Dynamic Code Management: Improving Whole Program Code Locality in Managed Runtimes Xianglong Huang Brian T Lewis Kathryn S McKinley ∗ The University of Texas at Austin Intel Corporation The University of Texas at Austin
[email protected] [email protected] [email protected] Abstract Keywords Locality, Code Layout, Code Generation, Performance Poor code locality degrades application performance by increasing Monitoring, Dynamic Optimization, Virtual Machines memory stalls due to instruction cache and TLB misses. This prob- lem is particularly an issue for large server applications written in 1. Introduction languages such as Java and C# that provide just-in-time (JIT) com- The gap between processor and memory speed continues to present pilation, dynamic class loading, and dynamic recompilation. How- a major performance bottleneck. Applications spend much of their ever, managed runtimes also offer an opportunity to dynamically time waiting because of cache and TLB misses. For example, on the profile applications and adapt them to improve their performance. Intel R Itanium R 2 processor, the SPEC JBB20001 server bench- This paper describes a Dynamic Code Management system (DCM) mark spends 54% of its cycles waiting for memory. While the ma- in a managed runtime that performs whole program code layout jority of stalls are from data access, instruction related misses such optimizations to improve instruction locality. as instruction cache misses, ITLB misses, and branch mispredic- We begin by implementing the widely used Pettis-Hansen algo- tions also contribute. Zorn [18] notes that Word XP, Excel Xp, and rithm for method layout to improve code locality. Unfortunately, Internet Explorer have large code working sets, and that much of this algorithm is too costly for a dynamic optimization system, the latency users perceive is caused by page faults fetching code.