Choosing Parameters
Problem size and number of processors
- Use inherent characteristics considerations as discussed earlier
- For example, low c-to-c ratio will not allow block transfer to help much
- Suppose one size chosen is 514-by-514 grid with 16 processors
Cache/Replication Size
- Choose based on knowledge of working set curve
- Choosing cache sizes for given problem and machine size analogous to choosing problem sizes for given cache and machine size, discussed
- Whether or not working set fits affects block transfer benefits greatly
- if local data, not fitting makes communication relatively less important
- If nonlocal, can increase artifactual comm. So BT has more opportunity
- Sharp knees in working set curve can help prune space
- Knees can be determined by analysis or by very simple simulation