22
Architecture (continued)
•Shared
memory
–Technically very difficult -- and therefore slow -- to keep memory
coherent
–DSM
can be implemented by a directory scheme
•Record
sharers/dirty features for each cache line
•Directory
can double the memory requirements of machines, though some simplifications are
possible
•Follow distributed version of coherency protocol because no bus for
defining “timing sequence”; use mem location
•Homework
showed: Even limited sharing can cause much traffic
•Invalidations,
acknowledgements increase with sharing
DSM best when not used, i.e. manage
mem yourself