A new architecture for real-time control in RFX-mod G. Manduchi, A. Barbalace Big Physics Symposium 1/16

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1 A new architecture for real-time control in RFX-mod G. Manduchi, A. Barbalace 2011 Big Physics Symposium 1/16

2 Current RFX control system MHD mode control Plasma position control Toroidal field control diagnostic s 2011 Big Physics Symposium 2/16

3 Distributed Key concepts of current system To account for different location of sensors and actuators To distribute computing load Modular The same basic software used on all nodes Only control algorithms are changed Configurable New I/O requirements can be fulfilled by configuring header files and NOT by writing new code 2011 Big Physics Symposium 3/16

4 Distributed Lessons learnt Advantages: computing load can be spread among computers, BUT no more necessary with current multicore technology. Drawbacks: network connection introduces delay in system response. The system, basically pipelined, achieves higher throughput, BUT shows NOT negligible overall latency. Modularity The use of a Software Framework for data handling proved a successful approach. Configurability Easy configuration proved a key factor in reducing development time for system evolution 2011 Big Physics Symposium 4/16

5 Latency Critical factors Current latency is around 1.5 ms. This represents a critical factor in quality of control leading sometimes to instabilities. Sampling frequency Current sampling frequency is 2.5 khz. A higher sampling rate improves the quality of integration/derivation. Computing power Operations such as sideband correction and sensor radius extrapolation are highly computing-intensive. Currently only most significant modes are considered. Reliability The possibility of dry runs of the system, automatically performed before the shot, allows the detection of possible malfunctions which may affect plasma performance and machine integrity Big Physics Symposium 5/16

6 New architecture - hardware Distributed à Centralized Multicore and multiprocessor computers provide now all required computing power. Modularity in architecture is retained, letting work carried out by former embedded computers be now executed by a dedicated core This allows for a drastic reduction in latency: communication is now carried out in shared memory, not on a network Data acquisition and reference waveform generation is still distributed. cpci or PXI racks will host ADCs and DACs and fiber-optic bus extenders, possibly using PCIe, will provide direct connection to the central multicore computer. This solution is cost-effective: a general-purpose multicore workstation is cheaper than dedicated embedded computers Big Physics Symposium 6/16

7 New architecture 2011 Big Physics Symposium 7/16

8 Hardware details: data acquisition MHD feedback control requires the acquisition of thee sets of 192 signals each. One PXI rack hosting three NI 6255 ADC devices will be used for every set. The NI PCI-6255 is a 16-Bit, 1.25 MS/s(max), 750 ks/s(scanning), 80 analog inputs (64 used) The driver nirlpk has been ported to Linux kernels >= (major fix in DMA transfer, provision for 64 bit kernel). User space interrupt synchronization has been added, retaining user space register access (via mmap) 2011 Big Physics Symposium 8/16

9 Simultaneous vs. non simultaneous sampling In simultaneous sampling all the samples acquired in an ADC refer to the same instant in time. In non-simultaneous sampling, a multiplexed ADC provides the conversion in turn of all the input channels. Conversion time slightly varies from channel to channel, and the introduced error depends on the dynamics of the input signal. However, if the conversion time instant is known and deterministic, it is possible to remap all channels to the same time via real-time Lagrange interpolation Relative error Relative error in delay reconstruction Frequency (Hz) "Linear interpolation" "Lagrange 8 Samples" "Lagrange 4 samples" 2011 Big Physics Symposium 9/16

10 Hardware details: DAC generation 192 reference waveforms are produced by the MHD mode control system to drive the 192 power units controlling radial magnetic field NI 6723 is used: Static and Waveform Analog Output Bit, 32 Channels The Linux driver has been ported to Linux kernels >= (National). No output DMA is available, so two PXI chassis performing in parallel the generation of 96 channels each will be used to sustain the data rate (8kHz) 2011 Big Physics Symposium 10/16

11 Software framework Framework: join efforts in fusion community The framework developed and currently used in RFX did a good job. However, it has been developed with a well-defined architecture in mind, so it is not so easily portable to new architectures and platforms Other general frameworks for real-time control have been developed, notably PCS and MARTe PCS has been developed at DIII-D and is used in a few machines (e.g. DIII-D, MAST, EAST) MARTe has been developed at JET and is used for JET vertical stabilization MARTe has been chosen. Even if with a more limited (but growing) user base, it implements modern concepts and is well tied for multicore systems Big Physics Symposium 11/16

12 Operating system issues The current system uses VxWorks Rugged real-time operating system Some important bugs in TCP/IP communication Expensive Linux is becoming a candidate for the next system Soft real-time system Real-time extensions of Linux make it a hard real-time system 2011 Big Physics Symposium 12/16

13 Pipelined multicore execution Real time computation for MHD control is carried out in a pipelined organization with three stages: Data Acquisition, Control Computation, and Reference Waveform Generation. New cores can be added to provide additional computation in the case it can be performed in parallel. CPU core 0 CPU core 1 CPU core 2 CPU core 3 CPU core 4 CPU core 5 acq Clock 0 acq Clock 1 acq Clock 2 acq Clock 3 Software PIPELINE ADC stage algo rithmic stage DAC stage 2011 Big Physics Symposium 13/16 data out 0 data out 1 time

14 Realtime performance RedHat Linux distribution from CERN with RT patches is used. The Linux RT patches provide finer granularity in kernel preemptability, thus guaranteeing a more deterministic response time. Most Interrupt Request Services (ISR) are made threaded, and therefore can be interrupted in the case a higher priority thread becomes ready Big Physics Symposium 14/16

15 Costs racks Qty Unit Price Total network Qty Unit Price Total IO Qty Unit Price Total cabling Qty Unit Price Total Computing Qty Unit Price Total PXI PXI-PCI PXI SHC server PXI PXI SHC rack mount SH68-c68-s racks 6234 network 3079 IO cabling 3180 computing 4336 total computing 10% cabling 7% racks 14% netw ork 7% IO New System Cost Estimation 62% racks Qty Unit Price Total network Qty Unit Price Total IO Qty Unit Price Total cabling Qty Unit Price Total Computing Qty Unit Price Total 0 budgetary VGX SHC CPU MPV SHC VME FPDP SH68-c68-s MPV racks network 5000 IO cabling 3180 computing RTOS total RTOS 10% computing 10% cabling 2% racks 11% netw ork 3% IO 64% Old System Cost Estimation 2011 Big Physics Symposium 15/16

16 Conclusions The feedback control of RFX introduces a new architecture, where distributed components communicating via a real-time network (Ethernet, Reflective Memory) are moved to the cores of a multicore server. COTS solutions for computing (multicore servers) ADC and DAC (PXI-Based solutions with bus extenders) proved cost-effective and reliable. Linux is becoming a very good candidate for real-time feedback control systems in fusion devices Big Physics Symposium 16/16