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Approximate Caches for Packet Classification

Approximate Caches for Packet Classification. Author: Francis Chang, Wu- chang Feng , Kang Li Publisher: INFOCOM 2004 Presenter: Yun -Yan Chang Date: 2010/12/01. Outline. Introduction The Bloom filter Extension of Bloom filter Bloom filter aging. Introduction.

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Approximate Caches for Packet Classification

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  1. Approximate Caches for Packet Classification Author: Francis Chang, Wu-changFeng, Kang Li Publisher: INFOCOM 2004 Presenter: Yun-Yan Chang Date: 2010/12/01

  2. Outline • Introduction • The Bloom filter • Extension of Bloom filter • Bloom filter aging

  3. Introduction • Provides a modified Bloom filterallowing a small amount of misclassification can decrease the size of packet classification cache without reducing hit rates.

  4. The Bloom Filter • A space-efficient data structure to store and query set-membership information. • The data structureconsists of M = N × L bins, bins are organized into L levels with N bins in eachlevel, to create NLvirtual bins. • Each L functions can address all M bit buckets.

  5. The Bloom Filter Figure 1: An example: A Bloom filter with N = 5 bins and L = 3 hash levels. Suppose we wish to insert an element, e .

  6. Extension of Bloom Filters • Multiple Predicates • Goal • To extend the storage capability. • Consider a router with Iinterfaces. The cache requires to store a routing interface number. • Construct a cache composed of IBloom filters to record I binary predicates. • Query all IBloom filters when query the cache for forwarding interface number of flow e.

  7. Extension of Bloom Filters • If e is a member of the ith Bloom filter, this implies flow e should be through the ith interface. • If e is not a member of any Bloom filter, e has not been cached. • In the unlikely event that if more than one Bloom filter claims e as a member. • One solution to this problem is to treat the cache lookup as a miss by reclassifying e.

  8. Extension of Bloom Filters • If e is a member of the ith Bloom filter, this implies flow e should be through the ith interface. • If e is not a member of any Bloom filter, e has not been cached. • In the unlikely event that if more than one Bloom filter claims e as a member. • One solution to this problem is to treat the cache lookup as a miss by reclassifying e.

  9. Extension of Bloom Filters Figure 5: An example: A modified Bloom filter with 5 buckets and 2 hash levels, supporting a router with 8 interfaces. Suppose we wish to cache a flow e that gets routed to interface number 2.

  10. Extension of Bloom Filters • Multi-Predicate Comparison Figure 7: Effect of storing routing information on effective cache size, p = 1e − 9 , using optimal Bloom filter dimensions.

  11. Bloom Filter Aging • Cold Cache • Empty the cache whenever the Bloom filter becomes full. • Advantage • Makes full use of all of the memory devoted to the cache. • Disadvantage • While the cache is being emptied, it cannot be used. • All cached flows must be re-classified after empty the cache will cause a load spike in the classification engine. • Zeroing out the cache may cause a high amount of memory access.

  12. Bloom Filter Aging • Double-Buffering • Partition the cache into two Bloom filters, active cache and warm-up cache. • Goal • To avoid the high number of cache misses immediately following cache cleaning. • Disadvantage • Double the memory requirement to store the same number of concurrent flows. • Zeroing out the expired cache still causes a load spike in the use of the memory bus. • Potentially double the number of memory accesses required to store a new flow.

  13. Bloom Filter Aging Double-Buffering Algorithm when a new packet arrives if the flow id is in the active cache if the active cache is more than ½ full insert the flow id into the warm-up cache allow packet to proceed otherwise perform a full classification if the classifier allows the packet insert the flow id into the active cache if the active cache is more than ½ full insert the flow id into the warm-up cache allow packet to proceed if the active cache is full switch the active cache and warm-up cache zero out the old active cache

  14. Bloom Filter Aging • Double-Buffering • Disadvantage • Double the memory requirement to store the same number of concurrent flows. • Zeroing out the expired cache still causes a load spike in the use of the memory bus. • Potentially double the number of memory accesses required to store a new flow.

  15. Bloom Filter Aging Figure 11: Average cache misses as a function of memory, M Figure 9: Cache hit rates as a function of memory, M For a memory-starved system, the cold-cache approach is more effective with respect to cache hit-rates.

  16. Bloom Filter Aging The variance in miss rates decreases much faster in the double-buffered case than in the cold-cache approach. Figure 12: Variance of cache misses as a function of memory, M (aggregate over 100ms timescales)

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