1. © 2009 IBM Corporation1 Feedback Directed Dynamic Recompilation for Statically Compiled Languages Dorit Nuzman, Sergei Dyshel, Revital Eres IBM Research, Haifa Thematic Session on Dynamic Compilation HiPEAC Computing Systems Week Paris, May 3 rd 2013

2. © 2009 IBM Corporation2 Motivating Scenario (IBMs) customer Independent Software Vendor Computer System Vendor (e.g., IBM) Third party software owned by some ISV Power780 server Increase target platform level? Performance problem Increase optimization level? Apply feedback directed optimization? No Nope Cant do

3. © 2009 IBM Corporation3 Fat Binary Runtime Engine Profiler Intermediate Representation Dynamic execution stage Program Source Code Static Compiler Motivating Scenario (IBMs) customer Independent Software Vendor Computer System Vendor (e.g., IBM) Power780 server Performance problem Native machine code JIT compiler opt = -O2 arch = common no-profile

4. © 2009 IBM Corporation4 Fat Binary Runtime Engine Profiler Intermediate Representation Dynamic execution stage Program Source Code Static Compiler Motivating Scenario (IBMs) customer Independent Software Vendor Computer System Vendor (e.g., IBM) Power780 server Performance problem Native machine code JIT compiler opt = -O2 arch = common no-profile

5. © 2009 IBM Corporation5 Fat Binary Runtime Engine Profiler Intermediate Representation Dynamic execution stage Program Source Code Static Compiler selective profile-driven recompilation Native machine code JIT compiler Our approach: Fat Binary based, feedback-directed, dynamic recompilation Used for years in dynamic languages & Java Needed also for static languages Opposed to dynamic binary optimization: includes high-level semantic information allows aggressive, speculative transformations

6. © 2009 IBM Corporation6 Background Modern compilers provide sophisticated optimizations. O3 (O4, O5) Inter-procedural Auto-vect/par Feedback-directed Hardware-specific Complicates build process Prolongs development & testing cycle Requires per-customer tuning – too costly No representative input We can gain back the lost performance benefit by applying the optimizations dynamically, at runtime. These optimizations are usually not used. –Only in benchmarking and HPC

7. © 2009 IBM Corporation7 Dynamic Recompilation Solves the static-compiler usability issue –Transparent feedback-directed optimization for current workload. –Tuning for current hardware –Separation of optimization from software production Allows adaptive optimization. Allows iterative optimization. Virtualization & Cloud: physical resources known only at runtime, and continuously change

8. © 2009 IBM Corporation8 Other Approaches: Focus only on very long running programs with heavy workloads to compensate for time spent profiling. Focus on optimization across consecutive runs of repetitive programs Domain specific (focus on a specific optimization, to a small pre-selected part of the code) Trace-based binary-optimization …Our Goal: Demonstrate an execution environment with overheads that are low enough to allow the dynamic optimizer to speed up execution of the current invocation, for regular programs/workloads. Dynamic Recompilation for Static Languages …Our Goal: Demonstrate an execution environment with overheads that are low enough to allow the dynamic optimizer to speed up execution of the current invocation, for regular programs/workloads.

9. © 2009 IBM Corporation9 Fat Binary Runtime Engine Profiler Split-IR Dynamic execution stage Program Source Code Static Compiler Native machine code JIT compiler Our approach: Fat Binary based, feedback-directed, dynamic recompilation

10. © 2009 IBM Corporation10 t0t0 t1t1 t2t2 t3t3 Execution and sampling thread t4t4 t5t5 t6t6 t7t7 t8t8 t9t9 Instrumentation-based profiling sampling-based profiling for method hottness Original method version Instrumented method version Optimized method version Instrumentation Optimization Recompilation thread Runtime Monitoring and Recompilation timeline Startup cost (loading & mapping) monitoring overhead Recompilation cost Slow instrumented execution Synchronization cost

11. © 2009 IBM Corporation11 SPECint2006: Dynamic Optimization Overheads – ref dataset Overall not degrading performance. Stress test1: using highly statically-optimized executable (–O3 -qhot)

12. © 2009 IBM Corporation12 SPECint2006: Dynamic Optimization Overheads – train dataset Works also for very short running programs. Stress test2: using highly statically-optimized executable (–O3 -qhot) Currently limited gain from FDO alone.

13. © 2009 IBM Corporation13 Optimization effect (isolated from overheads) (1) Similar impact gained using sampled profile as with using a perfect profiles. the problem is not it the profile quality (2) offline optimizer applies link- time FDO (cross methods and modules). Our optimizer limited currently to single module

14. © 2009 IBM Corporation14 Fat Binary Runtime Engine Profiler Intermediate Representation Dynamic execution stage Program Source Code Static Compiler (IBMs) customer Independent Software Vendor Computer System Vendor (e.g., IBM) Power780 server Native machine code JIT compiler opt = -O2 arch = common no-profile programs are statically under-optimized / moderately-optimized

15. © 2009 IBM Corporation15 SPECint2006: Overall Effect of Dynamic Execution (ref) Overall 7% improvement on average moderately-optimized scenario (program statically compiled with –O2) Selected methods from the program dynamically recompiled using a higher optimization level.

16. © 2009 IBM Corporation16 Selected methods from the program dynamically recompiled using a higher optimization level. Recompilation Statistics Default recompilation mode (default method hotness threshold) Aggressive recompilation mode (lower method hotness threshold) moderately-optimized scenario (program statically compiled with –O2) Overall 7% improvement on average Overall 8% improvement on average

17. © 2009 IBM Corporation17 More Benchmarks: SQlite SQlite: –Static version compiled with default compiler options: -O2 warm. –Using 1G of TPC-H tables. (smallest dataset) –Using TPC-H queries: Stream of 13 instances of query #1 13% improvement from dynamic FDO Most improvement comes from higher optimization level.

18. © 2009 IBM Corporation18 Overall cost of runtime optimization environment, including – environment startup cost – recompilation – profiling overheads is less than 2% on average (SPECint2006) For highly optimized native binaries, on average, there is no overall degradation These low overheads imply that the fat-binary based approach is practical for real-world use-cases and workloads –Feedback directed optimization can easily surpass these costs Aggressive optimization level for selected methods at runtime brings up to 20% speedup, and an 8% average speedup Much more potential available: – more aggressive optimizations: loop-nest, memory-hierarchy, parallelization – more profiling (event based?) – more synergy with static compiler more synergy with underlying (virtual) environment, to adapt to changes Summary and Conclusions

19. © 2009 IBM Corporation19 Thematic Session on Dynamic Compilation 1) What is the dynamic optimization stage? During program execution 2) What triggers the dynamic compilation cycle? A method gets warm 3) How are these triggers being detected? sampling execution/PCs (via time interrupts & code instrumentation) to monitor application behavior 4) How/when are the above triggers being inserted? at run-time 5) What is the recompilation scope/granularity? method 6) What is the target application domain? general purpose/commercial applications 7) What is the input code for the dynamic optimization? fat-binary (binary + IR) 8) What is the programming language of the target applications? statically compiled languages (C/C++...) 9) What specific adaptation / optimization / code-transformation is applied? general feedback-directed optimizations (BB ordering, …)