Reporting Snoop Results: When?
Memory needs to know what, if anything, to do
Fixed number of clocks from address appearing on bus
- Dual tags required to reduce contention with processor
- Still must be conservative (update both on write: E -> M)
- Pentium Pro, HP servers, Sun Enterprise
Variable delay
- Memory assumes cache will supply data till all say “sorry”
- Less conservative, more flexible, more complex
- Memory can fetch data and hold just in case (SGI Challenge)
Immediately: Bit-per-block in memory
- Extra hardware complexity in commodity main memory system