title: cache coherency : snooping v directory based
body: <p>from what I understand: directory based system is more server centric design and snooping is more peer to peer centric.</p>
  
  <p>That is why directory based requires less messages for any read-miss as it can reach the processor who has the valid data after checking the directory.</p>
  
  <p>Snooping just asks every other processor which is n-1 messages.</p>
  
  <p>So directory based systems scales more.</p>
  
  <p>Now my questions is : if there is only one central directory then doesn't it face a bottleneck when too many processors are asking after a read miss?</p>