Thread Models

1
Thread Models

• Preemptive threads
• Advantage Enforce fairness
• Cooperative threads
• Advantage Easy to understand
• Semi-cooperative threads
• Best of both worlds!

void thread_1(void) while (1)
list head NULL void thread_2(void) list
node mknode() node-gtnext head head
node
2
Semi-Cooperative Threads

• How it works
• Multiprocessor support
• Compiler tells runtime which code segments cannot
  be run concurrently
• Threads are scheduled accordingly
• Forced yields ) fewer constraints

Compiler
Inserts guarded yield points at regular intervals
Loader
Verifies that yield points are placed
appropriately
Runtime
Preempts by setting flag program yields at next
point
vs.
3
Applications

• Case 1 CCured
• Transformation can introduce bugs
• Case 2 Javas StringBuffer

int SEQ p1 int SEQ p2 p1 p2
seqp_int p1 seqp_int p2 p1._p p2._p p1._b
p2._b p1._e p2._e
public synchronized StringBuffer append(char ch)
ensureCapacity(count1) valuecount
ch return this
4
Building Better Threads

• Long-term goal Make threaded programming
  simple!
• Exploit semi-cooperative approach
• Commit changes to shared state using simple
  operations (e.g., pointer swap)
• Use optimistic concurrency control
• E.g., ensureCapacity()
• Improve communication between compiler and
  runtime system
• Semi-cooperative threads
• Linked stacks

Key Principle The compiler can make simplifying
assumptions on a per-program basis if it tells
the runtime system about those assumptions!