Basic Cache Coherency Methods

• Multiple copies of data, spread throughout the caches, lead to a coherence problem among the caches. The copies in the caches are coherent if they all equal the same value.
• However, if one of the processors writes over the value of one of the copies, then the copy becomes inconsistent because it no longer equals the value of the other copies.
• If data are allowed to become inconsistent (incoherent), incorrect results will be propagated through the system, leading to incorrect final results.
• Cache coherence algorithms are needed to maintain a level of consistency throughout the parallel system.
• Cache coherence schemes can be categorized into two main categories: snooping protocols and directory-based protocols.
  • Snooping protocols are based on watching bus activities and carry out the appropriate coherency commands when necessary.
  • In cases when the broadcasting techniques used in snooping protocols are unpractical, coherence commands need to be sent to only those caches that might be affected by an update.
  • This is the idea behind directory-based protocols. Cache coherence protocols that somehow store information on where copies of blocks reside are called directory schemes.