Abstract Modeling

Summary

• Multiple threads
  • Sometimes called processes
• Single shared memory
• Objects live in memory
• Unpredictable asynchronous delays

Programmers’ View

• Parallel composition, shared memory & interleaving semantics
• Locks & synchronization operations
• Concurrent objects and their clients

• Parrellel composition: $C1||C2$
• Shared memory & Interleaving semantics
  • Which is non-deterministic

<aside> 💡 Problems

Non-deterministic ⇒ Difficult to find & reproduce a bug

Solution ⇒ Using locks (Mutex = Mutual Exclusion)

</aside>

A concurrent program

= concurrent objects + their clients

Another example

• Concurrent Objects ⇒ written as sequential programs

  

<aside> 💡 Asynchrony

Sudden unpredictable delays

• Cache misses (short)
• Page faults (long)
• Scheduling quantum used up (really long) </aside>

Memory Models

A memory model describes the writes and reads of threads

Interleaving semantics ⇒ Sequential Consistency (SC) model

• Operational semantics
•  No multicore processor implements SC
•  Compiler optimizations invalidate SC

(Weak/Relaxed) Memory Models

• Every hardware architecture has its own WMM
• WMMs like SB and LB can cause result different than SC model