Many Cores

Question 1

What are the challenges and solutions for many cores?

Answer 1

1. Bus limitations => network on chip + directory coherence
2. Increase in off-chip traffic => distributed LLC
3. Large coherence dir => on chip directory
4. Power budget split among cores => turbo boost frequency
5. OS must arbitrate chips/cores

Question 2

What is network-on-chip and what does it solve?

Answer 2

Solves bus bottleneck problem. Create point-to-point network of buses

Question 3

What is a mesh?

Answer 3

grid, where each core is connected to its horizontal and vertical neighbors

Question 4

What is a torus?

Answer 4

mesh network + connect the ends of each row / column to each other

Question 5

What is a distributed LLC and what does it try to solve?

Answer 5

Reducing off-chip traffic. All cores have same LLC and it is distributed across all cores.

Question 6

Ways to distributed LLC?

Answer 6

1. round-robin by cache index => evenly distributed traffic, but bad for locality
2. round-robin by page number => good for locality

Question 7

What is an on-chip directory?

Answer 7

Partial directory. If something is not in the LLC, then it is not in the cache anywhere and don’t need an entry for it (only keep entries for L1 and L2 items). Must still use replacement policy

Question 8

How are entries replaced in the on-chip directory?

Answer 8

1. Pick an entry to replace (LRU)
2. Invalidate all tiles for old entry
3. Put new entry
   This counts as another type of miss