Problems in Hardware Prefetching

1
Problems in Hardware Prefetching

• Unnecessary data being prefetched will result in
  increased bus and memory traffic degrading
  performance for data not being used and for
  data arriving late
• Prefetched data may replace data in the processor
  working set Cache pollution problem
• Invalidation of prefetched data by other
  processors or DMA
• Summary Prefetch is necessary, but how to
  prefetch, which data to prefetch, and when to
  prefetch are questions that must be answered.

2
Problems Contd.

• Not all the data appear sequentially. How to
  avoid unnecessary data being prefetched? (1)
  Stride access for some scientific computations
  (2) Linked-list data how to detect and
  prefetch? (3) predict data from program behavior?
  EX. Mowrys software data prefetch through
  compiler analysis and prediction, Hardare
  Reference Table (RPT) by Chen and Baer, Markov
  model by
• How to limit cache pollution? Stream Buffer
  technique by Jouppi is extremely helpful. What is
  a stream buffer compared to a victim buffer?

3
Prefetching in Multiprocessors

• Large Memory access latency, particularly in
  CC-NUMA, so prefetching is more useful
• Prefetches increase memory and IN traffic
• Prefetching shared data causes additional
  coherence traffic
• Invalidation misses are not predictable at
  compile time
• Dynamic task scheduling and migration may create
  further problem for prefetching.