CMP Design Choices Finding Parameters that Impact CMP Performance Sam Koblenski and Peter McClone

Outline Introduction Assumptions Plackett & Burman Analysis Simulation methods Statistical Design Plackett & Burman Results Mean Value Analysis MVA Implementation MVA Results AMVA Implementation AMVA Results Complementary Results Conclusions

Introduction 2 part study Method 1 Design space is huge, how can we reduce it? Method 1 Plackett & Burman (PB) Analysis finds critical parameters Design uses extreme values of parameters Detailed architecture design can focus on a few parameters

Introduction (cont.) Method 2 Mean Value Analysis Model of a CMP Simply designed to compute throughput Design choices can be narrowed down quickly Intuition is gained and patterns/parameter relationships identified

Assumptions - PB Design In-Order approximated as OoO with small window Die Size = 300 mm2 (16 MB Cache @ 65nm) L2 Cache Size expanded to fill the die Discrete sizes: 4, 8, 12 MB Associativity can be non-power-of-2 Core size measured in Cache Byte Equivalents: Pipeline Width CBE In-Order 1 50 kB 4 100 kB Out-of-Order 75 kB 250 kB

Simulation Methodology Simics with Ruby & Opal 16P sims used cache warmup files 2P sims ran for more transactions Attempted OLTP and JBB benchmarks Benchmark Processors Transactions OLTP 2 200 16 100 JBB 20000 10000

Plackett & Burman Design Motivation Narrow a huge design space Minimize simulation runs (experiments) Preliminaries Performance Measure Extreme Parameter Values Number of Parameters (N < 4Xn-1)

PB Design Example A B C D E F G Time + - 9 11 2 1 74 7 4 17 76 6 31 19 33 112 191 111 -13 79 55 239

PB Design Parameter Values Low Value (-) High Value (+) Number of Cores 2 16 Pipeline Organization In-Order Out-of-Order Pipeline Width 1 4 L1 Cache Size 16 kB 128 kB L1 Associativity Direct Mapped 32-Way L2 Cache Size Die Area – Core Area L2 Associativity L2 Banks 32 L2 Latency 50 Cycles 12 Cycles L2 Directory Latency 25 Cycles 6 Cycles Pin Bandwidth 400 10000 Memory Latency 300 Cycles 100 Cycles

PB Results Extreme Values stressed the simulator Have not completed an entire set of runs, yet Possibly necessary to build a custom L2 network for each run

PB Results for JBB

Assumptions - MVA Distribution of time between memory requests is exponential Processor cores exhibit the same average behavior with respect to their service times and miss rates. Doubling the size of the cache reduces the miss rate by a factor of 1/√2 An inorder core takes approximately the same area as 50 KB of cache

MVA Design Simple Closed Model:

MVA Design Two phases of this Model design First: Use the exact MVA equations Use average time between memory access as an application parameter Solve for throughput Second: Use Approximate MVA (AMVA) Use an iterative method to converge on this service time Solve for throughput 

Exact MVA To solve for the MVA equations, we determine the mean residence time at all service centers: Rp – processor/L1 residence time RL2 – L2 residence time RM – memory residence time. The case with one core is trivial. Use this case to solve for additional cores Rn,p = Dp * (1 + Qn-1,p)

Exact MVA results Using data from simulation runs throughput was calculated Miss rates, number of memory requests Results are erratic Not consistent with simulation results Source of the problem is most likely processor service time!

Approximate MVA Design An iterative method can be used to converge on a service time Uses total R as an input parameter Iterative method works well with approximate MVA Goal is to match total average residence time of a memory request

Approximate MVA Results Convergence using the AMVA equations does not always occur Total measured residence time cannot be reached with this model and parameter set. Variation of input values without convergence implies flaws in the model structure There is a complex relationship between the memory system and the rate at which a core issues requests that must be modeled 

Complementary Results Initial goal to produce PB Results to find parameters to focus on for MVA Model Results from both approaches could cross-verify correctness

Conclusions Simics has a STEEP learning curve <5 weeks is not enough time for valid/any results Refinement of a PB Design leads to long lead times on valid results CMPs complicate the relationship between cores and memory subsystem Design methodologies that focus simulation runs are necessary More results and conclusions to follow

Questions Questions?