( block
  special)
• FIFO (named pipe)
• A file type used for interprocess communication
• Socket
• A file type used for network communication
  between processes

41
Unix I/O

• The elegant mapping of files to devices allows
  kernel to export simple interface called Unix
  I/O.
• Key Unix idea All input and output is handled in
  a consistent and uniform way.

• Low level Unix I/O API (system calls)
• Opening and closing files
• open()and close()
• Changing the current file position (seek)
• lseek()
• Reading and writing a file
• read() and write()

42
Opening Files

• Opening a file informs the kernel that you are
  getting ready to access that file.
• Returns a small identifying integer file
  descriptor
• fd -1 indicates that an error occurred
• Each process created by a Unix shell begins life
  with three open files associated with a terminal
• 0 standard input (read only)
• 1 standard output (write only)
• 2 standard error (write only)

int fd / file descriptor / if ((fd
open(/etc/hosts, O_RDONLY)) lt 0)
perror(open) exit(1)
43
Closing Files

• Closing a file informs the kernel that you are
  finished accessing that file.
• Always check return codes, even for seemingly
  benign functions such as close()

int fd / file descriptor / int retval /
return value / if ((retval close(fd)) lt 0)
perror(close) exit(1)
44
Reading Files

• Reading a file copies bytes from the current file
  position to memory, and then updates file
  position.
• Returns number of bytes read from file fd into
  buf
• nbytes lt 0 indicates that an error occurred.
• short counts (nbytes lt sizeof(buf) ) are possible
  and are not errors!

char buf512 int fd / file descriptor
/ int nbytes / number of bytes read / /
Open file fd ... / / Then read up to 512 bytes
from file fd / if ((nbytes read(fd, buf,
sizeof(buf))) lt 0) perror(read)
exit(1)
45
Writing Files

• Writing a file copies bytes from memory to the
  current file position, and then updates current
  file position.
• Returns number of bytes written from buf to file
  fd.
• nbytes lt 0 indicates that an error occurred.
• As with reads, short counts are possible and are
  not errors!
• Transfers up to 512 bytes from address buf to
  file fd

char buf512 int fd / file descriptor
/ int nbytes / number of bytes read / /
Open the file fd ... / / Then write up to 512
bytes from buf to file fd / if ((nbytes
write(fd, buf, sizeof(buf)) lt 0)
perror(write) exit(1)
46
Unix I/O Example

• Copying standard input to standard output one
  byte at a time.
• Note the use of error handling wrappers for read
  and write (Appendix B).

include "csapp.h" int main(void) char
c while(Read(STDIN_FILENO, c, 1) ! 0)
Write(STDOUT_FILENO, c, 1) exit(0)
47
Dealing with Short Counts

• Short counts can occur in these situations
• Encountering (end-of-file) EOF on reads.
• Reading text lines from a terminal.
• Reading and writing network sockets or Unix
  pipes.
• Short counts never occur in these situations
• Reading from disk files (except for EOF)
• Writing to disk files.
• How should you deal with short counts in your
  code?
• Use the RIO (Robust I/O) package from your
  textbooks csapp.c file (Appendix B).

48
The RIO Package

• RIO is a set of wrappers that provide efficient
  and robust I/O in applications such as network
  programs that are subject to short counts.
• RIO provides two different kinds of functions
• Unbuffered input and output of binary data
• rio_readn and rio_writen
• Buffered input of binary data and text lines
• rio_readlineb and rio_readnb
• Cleans up some problems with Stevenss readline
  and readn functions.
• Unlike the Stevens routines, the buffered RIO
  routines are thread-safe and can be interleaved
  arbitrarily on the same descriptor.
• Download from csapp.cs.cmu.edu/public/ics/code/src
  /csapp.c csapp.cs.cmu.edu/public/ics/code/include/
  csapp.h

49
Unbuffered RIO Input and Output

• Same interface as Unix read and write
• Especially useful for transferring data on
  network sockets
• rio_readn returns short count only if it
  encounters EOF.
• rio_writen never returns a short count.
• Calls to rio_readn and rio_writen can be
  interleaved arbitrarily on the same descriptor.

include csapp.h ssize_t rio_readn(int fd,
void usrbuf, size_t n) ssize_t rio_writen(nt
fd, void usrbuf, size_t n) Return num.
bytes transferred if OK, 0 on EOF (rio_readn
only), -1 on error
50
Implementation of rio_readn
/ rio_readn - robustly read n bytes
(unbuffered) / ssize_t rio_readn(int fd, void
usrbuf, size_t n) size_t nleft n
ssize_t nread char bufp usrbuf
while (nleft gt 0) if ((nread read(fd, bufp,
nleft)) lt 0) if (errno EINTR) /
interrupted by sig
handler return / nread 0 / and
call read() again / else return -1
/ errno set by read() / else if (nread
0) break / EOF / nleft -
nread bufp nread return (n -
nleft) / return gt 0 /
51
Buffered RIO Input Functions

• Efficiently read text lines and binary data from
  a file partially cached in an internal memory
  buffer
• rio_readlineb reads a text line of up to maxlen
  bytes from file fd and stores the line in usrbuf.
• Especially useful for reading text lines from
  network sockets.
• rio_readnb reads up to n bytes from file fd.
• Calls to rio_readlineb and rio_readnb can be
  interleaved arbitrarily on the same descriptor.
• Warning Dont interleave with calls to rio_readn

include csapp.h void rio_readinitb(rio_t rp,
int fd) ssize_t rio_readlineb(rio_t rp, void
usrbuf, size_t maxlen) ssize_t rio_readnb(rio_t
rp, void usrbuf, size_t n)
Return num. bytes read if OK, 0 on EOF, -1 on
error
52
RIO Example

• Copying the lines of a text file from standard
  input to standard output.

include "csapp.h" int main(int argc, char
argv) int n rio_t rio char
bufMAXLINE Rio_readinitb(rio,
STDIN_FILENO) while((n Rio_readlineb(rio,
buf, MAXLINE)) ! 0) Rio_writen(STDOUT_FILENO,
buf, n) exit(0)
53
Pros and Cons of Unix I/O

• Pros
• Unix I/O is the most general and lowest overhead
  form of I/O.
• All other I/O packages are implemented using Unix
  I/O functions.
• Unix I/O provides functions for accessing file
  metadata.
• Cons
• Dealing with short counts is tricky and error
  prone.
• Efficient reading of text lines requires some
  form of buffering, also tricky and error prone.
• Both of these issues are addressed by the
  standard I/O and RIO packages.

54
Wrapup

• Dynamic memory allocation
• Key policies
• Placement
• Splitting
• Coalescing
• UNIX I/O
• Everything's a file
• Accessed through descriptors
• read/write/close
• Short reads are an issue

• Reading
• 10.9 10.11
• 11.1 11.4
• Pickup HW4 Malloclab
• Have a good turkey day!
• No class on Wednesday