Multiprocessors II
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Lecture 25, 4/26/2005

Example of parallel program
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Sum of an array of number, pg. 9-11 from CD

. show a naive implementation (lots of false sharing)
. show better implementation
     show graphically how it works

Some synchronization models
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* Barrier -- everyone waits until they get there, then everyone continues
* Lock -- exclusive access (mutual exclusion)
* Producer/consumer -- pairwise ordering

Bus-based cache coherence
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* Figure of bus-based multiprocessor

* Talk about what happens if writes happen in multiple caches -- bad!

* Use bus to coordinate writes

* Two options:
  - write-update -- all writes go to bus
  - write-invalidate -- writing processor must acquire exclusive copy
		        prior to writing
			Conserves bus bandwidth, just like write-allocate cache

* MESI protocol
  M = modified
  E = exclusive
  S = shared
  I = invalid

* Go through example of a cache line, and its state for each cache
  - first its written by one processor
  - then read by lots of processors
  - then written by a different processor

* Multiprocessors also need support for atomic memory operations:
  e.g. test if 0, if so, set equal to 1.

Synchronization
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Atomic pair, pg. 9-20 from CD:
   . Load locked
   . Store conditional