Overview

References

• Slides adapted from CMU

Outline

• Course theme

• Realities of programming

Course Theme

• Systems Knowledge

  • How hardware combine to support the execution of application programs

  • How you as a programmer can best use these resources

• Useful outcomes from taking CSC 235

  • Become more effective programmers

  • Prepare for later systems courses

It is Important to Understand How Things Work

• Most CS courses emphasize abstraction

  • Abstract data types

  • Asymptotic analysis

• These abstractions have limits

  • Especially in the presence of bugs

  • Need to understand the details of underlying implementations

  • Sometimes the abstract interfaces do not provide the level of control or performance you need

Realities of Programming

• ints are not integers and floats are not reals

• knowing assembly is useful

• memory matters

• there is more to performance than asymptotic complexity

• computers do more than execute programs

ints and floats

• Example: is \(x^2 \geq 0\)?

  • float: yes

  • int: ?

• Example: is \((x + y) + z = x + (y + z)\)?

  • unsigned and signed ints: yes

  • floats: ?

Computer Arithmetic

• Arithmetic operations have important mathematical properties

• But, we cannot assume all the “usual” mathematical properties

  • due to finiteness of representations

  • integer operations satisfy “ring” properties: commutativity, associativity, and distributivity

  • floating point operations satisfy “ordering” properties: monotonicity and values of signs

• Observation

  • Need to understand which abstractions apply in which contexts

  • Import issues for compiler writers and serious application programmers

Knowing Assembly is Useful

• You will probably never write programs in assembly

  • compilers are typically much better and more patient than you are

• But, understanding assembly is key to the machine-level execution model

  • behavior of programs in the presence of bugs

  • tuning program performance

  • implementing system software

  • creating / fighting malware

Memory Matters

• Memory is not unbounded

  • it must be allocated and managed

  • many applications are memory dominated

• Memory referencing bugs are especially pernicious

  • effects are distant in both time and space

• Memory performance is not uniform

  • cache and virtual memory effects can greatly affect program performance

  • adapting program to characteristics of memory system can lead to major speed improvements

Memory Referencing Bug Example

• Code with a bug:

  typedef struct {
      int a[2];
      double d;
  } struct_t;
  
  double fun(int i) {
      volatile struct_t s;
      s.d = 3.14;
      s.a[i] = 1073741824; /* Possibly out of bounds */
      return s.d;
  }

• What is the result of fun(6)?

Memory Referencing Bug Example

Memory Referencing Errors

• C and C++ do not provide any memory protection

  • out of bounds array references

  • invalid pointer values

  • abuses of malloc/free

• Can lead to nasty bugs

  • Whether or not a bug has any effect depends on the system and compiler

• How do we deal with this?

  • program in a memory safe language

  • understand what possible interactions may occur

  • use or develop tools to detect referencing errors

Asymptotic Complexity and Performance

• Constant factors matter

• Exact operation count does not predict performance

  • must optimize at multiple levels: algorithm, data representation, procedures, and loops

• Must understand the system to optimize performance

  • how programs are compiled and executed

  • how to measure program performance and identify bottlenecks

  • how to improve performance without destroying code modularity and generality

Memory System Performance Example

• Slower

  void cpy(int src[2048][2048], int dst[2048][2048]) {
      for (int j = 0; j < 2048; j++)
          for (int i = 0; i < 2048; i++)
              dst[i][j] = src[i][j];
  }

• Faster

  void cpy(int src[2048][2048], int dst[2048][2048]) {
      for (int i = 0; i < 2048; j++)
          for (int j = 0; j < 2048; i++)
              dst[i][j] = src[i][j];
  }

Computers do more than execute programs

• They need to get data in and out

  • I/O system critical to program reliability and performance

• They communicate with each other with each other over networks

  • Many system-level issues arise in the presence of a network

    • concurrent operations by autonomous processes

    • coping with unreliable media

    • cross platform compatibility

    • complex performance issues

Course Perspective

• This course is programmer-centric

  • By knowing more about the underlying system, you can be more effective as a programmer

  • Enable you to write programs that are more reliable and efficient