So based on the second bullet I figure that directories are scalable because if there are, say, hundreds of processors with their individual caches, you only need to send a signal to the processors that are sharing that line in the cache. So if there are hundreds of processors you won't ALWAYS need to broadcast to every other processor, but may need to in the case that every processor happens to be sharing that line in the cache.
Now if that's the case, this seems like a major benefit. Is the only benefit to snooping that you don't need to have that space on the chip for the directory? I'm sure there are studies on this, but I feel like the positives of having extra memory outweigh the negatives of clogging the bandwidth on the bus.
So based on the second bullet I figure that directories are scalable because if there are, say, hundreds of processors with their individual caches, you only need to send a signal to the processors that are sharing that line in the cache. So if there are hundreds of processors you won't ALWAYS need to broadcast to every other processor, but may need to in the case that every processor happens to be sharing that line in the cache.
Now if that's the case, this seems like a major benefit. Is the only benefit to snooping that you don't need to have that space on the chip for the directory? I'm sure there are studies on this, but I feel like the positives of having extra memory outweigh the negatives of clogging the bandwidth on the bus.