PINTOS: An Execution Phase Based Optimization and Simulation Tool) PINTOS: An Execution Phase Based Optimization and Simulation Tool) Wei Hsu, Jinpyo Kim, Sreekumar Kodak Computer Science Department University of Minnesota October 9, 2004 PIN Tutorial at ASPLOS`04

Outline What is Pintos? What can Pintos do? Phase detection for optimization and simulation Optimization (instruction prefetching) Fast Simulation Summary

What is Pintos? PINTOS is a PIN based Tool for Optimization and Simulation A research framework supports adaptive object code optimization –Supports deep analysis of run-time program behavior for object code optimization (e.g. instruction, data prefetching) –Integrates HPM performance monitoring (Pfmon) with dynamic instrumentation (PIN). Also supports fast performance simulation –Identifies program phases (with coarse and fine granularity) –Generates simulation strings that capture representative program behaviors

Pintos Framework program pfmon profile analysis Opt targets program pfmon profile analysis phase targets PIN-based Analysis control flow Cache Sim PIN-based Phase Detection Simulation String Gen Optimization Simulation Filtered Opt Targets Simulation Strings Phase Info

Our Background ADORE dynamic optimization system Main Thread Kernel / Pfmon Hardware Performance Monitoring Unit Dynamic Optimization Thread Code Cache Trace Selection Optimization Deployment Phase Detection

ADORE Performance: Speedup of ORC2.1 +O2 Compiled SPEC2000 Benchmarks

ADORE Performance at Different Sampling Rates

Future Enhancements to ADORE I-cache prefetching Help thread based optimizations Value prediction based optimizations Dynamically undo aggressive optimizations (e.g. control/data speculations, indirect array prefetches) Software Branch Predictions

What can Pintos do for us? Pintos uses pfmon to identify high-level performance problems (e.g. I-cache miss) and locate target code (phases) for optimization Pintos then uses PIN-based analysis tool to focus on target code (phases) to conduct deep analysis Pintos provides a framework to support deep analysis of program behavior so that we may experience with new object code optimization techniques and feed them to ADORE. Simulation strings can be generated by Pintos and used for more efficient micro-architecture simulations

Phase based Optimization and Simulation Phase is a sequence of code that consistently exhibits certain performance behaviors in Pintos, for example –Gzip shows consistent and repeated data cache miss patterns –Crafty exhibits consistent I-cache misses A repeating phase can serve as an unit for dynamic and adaptive optimization, or for fast performance simulations. –Optimization unit can be basic block, trace, procedure and region (loops and loop nests including complex control transfers) –Simulation unit can be an extended code sequence

Phase Detection One phase detection method doesn’t fit all needs. –Dynamic data cache prefetching requires coarse grain phases (e.g. loops) while dynamic I-cache prefetching requires fine-grain phases (e.g. frequent calling paths). A phase tuple is used to determine the current point of execution in PIN instrumentation –Phase tuple: (phase ID #, ip addr, # of retired insts)

Pintos for Optimization (I-Prefetch) Many applications still suffer from significant I-cache misses (e.g. data base apps, some SPEC CPU2000 benchmarks, etc) L1I miss rate (%) L1I Prefetch miss rate L2I miss rate 176.gcc crafty eon perlbmk vortex Complex control flows cause high miss rate from streaming prefetches Predictable call sequence results in relatively low miss rate

I-Cache Miss Analysis (pfmon) Miss address based info –Crafty (2125/ ) 25% 30 (1.41%) Each top miss PC was caused by % 91 (4.28%) different paths. 75% 228 (10.73%) 90% 442 (20.80%) Path based info –Crafty (8016/ ) Each top path leading to I-cache 25% 28 (0.34%) miss has 1-2 possible prefetch targets 50% 126 (1.57%) 75% 436 (5.43%) Data show we can reduce points of 90% 1118 (13.94%) interest for inst prefetching

