3.2.3 Data coherency

Data coherency :
All copies of on data set include always the same data values.

• On distributed memory machines, the programmer is responsible for data coherency, see Sec. 4 and 5.
• On classical (elder) machines with huge shared memory, data coherency was guaranteed by semaphores from Sec. 3.2.2.
• Nowadays, also shared memory computers have a local cache on each processor (sometimes a distributed shared memory model is used for programming).
  Here, cache coherency is additionally required which is much harder to realize than the simple coherency. Several mechanisms/protocols are used to guarantee that cache coherency. Especially, the most recent distributed shared memory machines use an appropriate Cache Coherency NUMA model.
  
  snoopy-based protocol: Each process sends its memory accesses to the whole network. This leads to problems with the communication bandwidth.
  
  directory-based protocol: Each memory block (cache-line) possesses an entry in a table (KSR: sea of addresses) which stores the state of that block and a bit-vector including all processes with copies of that block. It can be determined by analysis of these entries, which cache have to be updated [SGI97].