Supercomputing Challenges at the

National Center for Atmospheric

ResearchDr. Richard Loft

Computational Science Section

Scientific Computing Division

National Center for Atmospheric Research

Boulder, CO USA

Talk Outline

Supercomputing Trends and ConstraintsObserved NCAR Cluster Performance (Aggregate)Microprocessor efficiency: what is possible?Microprocessor efficiency: recent efforts to improve CAM2 performance.Some RISC/Vector Cluster ComparisonsConclusions

The Demand: High Cost of Science Goals

Climate scientists project a need for 150x more computing power of the next 5 years.T42->T85. Doubling horizontal resolution increases computational cost eightfold.Many additional constituents will be advected.New physics: computational cost of CAM/CCM, holding resolution constant, has increased 4x since 1996. More coming…Future: introducing super-parameterizations of moist processes would increase physics costs dramatically.

Existing Infrastructure Limits at NCAR

Cooling Capacity– 450 tons (1.58 megawatts)– Most limiting– One P690 node ~ 7.9 KW ~ 2.5 tons– Balance cooling with power

Power ~ 1.2 MW without modifications– Second most limiting– Currently NCAR computer room draws 602 KW– About 400 kw from IBM clusters

Space ~ 14,000 sq.ft.– P690 ~ 196 W/sq. ft.– Least limiting based on current trends

Mass Storage Growth

1.3 Pbytes total Adding ~3 Tbytes/day5 year doubling times -– Unique files: 2.1 years– File size: 10.4 years– Media performance (GB/$) 1.9 years

Alarming trends– MSS growth rate doubling time has

accelerated over past year. Now 18 months.

– MSS costs are increasing…

NCAR Mass Storage System Growth

1.0E+07

1.0E+08

1.0E+09

1.0E+10

01/02/0207/02/0201/02/0307/02/0301/02/0407/02/0401/02/0507/02/0501/02/0607/02/0601/02/0707/02/07

MBytes

unique_MB

total_MB

Observed Cluster Performance (Aggregate)

IBM Clusters at NCAR

Bluesky: 1024 IBM 1.3 GHz Power-4 cluster– 32 P690/32 compute servers– 736 in 92, 8 way “nodes” (bluesky8)– 288 in 9, 32 way “nodes” (bluesky32)– Peak: 5.234 TFlops– Dual “Colony” interconnect

Blackforest: IBM 375 MHz Power-3 cluster– 283 “winterhawk” 4-way SMP’s– Peak: 1.698 TFlops– TBMX interconnect

Observed IBM Cluster Efficiencies

System Application efficiency

(% of peak)

bluesk8 4.1%

bluesk32 4.5%

blackforest 5.7%

•Newer systems are less efficient.•Larger nodes are more efficient.•Max sustained performance: 320.3 GFlops

Why is workload efficiency low?

Computational character of workload average:– L3 cache miss rate 31%– computational intensity is 0.8

Applications are memory bandwidth limited.– Simple BW model predicts 5.5% for bluesky32.

A good metric of efficiency is Flop/cycle.– Factors out dual FPU’s.– Bluesky32: 0.18 Flop/cycle– Blackforest: 0.23 Flop/cycle

RISC Cluster Network Comparison

IBM Power-4 cluster with dual “Colony” network.IBM Power-3 cluster with single TBMX network.Compaq Alpha cluster with Quadrix network.Bisection Bandwidth– Important for global communications– XPAIR benchmark initiates all to all

communication.– Dual Colony P690 local:global BW ratio 50:1

Global Reductions – For P processors these should scale as log(P).– Actually scales linearly.

Cluster Network Performance

Microprocessor efficiency:

What is possible?

Example: 3-D FFT Performance

Hand tuned multithreaded, 3-D FFT (STK)Three 1-D FFT on each axis with transpositionsFFTs are memory bandwidth intensive– Both loads and Flop’s scale like N*log(N)

The FFT is not multiply-add dominated The FFT butterfly is a non local, strided calculation.Gets more non local as size of FFT increases1024^3 Transforms on P690 (IBM Power-4)

Microprocessor efficiency:

Recent efforts to improve CAM2 performance…

CCM Benchmark Performance on Existing Multiprocessor Clusters

Some RISC/Vector Cluster Comparisons…

Processor Comparison

Power 4(2 cores)

Pentium 4

Itanium II SX-6

Process .18 µ Cu/7l

.13 µ Cu

0.18 µ Al/ 6l

0.15 µ Cu/ 9l

Mhz 1300 2800 1000 500/1000

Peak GF 5.2 2.8/5.6 4.0 8.0

Die area 400 mm2 145 mm2

421 mm2 420 mm2

Trans. 170 M 55 M 221 M 57 M

On-Chip cache

1.77MB 512 KB 3.3 MB

On-Chip bandwidth

41 GB/s (per core)

89.6 GB/s

64 GB/s

Memory Bandwidth

5.8 GB/s 4.3 GB/s

6.4 GB/s 32 GB/s

IBM P690 Cluster

5.3 TFlops peak1024 processors (32, 32 way P690 nodes)5.2 Gflops/processorObserved 4.1-4.5% of peak on NCAR codesMax sustained on workload: 213.5 GFlopsEst. Peak Price Performance: $2.6/MFlopsSustained Price Performance: $59/MFlopsSustained Power Performance: 0.7 Gflops/KW

Earth Simulator

40.96 Tflops peak5120 Processors (640, 8 processor GS40 nodes)8 Gflops/processorEstimate 30% of peak on NCAR codes

Est. Max sustained on workload: 12,200 GFlopsEst. Peak Price Performance: $8.5/MFlopsEst. Sustained Price Performance: $28/MFlopsEst. Sustained Power Performance: 1.525 Gflops/KW

Power 4 die floor plan

Power 4 cache/CPU area comparison

Conclusions

Infrastructure (power, cooling, space) are becoming critical constraints.NCAR IBM clusters sustain 4.1%-4.5% of peak.Workload is memory bandwidth limited.RISC cluster interconnects are not great.We’re making steady progress learning how to program around these limitations.At this point, vector systems appear to be about 2x more cost effective in both price and power performance.

Pentium-4 die floor plan

Pentium-4 cache/CPU comparison

Itanium II die floor plan

Itanium II CPU/cache area comparison