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An FX software correlator for VLBI

An FX software correlator for VLBI. Adam Deller Swinburne University Australia Telescope National Facility (ATNF). Outline. Function of a correlator Reasons for software correlation Code and platforms Progress and current performance Immediate and near future plans

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An FX software correlator for VLBI

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  1. An FX software correlator for VLBI Adam Deller Swinburne University Australia Telescope National Facility (ATNF)

  2. Outline • Function of a correlator • Reasons for software correlation • Code and platforms • Progress and current performance • Immediate and near future plans • Facilitation of real-time gigabit eVLBI

  3. Correlator functionality • Averaging the product of signals at two telescopes gives visibility

  4. Correlator functionality • Averaging the product of signals at two telescopes gives visibility • To get frequency information, average additional multiplications by lagged signal, then FFT (or reverse for FX) • What does it all boil down to? • Gigabit interferometry, 1024 channels with 6 telescopes, FFT cost ~ 200 Gflops (FX)

  5. Why software correlate? • Flexibility • Handle disparate file formats and bands • High spectral resolution + high b/w • Real time fringe checking • Rapid development • For Australian LBA • 8 x bandwidth, using disk-based recorders instead of S2 tape system

  6. fxcorr Telescope Baseline Mode InputStream Correlator code structure • Developed in C++ • Disk read/write threaded to hide latency • Currently using IPP for vector mathematics

  7. Cray XD-1 chassis: 12 AMD Opteron processors, 6 Xilinx FPGAs Platform options • Initial development to run on Swinburne cluster: 200+ 3GHz Pentium 4 machines

  8. Hybrid Architecture • PC flexibility with FPGA power • FPGA used for unpacking/fringe rotating and FFT • Possibly incorporate cross-multiply • FPGA code compiled into library, available as C subroutine call • Cray contributing via development of FPGA subroutine library

  9. Milestones to date • ‘Basic’ and ‘complex’ FX algorithms implemented and verified • Fortran driver for CALC ported to C for delay modeling • Used in search for first trans-Tasman fringes to 6m antenna in NZ • Range of support/analysis packages created

  10. Correlator output

  11. Current performance • LBA: Complex correlation, 6 telescopes, 2x 16MHz bands (128 Mb/s), 4 products/band, one node of cluster: • 512 channels: 21x real time • 2048 channels: 25x real time • 32786 channels: 32x real time • 15 telescopes, 128 Mb/s, 4 products • 512 channels: 88x real time

  12. Immediate applications • Improved LBA sensitivity: parallax of double pulsar J0737-3039 • High time and frequency resolution + high bandwidth: Wide field imaging • Masers, SNRs, AGNs… NGC253 image credit: Emil Lenc (Swinburne)

  13. Real-time gigabit eVLBI? • All ATNF telescopes of LBA capable of 1 Gb/s, others 512 Mb/s • Technical/cabling issues to overcome: disk-based recorders • Software correlator ready now • Simulated Gb/s data, 512 channels <160x real time • Current system has disk transfer limitation

  14. In the near future… • Fibre links overcome disk limitations? • Investigate alternate architecture: ‘Cell’ processor array at VPAC • Code improvements: • Sophisticated pulsar gating • Polyphase filterbank alternative to FFT • Automated configuration from schedule, GUI frontend, output FITS compatible…

  15. Integration into the LBA • Available for use from now on a limited basis • Final release planned before March 2006 observing session • Will incorporate automatic configuration, GUI frontend etc • Feedback on desired features encouraged

  16. Applications outside the LBA Existing correlator limited + disk space available + processors = SOFTWARE CORRELATE • Potentially useful for niche applications anywhere • Applicable to connected-element as well as VLBI • “Piggy-backing” on existing interferometers - only need facility to get data to disk, and some processors

  17. Conclusions • Fast, flexible new FX software correlator for generic clusters developed • Hybrid architecture will be explored • Will provide significant increase in LBA sensitivity until next upgrade • Real-time gigabit eVLBI using the software correlator is feasible on Swinburne cluster • Final release planned in early 2006

  18. Questions??

  19. XD-1 Specifications • 12x 2.2GHz Opteron processors - 106 Gflops total • 1-8 GB RAM per processor, 12.8GB/sec • 4-8 GB/s interconnect (between nodes) • 6 Xilinx Virtex 4 FPGAs • 4 PCI-X slots for ethernet/fibre channel • Max 1.5 TB local storage

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