buzzword
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buzzword [2015/03/30 23:15] rachata |
buzzword [2015/04/01 18:17] rachata |
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| * Need runahead cache to handle load/store in Runahead mode (since they are speculative) | * Need runahead cache to handle load/store in Runahead mode (since they are speculative) | ||
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| + | ===== Lecture 25 (4/1 Wed.) ===== | ||
| + | |||
| + | * More Runahead executions | ||
| + | * How to support runahead execution? | ||
| + | * Need a way to checkpoing the state when entering runahead mode | ||
| + | * How to make executing in the wrong path useful? | ||
| + | * Need runahead cache to handle load/store in Runahead mode (since they are speculative) | ||
| + | * Cost and benefit of runahead execution (slide number 27) | ||
| + | * Runahead can have inefficiency | ||
| + | * Runahead period that are useless | ||
| + | * Get rid of useless inefficient period | ||
| + | * What if there is a dependent cache miss | ||
| + | * Cannot be paralellized in a vanilla runahead | ||
| + | * Can predict the value of the dependent load | ||
| + | * How to predict the address of the load | ||
| + | * Delta value information | ||
| + | * Stride predictor | ||
| + | * AVD prediction | ||
| + | * Questions regarding prefetching | ||
| + | * What to prefetch | ||
| + | * When to prefetch | ||
| + | * how do we prefetch | ||
| + | * where to prefetch from | ||
| + | * Prefetching can cause thrasing (evict a useful block) | ||
| + | * Prefetching can also be useless (not being used) | ||
| + | * Need to be efficient | ||
| + | * Can cause memory bandwidth problem in GPU | ||
| + | * Prefetch the whole block, more than one block, or subblock? | ||
| + | * Each one of them has pros and cons | ||
| + | * Big prefetch is more likely to waste bandwidth | ||
| + | * Commonly done in a cache block granularity | ||
| + | * Prefetch accuracy: fraction of useful prefetches out of all the prefetches | ||
| + | * Prefetcher usually predict based on | ||
| + | * Past knowledge | ||
| + | * Compiler hints | ||
| + | * Prefetcher has to prefetch at the right time | ||
| + | * Prefetch that is too early might get evicted | ||
| + | * It might also evict other useful data | ||
| + | * Prefetch too late does not hide the whole memory latency | ||
| + | * Previous prefetches at the same PC can be used as the history | ||
| + | * Previous demand requests also is a good information to use for prefetches | ||
| + | * Prefetch buffer | ||
| + | * Place the prefetch data to avoid thrashing | ||
| + | * Can treat demand/prefetch requests separately | ||
| + | * More complex | ||
| + | * Generally, demand block is more important | ||
| + | * This means eviction should prefer prefetch block as oppose to demand block | ||
| + | * Tradeoffs between where do we place the prefetcher | ||
| + | * Look at L1 hits and misses | ||
| + | * Look at L1 misses only | ||
| + | * Look at L2 misses | ||
| + | * Different access pattern affect accuracy | ||
| + | * Tradeoffs between handling more requests (seeing L1 hits and misses) and less visibility (only see L2 miss) | ||
| + | * Software vs. hardware vs. execution based prefetching | ||
| + | * Software: ISA previde prefetch instructions, software utilize it | ||
| + | * What information are useful | ||
| + | * How to make sure the prefetch is timely | ||
| + | * What if you have a pointer based structure | ||
| + | * Not easy to prefetch pointer chasing (because in many case the work between prefetches is short, so you cannot predict the next one timely enough) | ||
| + | * Can be solved by hinting the nextnext and/or nextnextnext address | ||
| + | * Hardware: Identify the pattern and prefetch | ||
| + | * Execution driven: Oppotunistically try to prefetch (runahead, dual-core execution) | ||
| + | * Stride prefetcher | ||
| + | * Predict strides, which is common in many programs | ||
| + | * Cache block based or instruction based | ||
| + | * Stream buffer design | ||
| + | * Buffer the stream of accesses (next address) | ||
| + | * Use the information to prefetch | ||
| + | * What affect prefetcher performance | ||
| + | * Prefetch distance | ||
| + | * How far ahead should we prefetch | ||
| + | * Prefetch degree | ||
| + | * How many prefetches do we prefetch | ||
| + | * Prefetcher performance | ||
| + | * Coverage | ||
| + | * Out of the demand requests, how many are actually from the prefetch request | ||
| + | * Accuracy | ||
| + | * Out of all the prefetch requests, how many are actually getting used | ||
| + | * Timeliness | ||
| + | * How much memory latency can we hide from the prefetch requests | ||
| + | * Cache pullition | ||
| + | * How much did the prefetcher cause misses in the demand misses? | ||
| + | * Hard to quantify | ||
buzzword.txt · Last modified: 2015/04/27 18:20 by rachata
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