Exploring prospective points of instruction prefetching (PIN) B2 B1 B3 B6 B4 B5 B7 B8 Instruction Cache Simulator Control flow graph Pintos generates prospective paths leading to frequent I- cache misses by analyzing pfmon profile PIN instrumentation routine constructs control flow graph and simulates instruction cache along execution It inserts I-cache prefetching instructions for the prospective paths based on control flow edge weight and estimated cache replacement Paths frequently causing I-cache misses

Exploring prospective points of instruction prefetching (PIN) B2 B1 B3 B6 B4 B5 B7 B8 Instruction Cache Simulator Control flow graph Key observation –Most I-cache misses happen in the following cache lines after the entry or the return of a function call. –L1I cache misses are mostly capacity misses. We need to estimate how prefetch affect incoming instruction stream. Key idea –Run ahead by exploring CFG and I-cache simulator –Evaluate prospective paths given by Pintos Paths frequently causing I-cache misses

Pintos for Fast Simulation Execution driven micro-architectral simulation is commonly used for evaluating new micro- architecture features and respective code optimizations. Simulation time is often too long for a complete simulation. New methods for fast simulations such as Simpoint and Smarts have been proposed. PASS (Phase Aware Stratified Sampling) is a different way to generate representative and customized traces for targeted simulations

Fast Simulation Techniques Truncated Execution -Run Z, FastFoward-W-R Sampling -SMARTS -SIMPOINT -Stratified Sampling Reduced Input Sets -MinneSPEC

Problems of Previous Works Truncated Execution gives very inaccurate results Reduced Input sets do not always behave the same as reference inputs so the performance estimation based on reduced input sets may be misleading.

Mechanism of SMARTS UWWU (K-1) * U Program Run Time W: Warm up time (Fixed to 2000 instructions for SPEC 2000) U: Detailed Simulation (Fixed to 1000 instructions for SPEC2000) (K-1)*U: Function Simulation with Functional Warming (The tool gives the value of K for which the IPC will be within + 3% of the actual value with 99.7% confidence interval)

Issues in Previous Work SMARTS Value of U and W fixed for SPEC 2000 suite. Have to identify them for every new benchmark suite (Very time consuming) Over sampling in steady phases. Does not effectively exploit the existence of phases in programs SIMPOINT The user chooses the length of simulation point (100 million, 10 million, 1 million) Provides Simulation Points based on Clustering of Basic Block profiles which is generated using sim-fast or ATOM

Phase Aware Stratified Sampling (PASS) Deploy a hierarchical method to detect coarse and fine grain program phases (1) Tracking calling stack (stable bottom = coarse grain phase)  inter-procedure (2) Detecting loops within the procedure  intra- procedure (3)Tracking data access pattern such as stride within loops (fine grain phases) Select stratified samples from each phase until getting high statistical confidence

IPC vs SimPoint (cc1-166, 1 million insts) simpoint IPC

IPC vs Phase Classification on PASS (cc1-166, 1 million insts)

IPC vs SimPoint (cc1-166, 250 million insts)

IPC vs SimPoint (gzip-source, 1 million insts) simpoint IPC

IPC vs Phase Classification on PASS (gzip-source, 1 million insts)

IPC vs SimPoint (gzip-source, 250 million insts)

IPC vs SimPoint (mcf-ref, 1 million insts) simpoint IPC

IPC vs Phase Classification on PASS (mcf-ref)

IPC vs SimPoint (mcf-ref, 250 million insts)

IPC vs Phase Classification on PASS (gap-ref, 1 million insts)

IPC vs SimPoint (gap-ref, 250 million insts)

Summary We show the combination of HPM sampling (Pfmon) and dynamic instrumentation (Pin) in our research framework (Pintos) for adaptive object code optimization and micro-architectural simulation. PASS (Phase Aware Stratified Sampling) may lead to a more efficient way in simulating the interaction between compiler optimizations and new micro-architectural features.