TddAndConcurrency.Slides.RandomStuff

TDD & Concurrency

Modern Processors have a single instruction

IncrementValue() {
   int v;
   do {
      v = get current value
   } while(v != compareAndSwap(v, v+1))
}

Keep attempting to increment value until it actually happens
If processor does not support CAS operation, Java compiler simulates it.
- If this is the case, high-contention systems will burn CPU time
  
  Conditions for Deadlock
Mutual Exclusion
- Multiple threads want to use same (limited) resource
- Cannot be used at same time
- E.g. A database connection
Lock and Wait
- Require multiple non-sharable resources
- Grab one at a time
- Hold until have them all
No Preemption
- One process cannot take another process’s resource
- Or force another process to release resources
Circular Wait
- At least two resources required by at least two threads
  
  Avoiding Deadlock
Break any of the 4 conditions
- Makes deadlock impossible
- Has performance and responsiveness ramifications
Mutual Exclusion
- Don’t lock mutual resources
- Don’t share
- Make more available
- Only lock one at a time
Lock and Wait
- Don’t wait – back-off if everything not available
No preemption
- Demand threads to release
- DO NOT use Thread.stop() – can leave resources locked
Circular Wait
- Order your locks