Introduction#

1. Overview#

  • Systems knowledge is power!

    • How hardware (processors, memories, disk drives, network infrastructure) plus software (operating systems, compilers, libraries, network protocols) combine to support the execution of application programs.

    • How general-purpose software developers can best use these resources.

    • A new specialization: systems programming.

2. Understand how things work#

  • Why do I need to know this stuff?

    • Abstraction is good, but don’t forget reality

  • Most CS courses emphasize abstraction

    • Abstract data types

    • Asymptotic analysis

  • These abstractions have limits

    • Especially in the presence of bugs

    • Need to understand details of underlying implementations

    • Sometimes the abstract interfaces don’t provide the level of control or performance you need

3. Hands-on: Getting started#

Log into molly and run the following commands:

  • Create a directory called csc231 in your home directory using the mkdir command.

  • Change into that directory using the cd command.

mkdir csc231
cd csc231

  • Check that you are inside csc231 using the pwd command.

  • Clone the Git repository for the class’ examples.

pwd
git clone https://github.com/CSC231-WCU/examples.git

  • The git clone command will download the repository into a directory called examples inside your current directory, which is csc231.

  • Run the command ls -l to confirm that examples exists.

  • Change into examples using cd.

  • Run ls -l to see how many subdirectories there are inside examples.

ls -l
cd examples
ls -l

  • Change into the directory 01-intro.

  • Compile and run the example code nums.c.

cd 01-intro
gcc -Wno-overflow nums.c
./a.out

Compile and run nums.c

4. Computer arithmetic#

  • Does not generate random values

    • Arithmetic operations have important mathematical properties.

  • Cannot assume all usual mathematical properties.

    • Due to finiteness of representations.

    • Integer operations satisfy ring properties: commutativity, associativity, distributivity.

    • Floating point operations satisfy ordering properties: monotonicity, values of signs.

  • Observation

    • Need to understand which abstractions apply in which contexts.

    • Important issues for compiler writers and application programmers.

5. Assembly#

  • You are more likely than not to never write programs in assembly.

    • Compilers take care of this for you.

  • Understand assembly is key to machine-level execution model.

    • Behavior of programs in presence of bugs

      • High-level language models break down

    • Tuning program performance

      • Understand optimizations done / not done by the compiler

      • Understanding sources of program inefficiency

    • Implementing system software

      • Compiler has machine code as target

      • Operating systems must manage process state

    • Creating / ../fighting malware

      • x86 assembly is the language of choice!

6. Memory Matters#

  • Random Access Memory is an un-physical abstraction.

  • Memory is not unbounded.

    • It must be allocated and managed.

    • Many applications are memory dominated.

  • Memory referencing bugs are especially pernicious

    • Pernicious: having a harmful effect, especially in a gradual or subtle way.

    • 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

7. Hands-on: Memory referencing bug#

  • We are still inside examples\intro-01 directory from Hands-on 1.

gcc mem1.c
./a.out

Compile and run mem1.c

8. Memory referencing errors#

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

    • Out of bounds array references

    • Invalid pointer values

    • Abuses of malloc and free

  • Can lead to nasty bugs

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

    • Action at a distance

      • Corrupted object logically unrelated to one being accessed

      • Effect of bug may be first observed long after it is generated

  • How can I deal with this?

    • Program in Java, Ruby, Python, ML, …

    • Understand what possible interactions may occur

    • Use or develop tools to detect referencing errors (e.g. Valgrind)

9. Beyond asymptotic complexity#

  • Constant factors matter!

  • Exact op count does not predict performance.

    • Possible 10:1 performance range depending on how code written (given same op count).

    • Optimizations must happen at multiple level: algorithm, data representations, procedures, and loop.

  • Must understand system to optimize performance

    • How programs compiled and executed.

    • How to measure program performance and identify bottlenecks.

    • How to improve performance without destroying code modularity and generality.

10. Hands-on: Memory system performance#

  • We are still inside examples\intro-01 directory from Hands-on 1.

gcc mem2.c
./a.out

Compile and run mem2.c

11. Does computer just execute arithmetic and control flow operations?#

  • They need to get data in and out

    • I/O system critical to program reliability and performance

  • They communicate with each other over networks

    • Many system-level issues arise in presence of network

    • Concurrent operations by autonomous processes

    • Coping with unreliable media

    • Cross platform compatibility

    • Complex performance issues

12. Layered Services#

  • Direct communication between applications and hardware components are impractical due to complexity.

  • Operating systems provide much-needed interfaces between applications and hardware through:

    • OS/application interface: system calls.

    • HW/SW interface: x86 standard and device drivers.

  • Systems programming: develop software systems that …

    • are composed of multiple modules

    • are complex

    • meets specific requirements in aspects such as performance, security, or fault tolerance.

Layered service design.c