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A Component-based Definition of Spatial Locality

A Component-based Definition of Spatial Locality. Traditional Definition of Spatial locality. When a data element is accessed, the nearby data elements will also be accessed Overall miss rate The fewer the misses, the better the layout. Questions.

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A Component-based Definition of Spatial Locality

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  1. A Component-based Definition of Spatial Locality

  2. Traditional Definition of Spatial locality • When a data element is accessed, the nearby data elements will also be accessed • Overall miss rate • The fewer the misses, the better the layout

  3. Questions • Can the overall spatial locality be decomposed into finer components? • How much can the locality of a given data layout be improved? • Can a data layout be improved if the miss rate cannot be lowered?

  4. A Component-based Definition of Spatial Locality • Based on the reuse distance • Based on components

  5. Reuse Distance • The reuse distance of a memory access is the number of distinct data elements accessed between this and the previous access to the same data.

  6. Reuse Signature • The distribution of all reuse distances • In our experiment, we use log sized bins

  7. A Component-based Definition ofSpatial Locality • Spatial locality measures the change in reuse distance when the data block size changes from b1 to twice the size b2 = 2*b1. b1 b2

  8. Cases of Change Data layout b1 Trace x ........................................... x Data layout Case 1: x ........................................... x b2 Case 2: x y x ...........................................

  9. Changes of Reuse Signature b1 b2

  10. Data Layout Quality • Effective spatial reuse: the bin they are located has been moved to left by at least C bins, when the block size is doubled • Ineffective spatial reuse: the other reuses • In our experiment, we pick C to be 3 • The data layout quality of a bin: • 2 * effective spatial reuse / total spatial reuse

  11. Component • Data reuses of nearby bins that have a similar portion of effective spatial reuse • The spatial locality of a component is the weighted average of the spatial locality of each bin • We manually examine the bins and groups them into components

  12. Experimental Setting • 7 SPEC2000 (equake, art, swim, gzip, mcf, crafty, twolf) and 1 SPEC2006 (milc) • Data size varies from 1.2MB to 72MB • Trace length varies from 7.7 billion to 400 billion • Valgrind[NethercoteSeward’07] to collect traces • Augmented reuse distance analyzer [DingZhong’03] • Several hundred times slower

  13. Swim

  14. Why Optimized Swim Better L1 miss rate reduced by 6.7% Performance increased by 8.1% 4% reference: 0.16 -> 0.99

  15. Good spatial locality Good temporal locality Poor Spatial locality

  16. Different Inputs

  17. Possible Uses • User tuning [Levinthal] • Superpage management • Data-based cache hints [ Fang’05, BeylsD’Hollander’05]

  18. Related Work • Spatial locality [BuntMurphy’84,Weinberg+’05] • Component-based analysis [DingZhong’03,Shen+’03] • Spatial uses within a loop [WolfLam’91,McKiney+’96]

  19. Summary • A quantitative model of spatial locality • We have tested our new model on 8 SPEC benchmark programs • Among 18 components, 2 have good temporal locality, 6 have good spatial locality, 4 have poor spatial locality/poor temporal locality/large • Possible uses

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