Scope-based Method Cache Analysis Benedikt Huber 1 , Stefan Hepp 1 , - PowerPoint PPT Presentation

Scope-based Method Cache Analysis Benedikt Huber 1 , Stefan Hepp 1 , Martin Schoeberl 2 1 Vienna University of Technology 2 Technical University of Denmark 14th International Workshop on Worst-Case Execution Time Analysis (WCET 2014) July 8, 2014

Outline  Motivation  The method cache  Scope-based Cache Analysis  Basic principle  Confmict detection  Evaluation 2

Motivation: The T-CREST Project T-CREST: The quest for a time-predictable multicore platform .c Compiler WCET Analysis RTOS ALU LM M$ ALU LM M$ ALU LM M$ D$ S$ D$ S$ D$ S$ Patmos Patmos Patmos NOC Memory controller FPGA Memory 3

Motivation: The T-CREST Project T-CREST: The quest for a time-predictable multicore platform .c Compiler WCET Analysis Method Split Cache Caches RTOS ALU LM M$ ALU LM M$ ALU LM M$ D$ S$ D$ S$ D$ S$ Patmos Patmos Patmos NOC Memory controller Time-composable memory FPGA hierarchy Memory 4

Motivation: The Method Cache  Instruction cache that holds blocks of code of variable size  Functions or parts of functions (compiler defjned)  Cache updates only at call, return and special branches  Fully associative tag memory  More dynamic than I-SPM  Address translation, replacement done in hardware  Less hardware costs than n-way LRU instruction cache  Instructions only stall in memory stage  Smaller tag memory T ag memory main: 2 g: 10 f: 14 - main() g() f() Cache 5

Motivation: Analysis of the M$  But : Only FIFO replacement (fragmentation)  But : Variable sized cache blocks  Path-sensitive analysis techniques based on abstract interpretation do not work well for the method cache  Instead: Generalized persistence analysis T ag memory main: 2 g: 10 f: 14 - main() g() f() Cache 6

Cache Analysis T echniques  Always-miss/always-hit classifjcation  [Grund10]: Shown to work for FIFO  [Reineke08]: bound FIFO caches based on smaller LRU cache performance  Persistence Analyses ( [Cullmann13], [Huynh11])  Deal with LRU caches (fjrst-miss)  Use functions and loops as scopes  Scope-based Cache Analysis  Find at-most-one-miss scopes  Find fjne-grained persistence scopes 7

Scope-based cache analysis 1) Build scope graph  Scope graph: Functions, loops, callsites  Access graph (per scope graph node): control fmow, memory accesses, subscopes and backedge nodes 2) For each memory block B :  Find set of single-entry scopes S( B ) (regions) on the access graphs s.t. : • every access to B is within a scope • B is confmict-free / missed at most once within each scope 3) Maximum number of misses of B ≤ sum of maximum number of executions of all scopes S(B)  Extend IPET by cache miss frequency variables and bound sum of variables by ∑ f(S(B)) 8

Example: Building the Scope Graph g() { for (;;) { access(X); } } f() { for (;;) { access(Y); } c3: g(); } main() { access(Y); c2: if (?) f(); access(Z); c1: g(); Scope graph access(Y); } 9

Example: Building the Scope Graph g() { for (;;) { access(X); } } f() { for (;;) { access(Y); } c3: g(); } main() { access(Y); c2: if (?) f(); access(Z); c1: g(); Access graph Scope graph access(Y); for main() } 10

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: OK OK Scope graph 11

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: OK OK OK OK Scope graph 12

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: OK OK OK OK OK OK Scope graph 13

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK OK OK OK Scope graph 14

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK OK OK OK Access graph Scope graph for main() 15

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK OK OK OK Access graph Scope graph for main() 16

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK OK OK OK Access graph Scope graph for main() 17

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK Confmict! OK OK OK Access graph Scope graph for main() 18

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK OK OK OK Access graph Scope graph for main() 19

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK OK OK OK Access graph Scope graph for main() 20

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! OK OK OK OK OK OK Access graph Scope graph for main() 21

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! S1: {Y,X} OK OK OK S2: OK {Z,X} OK S3: OK {Y} Cache misses of X ≤ f(S1) + f(S2) Cache misses of Y ≤ f(S1) + f(S3) Cache misses of Z ≤ f(S2) 22

Example: Finding confmict-free scopes Assuming a 2-way set-associative cache: Confmict! S1: {Y,X} OK OK OK S2: OK {Z,X} OK S3: OK {Y} Cache misses of X ≤ f(S1) + f(S2) Cache misses of Y ≤ f(S1) + f(S3) => T otal cache misses ≤ 5 f(main) Cache misses of Z ≤ f(S2) 23

Confmict Detection  For FIFO and LRU n-way set-associative caches:  confmict-free if at most n distinct cache lines are accessed  path-insensitive , one analysis per cache set  For method cache :  confmict-free if size and number of accessed functions does not exceed capacity of cache  path-insensitive , one single analysis  For LRU n-way set-associative caches:  confmict-free if at most n-1 distinct cache lines accessed between two accesses of a block B  path sensitive , per block, abstract interpretation • similar to [Cullmann13] or [Huynh11], but more fjne- grained scopes 24

Implementation  Implemented in the platin analysis tool  Persistence analysis based on [Huynh11]  Scope-based analysis for set-associative FIFO/LRU caches  Scope-based analysis for method cache  'Simple' scope-based analysis: only functions and loops as scopes, no construction of fjne-grained scopes  Evaluated on Mälardalen and PapaBench benchmarks 25

Evaluation Results for a 1 kB 8-way LRU set-associative cache

Evaluation Results for a 1 kB method cache

Conclusion  Scope-based cache analysis framework  M$ or set-associative caches, FIFO or LRU  Generalization of Persistence Analysis  Improved concept of scopes  Evaluation promises good analysis precision  Implemented in platin , available at: http://patmos.compute.dtu.dk/ http://github.com/t-crest/ Thanks! Questions? 28