1. © 2010 IBM Corporation What computer architects need to know about memory throttling WEED 2010 June 20, 2010 IBM Research – Austin Heather Hanson Karthick Rajamani

2. © 2010 IBM Corporation2 Outline Memory throttling overview Experimental platform –System configuration –Memory throttling implementation Memory throttling characterization –Bandwidth –Power –Performance Summary

3. © 2010 IBM Corporation3 Memory throttling in a nutshell Memory throttling is a power-performance knob that: –Impacts memory reference rates of both instruction and data streams –controls power –can be used for safety or optimization regulate DIMM temperatures enforce memory power budgets Memory throttling restricts read & write traffic –directly controls memory power –indirectly affects processors and other components Several implementation styles in commercial systems –insert periodic idle cycles –allow arbitrary number of transactions up to power (estimated) threshold –run + hold windows –enforce read & write quotas [this paper] first N transactions to proceed in time window any further requests wait until next time period

4. © 2010 IBM Corporation4 Comparison to clock throttling run-hold clock throttling regular frequency during run portion; clock halted during hold portion quota-style memory throttling reads & writes proceed as requested up to N requests per period Example: N = 6 Up to 6 transactions serviced per period, regardless of request timing Nth request in each period; additional requests would be queued for later service

5. © 2010 IBM Corporation5 POWER6 Memory Throttling IBM JS12 blade system –Processor POWER6 1 socket x 2 cores per processor socket 3.8 GHz frequency (fixed in these experiments) SLES10 linux –Memory 16 GB capacity 8 DIMMS x 2 GB each DDR2 667 MHz bus Quota-style memory throttling –N transactions per M memory cycles 100% throttle level == unthrottled –Time period is faster than thermal and power supply timescales

6. © 2010 IBM Corporation6 Memory throttle characterization methodology 1.Sweep throttle settings Set throttle Run steady-behavior benchmark DAXPY (double A * X plus Y) FPMAC (floating-point multiply accumulate) RandomMemory (generate random addresses) SPECPower_ssj2008 calibration phase (peak throughput for warehouse transactions) Record sensor data, 256ms per sample Memory power Memory reads & writes Instruction throughput And other sensors not shown here Decrement throttle Repeat for full range of throttle settings 2.Repeat throttle sweep for multiple benchmarks and memory footprints –Microbenchmarks: L1 cache contained and main memory footprints –SPECPower_ssj2008: behaves as nearly contained in on-chip caches 3.Calculate median sensor data for each permutation {benchmark, footprint, throttle}

7. © 2010 IBM Corporation7 saturated linear Memory throttle effect on bandwidth transition between linear & saturated regions

8. © 2010 IBM Corporation8 A closer look at RandomMemory-DIMM uses less bandwidth than other benchmarks at same throttle levels also less bandwidth than its own saturation level Simply measuring bandwidth at a single/current throttle level is not enough to identify a region of operation less than max could be saturated or transition region ….a controller will not be able to accurately predict the effect on bandwidth of a throttle level change …or predict the effect on power or performance Subtle but very important point about transition region Actual bandwidth < max bandwidth bandwidth restrictions pipeline starvation reduced request rate

9. © 2010 IBM Corporation9 Memory Power is basically linear with bandwidth, so this chart looks familiar….

10. © 2010 IBM Corporation10 power performance Throttling effects relative to each benchmark L1-contained DAXPY: throttling has no effect DIMM-sized DAXPY: drastic effect Generally more performance reduction than power reduction (in %) –Throttling alone doesnt affect static portion of memory power Leveraging idle low-power modes of memory can alter positively the power-performance curve for memory request rate throttling. –Possible to waste energy from longer execution time Larger bandwidth demands larger effect from throttling –Conversely, power reduction only when performance is impacted.

11. © 2010 IBM Corporation11 Summary Memory throttling is a power-performance knob available in commercial systems Memory controller restricts read & write bandwidth –caps memory power –controls DIMM temperature Mileage may vary –power and performance management depend on bandwidth demand throttling a low-bandwidth workload doesnt reduce much power –potential to use more energy due to increased execution time use highly throttled settings with caution Effective tool for power capping –power constrained configurations –thermal safety –power shifting

12. © 2010 IBM Corporation12 Acknowledgements IBM Research – Austin IBM Systems & Technology Group –Memory characterization: Joab Henderson, Kenneth Wright –EnergyScale firmware: Guillermo Silva, Andrew Geissler

13. © 2010 IBM Corporation13 Throughput: Instructions per Second Performance normalized to unthrottled, per benchmark L1-contained DAXPY: throttling has no effect DIMM-sized DAXPY: drastic effect

14. © 2010 IBM Corporation14 Memory Power: normalized to per-benchmark maximum