Speculative Multiprocessing

1
Speculative Multiprocessing

• 15-740 Computer Architecture
• Alex Groce
• Ioannis Koutis
• Martin Zinkevich
• November 6, 2000

2
What is the problem?

• We want to make a serial program run fast on
  several processors.
• It MUST appear to the user as if it was all run
  sequentially as written.
• Sometimes, dependencies are predictable
  simulations, array multiplications, etc. Why?
• When is it not so easy?

3
Possible Parallelism

• Hashing you are doing multiple manipulations on
  a hash table. They sometimes collide, but not
  often.
• Hash H
• H.add(dog, brown)
• H.add(cat, black)
• H.add(bag, green)
• H.add(house, blue)
• H.get(bag)

4
Possible Parallelism

• DAGs you want to do a depth-first traversal, and
  it is a tree-like structure, but not exactly.
• void traverse(TreeNode N)
• if (Nnull)
• return
• traverse(N.left)
• traverse(N.right)
• doStuff(N)

5
Thread-Level Speculation

• Thread-Level Speculation is taking sequential
  code and breaking it down into a sequence of
  epochs, and executing these epochs in parallel.
• How do we form the epochs?
• How do we detect data dependencies at runtime?