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ECE 526 – Network Processing Systems Design Network Processor Architecture and Scalability Chapter 13,14: D. E. Comer
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Ning WengECE 5262 NP Architectures Last class: ─ Key requirement of network processor: flexibility and scalability ─ Optimized instruction set and parallel processing using multiprocessors This class: ─ Internal organization of NP: Computation, storage and communication Operating support Content addressable memory (CAM) ─ NP scaling issues
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Ning WengECE 5263 NP Architectures NP architecture characteristics ─ Computation Processor hierarchy Special-purpose functional units ─ Storage Memory hierarchy Content addressable memory (CAM) ─ Communication Internal buses External interfaces ─ Operation support Concurrent/parallel execution support Programming models Dispatch mechanisms
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Ning WengECE 5264 Processor Functionality
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Ning WengECE 5265 Processor Pyramid
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Ning WengECE 5266 Packet Flow through Hierarchy Accommodating tasks of different complexity and frequency ─ Low level: simple and frequent processing ─ High level: occasional and complex processing Computation scaling ─ Faster processor ─ More concurrent threads ─ More processors ─ More processor types
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Ning WengECE 5267 Memory Hierarchy Different memory technologies used for performance, cost and area Conventional Approach: ─ Register + cache + off-chip DRAM Exploiting locality: temporal and spatial ─ Optimized for average case ─ Transparent to programmer Network Processors: ─ Register, scratch pad, control store, onboard RAM, CAM/TCAM, SRAM and SDRAM ─ Specialized for network processing application Little temporal locality ─ Explicit to application developer Different to programming More control ─ Memory hierarchy is not “cached” but used explicitly
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Ning WengECE 5268 Memory Technology Characterized by access latency, area ─ SRAM: 2-10 ns, 4-6 transistors ─ DRAM: 50-70 ns, 1 or 3 transistors What data should be store where? ─ Instruction data ─ Packets data: header, payload and meta-data ─ Temporal data: data structure allocated on the stack ─ Application data: persistent data, e.g., routing table, rule file
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Memory Size Example Consider a network system that processes IP datagram. Assume the system executes 5,000 instructions per packet, each instruction occupies 4 bytes, 10% of instructions need to access 4-byte value memory, each datagram consists of 1500 bytes, a lookup examines 10 4-byte values on average in an IP routing table, and a datagram arrives and leaves in an Ethernet frame. Compute the total number of memory locations accessed to process on datagram. Assume no memory caching. ─ Instruction Memory: ─ Packet Memory: ─ Application Memory: ─ Temporary Memory: Total: Ning WengECE 5269
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Ning WengECE 52610 Memory Scaling Memory access time: raw access speed ─ Technology dependent ─ Important for random access Memory bandwidth ─ Important for overall system performance ─ Scale with Multiple ports Multiple banks Wider bus ─ Limits by Pins and package cost
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Ning WengECE 52611 Content Addressable Memory Not using address to locate content CAM using content as input in a query-style format Organized as array of slots Combination of mechanisms ─ Random access storage ─ Exact-match pattern search Rapid search enabled with parallel hardware
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Ning WengECE 52612 Lookup using Conventional CAM Given ─ Pattern for which to search ─ Known as key CAM returns ─ First slot that match key or ─ All slots that match key Algorithm for each slot do { if (key == slot) { declare key matches slot; } else { declare key does not match slot; }
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Ning WengECE 52613 Ternary CAM (TCAM) Regular CAM ─ Binary value: 0 and 1 ─ Requiring key to match all the content in one slot ─ Not flexible TCAM ─ Ternary value: 0, 1 and don’t care ─ Implemented using masking of entries Good for network processor flow classification
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Ning WengECE 52614 TCAM Lookup Each slot has bit mask Hardware uses mask to decide which bits to test Algorithm for each slot do { if (key & mask ) == (slot & mask)) { declare key matches slot; } else { declare key does not match slot; }
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Ning WengECE 52615 Partial Matching using TCAM Key matched slot 1 Packet belonging to flow ID: 00.02 Here “additional information” stored in each slot
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Ning WengECE 52616 Classification using TCAM Flexibility: “additional information” stored in separate memory Extracting values from fields in headers Forming values in contiguous string Using a key for TCAM lookup Storing classification in slot
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Ning WengECE 52617 Communication Internal interfaces: channels between processing elements, memories ─ Internal bus ─ Hardware FIFO: sequential access ─ Transfer register: random access ─ Onboard shared memory: shared random access External interfaces ─ Memory interfaces: accesses to larger off-chip memory ─ Direct I/O interfaces: e.g., access to link interfaces ─ Bus interfaces: accesses to other devices, e.g., control CPU ─ Switching fabric interface Access to switching fabric Several standards (e.g., CSIX by NP Forum)
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Communication Cost Example Consider a second generation network system that forwards IP datagram. If the system has 16 interfaces that each connect to an OC-192 line (data rate is 10 Gbps). These 16 interfaces are interconnected with a shared communication channel. The packet size is in the range of 40 bytes to 1500 bytes. What aggregate bandwidth is needed on the communication channel for the two design scenarios: ─ Every bit of a packet transfers through the shared communication channels. ─ Only a 4-byte packet memory address transfers through the shared communication channels. Ning WengECE 52618
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Ning WengECE 52619 NP Operating Support Programming model: interrupt, event vs. thread based Parallel and concurrent execution support Dispatch mechanism: how threads are initiated
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Ning WengECE 52620 Summary NP scaling by ─ Heterogeneous multiprocessors structured hierarchically ─ Mixed memory technologies explicitly available to programmer ─ Different communication mechanisms ─ Operating support important to achieve high system performance NP scaling limited by ─ Physical space: chip area (less than 400 mm 2 ) ─ Pin limits and packaging technology ─ Power consumption and heat dissipation
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Ning WengECE 52621 For Next Class and Reminder Read Comer: chapter 15 and 16 Homework solution on-line by Friday Midterm: 10/6 Project ─ topic finalized 10/5 (group leader email me) ─ proposal presentation 10/22
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