Cache-ConsciousStructure Definition

By Trishul M. Chilimbi, Bob Davidson, and James R. Larus

Presented by Shelley ChenMarch 10, 2003

Motivation

Processor-Memory Performance Gap Growing at 53% per year!

Chilimbi and Larus previous work Placed objects with high temporal

locality in the same cache block Works best with objects < ½ cache block

Current paper proposes techniques for larger data structures

Improving Cache Performance

Two Cache-Conscious Definition Techniques

Structure Splitting Split large data structures into two

smaller structures

Field Reordering Group fields in a structure with high

temporal locality into the same cache block

Structure Splitting

Split large structures into two smaller structures “hot” structure contains frequently accessed

fields “cold” structure contains rarely accessed fields

Allow more “hot” structures to fit into the cacheHas been done manually in the past, this paper is first to automate

Class Splitting Overview

Experimental Setup

Ran compiled programs on Sun Ultraserver E50002 GB of memoryL1 dcache, 16 KB DM, 16 byte blocks L2 cache, 1 MB DM, 64 byte blocks

Results: Structure SplittingReduces L2 miss rates by 10-27%; improves execution time by 10-20%

Field ReorderingLogical ordering of the program is not usually consistent with its data access patterns Frequently accessed fields may be coded next to

rarely accessed fields, putting them in the same cache block

cause excessive cache misses

Reorder field definitions of structure fields with high temporal affinity in same cache

block

bbcache

Tool that produces structure field reordering recommendations Construct a database containing both static and

dynamic information about the structure field accesses

Process database to construct field affinity graphs for each structure

Produce the structure field order recommendations for the affinity graphs

Attempts to group fields with high temporal affinity into the same cache block

Experimental Setup

4 processor 400MHz Pentium II Xeon system1MB L2 cache/processor4GB memoryRan TPC-C on Microsoft SQL Server 7.0 to collect traces as input to bbcacheChose 5 structures which showed largest potential from the benefits of reordering

Results: Field Reordering

Cache block pressure =Σ (b1, …,bn)/n

Cache block utilization = Σ(f11, …, fnbn)/ Σ(b1, …,bn)

Modified SQL Server was consistently better by 2-3%

Conclusion

2 techniques to improve cache performance change the internal organization of

fields in a data structure Structure splitting Field reordering

Structure Layouts better left to compiler Easier to determine field access order

Questions?

Class Splitting Algorithm

Structure Access Database

Building Field Affinity Graphs