Code Camp: The Basics of Threading

The Basics of Threading

Speaker: Kim Greenlee

Started with an overview of Threads and the concept of a process space

• A Process consists of :
  • Heap
  • Code
  • Multiple Threads, each with a stack and registers
• Each app runs in its own process space
• Every process space gets at least one thread
• Each process space has a heap and code
• Process space creation is heavy (resource intensive)

If you want to share data between the process space, need some way to explicitly do it
i.e. need a way to allow different process spaces to share their heap and code

She talked about Replay Solutions (http://www.replaysolutions.com/) – who provide thread monitoring/debugging tools.

Issues with Threading

• Race condition – who gets there first
• Deadlock – one thread wating forever on another (e.g. thread A has x and needs y; Thread B has y and needs x)
• Starvation – Pick me! I need a turn
• Livelock – you go, no you go, no I insist you go… (Thread A has x and needs y; Thread B has y and needs x; So Thread A out x back, tries whole thing again, but with no success?) [Didn’t quite understand this one]
• Memory reordering – things aren’t always what they seem. [Didn’t quite understand this one]

Thread Costs (in .Net)

• Creation costs = 200,000 cycles
• Destruction Cost: 100,000 cycles
• i.e. super high!

Decomposition

• Task decomposition – break the task down in to multiple parts. e.g. do the slow, file based parts in a separate thread
• Data decomposition – break the data up in to chunks
• Messaging – passing messages back and forward? Rarely used outside of scientific circles. e.g. used in physics/fluid dynamics tests

Tips and Tricks

• Implementation Plan
• Plan for problems – Log files – i.e. have a logging system built in
• Testing Strategies –
  • debug in AND out of debugger as using debugger may cause different behaviour
  • Test on single and multi-processor testing
• Code Review – pair programming
• 3rd party tools – coverity? Replay solutions; Intel;

Overall:

A good overview/intro to threading. Kim clearly has a lot of hands of experience. All examples were in .Net/C#/VisualStudio, would have preferred Java, but obviously that just my personal bias.

Links

http://krgreenlee.blogspot.com/

Replay Solutions