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Why it might be interesting to look at ARM

Why it might be interesting to look at ARM. Ben Couturier, Vijay Kartik Niko Neufeld , PH - LBC SFT Technical Group Meeting 08/10/2012. The challenge for LHCb. Major upgrade during LS2 Read out detector at bunch- xing rate 40 MHz

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Why it might be interesting to look at ARM

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  1. Why it might be interesting to look at ARM Ben Couturier, Vijay Kartik Niko Neufeld, PH-LBC SFT Technical Group Meeting 08/10/2012

  2. The challenge for LHCb • Major upgrade during LS2 • Read out detector at bunch-xing rate 40 MHz • No more hardware based trigger – need to filter 40 Million events / s (32 Tbit/s) in software Why look at ARM? N. Neufeld

  3. Dataflow Detector • GBT: custom radiation- hard link over MMF, 3.2 Gbit/s (about 10000) • Input into DAQ network (10/40 Gigabit Ethernet or FDR IB) (1000 to 4000) • Output from DAQ network into compute unit clusters (100 Gbit Ethernet / EDR IB) (200 to 400 links) Readout Units 100 m rock DAQ network Compute Units Why look at ARM? N. Neufeld

  4. What will be the Compute Unit? • Baseline could possibly be augmented with a co-processor card (like Intel MIC or a GPU)  lots of interest from various groups • Alternative 1: Use lower-power, cheaper x86 processors such as Intel Atom, AMD • Optimize HEPSpec/CHF/W • Alternative 2: Or use non-Intel processors. Try to profit from the highly competitive and innovative market for processors for portable devices  ARM • A compute unit is a destination for the event-data fragments from the readout units • It assembles the fragments into a complete “event” and runs various selection algorithms on this event • About 0.1 % of events is retained • Baseline option: a high-density server platform (mainboard with standard CPUs) using Moore’s law and some estimates on the algorithms need 4000 to 5000 servers of the 2018 type! Why look at ARM? N. Neufeld

  5. ARM • A “pure” RISC architecture (with some enhancements) • A long tradition in the embedded market • Billions of cores sold • in many variants • # cores / power vs performance • Produced by various licensees • Has a reputation of the best power-efficiency in the market We are here 32-bit IEEE floats SIMD native Javaoffload Announced: 64-bit SIMD with DP floats Why look at ARM? N. Neufeld

  6. So what would a compute unit look like? Why look at ARM? N. Neufeld

  7. Operational constraints • The Online farms are very big • O(2000) servers, of different generations, vendors, • Like a traditional data-centre with all the problems, and very few administrators and some simplifications: • A single client • In Online operation at least mostly a single work-load • But want rack-mountable, remote-manageable, good mechanics, decent powering, vendor support etc… and of course low cost! • Don’t want to build this ourselves  needs to fit in traditional data-centre structure Why look at ARM? N. Neufeld

  8. Embedded in the data-centre • Boston Viridis (projects alsofrom DELL and HP) • Consists of 48 SoC • 4 cores 4 GB RAM • ARM A9 Cortex 1.4 GHz • 80 Gb Ethernet switch • Total 192 cores / 192 GB RAM / 300 Watt • Exists also from DELL/HP Why look at ARM? N. Neufeld

  9. How fast is a core? So we’ll need many Why look at ARM? N. Neufeld

  10. Is it worth it? • ARM v7: 192 cores need 300 W and 2 U for about 520 HepSpecs • X5650: 96 hyperthreads need about 1400 W and 2 U for 900 HEPSpecs • If this ratio continues to hold into 2018 LHCb could do the upgrade with a 600 kW data-centre instead of a new (!) 2 MW one • And maybe at some point we need to pay for the power  Why look at ARM? N. Neufeld

  11. The acid test • HepSPEC is not necessarily a good test for Online usage • Online we (currently) run n instances of the same application in parallel, where n is the number of cores/hyperthreads • No “mixed” work-load – hyperthreading typically adds more in the Online “mono-culture” • Need to benchmark using the High Level Trigger code Why look at ARM? N. Neufeld

  12. Project: “Moore on ARM”  • Need to compile the LHCb software-stack (beginning from Root) • Can compare with natively compiled code – everything works fine on the FC17 test-node, but compilation is slow • Root 5.34.02 ./configure linuxarm --enable-c++11;make –j 4 takes 30m43s • Team (part-time only) Ben Couturier, Vijay Kartik, Niko Neufeld Why look at ARM? N. Neufeld

  13. Future plans • X-compiler chain ready • Will now go on to compile stack • Verification and bench-marking • Then: full-scale test on fully loaded 192 core system (with a faster ARM – currently use A8 – will have A9 or A15), possibly including real network input (for fun) Why look at ARM? N. Neufeld

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