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Operation of the SM Pipeline
©Sudhakar Yalamanchili unless otherwise noted
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Objectives Cycle-level examination of the operation of major pipeline stages in a stream multiprocessor Understand the type of information necessary for each stage of operation Identification of performance bottlenecks Detailed implementations are addressed in subsequent modules
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Reading Documentation for the GPGPUSim simulator
Good source of information about the general organization and operation of a stream multiprocessor Operation of a Scoreboard X. Xiang, Y. Yiang, H. Zhou, “Warp Level Divergence in GPUs: Characterization, Impact, and Mitigation,” International Symposium on High Performance Computer Architecture, 2014. D. Tarjan and K. Skadron, “On Demand Register Allocation and Deallocation for a Multithreaded Processor,” US Patent 2011/ A1, June 2011
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NVIDIA GK110 (Keplar) Thread Block Scheduler
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SMX Organization : GK 110 Multiple Warp Schedulers 64K 32-bit registers 192 cores – 6 clusters of 32 cores each What are the main stages of a generic SMX pipeline? Image from
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A Generic SM Pipeline Scalar Fetch & Decode
Warp 6 Warp 1 Warp 2 Decode RF PRF D-Cache Data All Hit? Writeback Pending Warps Pipeline scalar pipeline Issue I-Buffer I-Fetch Miss? Scalar Fetch & Decode Instruction Issue & Warp Scheduler Front-end Predicate & GP Register Files Scalar Cores Scalar Pipelines Data Memory Access Back-end Writeback/Commit
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Single Warp Execution PC AM WID State warp state Thread Block
setp.lt.s32 %p, %r5, %rd4; //r5 = index, rd4 = N @p bra L1; bra L2; L1: ld.global.f32 %f1, [%r6]; //r6 = &a[index] ld.global.f32 %f2, [%r7]; //r7 = &b[index] add.f32 %f3, %f1, %f2; st.global.f32 [%r8], %f3; //r8 = &c[index] L2: ret; PTX (Assembly): Grid
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Instruction Fetch & Decode
I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps Examples from Harmonica2 GPU PC AM WID State Instr Warp 0 PC Warp 1 PC To I-Cache Warp n-1 PC May realize multiple fetch policies Next Warp From GPGPU-Sim Documentation
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Instruction Buffer Buffer a fixed number of instructions per warp
I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps Example: buffer 2 instructions/warp Decoded instruction V Instr 1 W1 R Instr 2 W1 Instr 2 Wn Instr 1 W2 Scoreboard ECE 6100/CS 6290 Buffer a fixed number of instructions per warp Coordinated with instruction fetch Need an empty I-buffer for the warp V: valid instruction in the buffer R: instruction ready to be issued Set using the scoreboard logic From GPGPU-Sim Documentation
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Instruction Buffer (2) Scoreboard enforces and WAW and RAW hazards
I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps V Instr 1 W1 R Instr 2 W1 Instr 2 Wn Instr 1 W2 Scoreboard Scoreboard enforces and WAW and RAW hazards Indexed by Warp ID Each entry hosts required registers, Destination registers are reserved at issue Reserved registers released at writeback Enables multiple instructions to be issued from a single warp From GPGPU-Sim Documentation
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Instruction Buffer (3) Scoreboard Generic Scoreboard Name Busy Op Fi
I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps V Instr 1 W1 R Instr 2 W1 Instr 2 Wn Instr 1 W2 Scoreboard Generic Scoreboard dest reg src1 src2 Source Registers have value? Function unit producing value Name Busy Op Fi Fj Fk Qj Qk Rj Rk Int Yes Load F2 R3 No From GPGPU-Sim Documentation
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Instruction Issue I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps pool of ready warps Warp 3 Warp 8 Warp 7 instruction Warp Scheduler Manages implementation of barriers, register dependencies, and control divergence From GPGPU-Sim Documentation
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Instruction Issue (2) warp I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps barrier Warp 3 Warp 8 Warp 7 instruction Warp Scheduler Barriers – warps wait here for barrier synchronization All threads in the CTA must reach the barrier From GPGPU-Sim Documentation
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Instruction Issue (3) I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps Scoreboard V Instr 1 W1 R Instr 2 W1 Instr 2 Wn Instr 1 W2 Warp 3 Warp 8 Warp 7 instruction Warp Scheduler Register Dependencies - track through the scoreboard From GPGPU-Sim Documentation
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Instruction Issue (4) Control Divergence - per warp stack
I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps divergent warps Warp 3 Warp 8 Warp 7 instruction Keeps track of divergent threads at a branch Warp Scheduler SIMT Stack (per warp) Control Divergence - per warp stack From GPGPU-Sim Documentation
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Instruction Issue (5) Scheduler can issue multiple instructions from a warp Issue conditions Has valid instructions Not waiting at a barrier Scoreboard check Pipeline line is not stalled: operand access stage (will get to it later) Reserve destination registers Instructions may issue to memory, SP or SFU pipelines Warp scheduling disciplines more later in the course
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Single ported Register File Banks
Register File Access Banks 0-15 I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps Arbiter RF n-1 RF n-2 RF n-3 RF n-4 RF1 RF0 RF n-1 RF n-2 RF n-3 RF n-4 RF1 RF0 RF n-1 RF n-2 RF n-3 RF n-4 RF1 RF0 Single ported Register File Banks 1024 bit Xbar OC OC OC OC Operand Collectors (OC) DU DU DU DU Dispatch Units (DU) ALUs L/S SFU
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Scalar Pipeline Functional units are pipelined
I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps Functional units are pipelined Designs with multiple issue Dispatch ALU FPU LD/SD Result Queue A Single Core
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Shared Memory Access Multiple bank organization
I-Fetch Decode RF PRF D-Cache Data All Hit? Writeback Pipeline scalar pipeline Issue I-Buffer pending warps 2-way Conflict access Conflict free access Multiple bank organization Data is interleaved across banks Bank conflicts extend access times
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Memory Request Coalescing
Memory Requests Tid RQ Size Base Add Offset Pending Request Table Memory Address Coalescing Pending RQ Count Addr Mask Thread Masks PRT is filled whenever a memory request is issued Generate a set of address masks one for each memory transaction Issue transactions From J. Leng et.al., “GPUWattch : Enabling Energy Optimizations in GPGPUs,’ ISCA 2013
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Case Study: Keplar GK 110 From GK110: NVIDIA white paper
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Keplar SMX Up to two instruction can be issued per warp
A slice of the SMX From GK110: NVIDIA white paper Up to two instruction can be issued per warp E.g., LD and SFU More flexible instruction paring rules More efficient support for atomic operations in global memory – both latency and throughput E.g., atomicADD, atomicEXC
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Shuffle Instruction Permits threads in a warp to share data
From GK110: NVIDIA white paper From GK110: NVIDIA white paper Permits threads in a warp to share data Avoid a load-store sequence Reduce the shared memory requirement per TB increase occupancy Data exchanged in registers without using shared memory Some operations become more efficient
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Memory Hierarchy Configurable cache/shared memory configuration for L1
warp Configurable cache/shared memory configuration for L1 Read-only cache for compiler or developer (intrinsics) use Shared L2 across all SMXs ECC coverage across the hierarchy Performance impact L1 Cache Shared Memory Read-Only Cache L2 Cache DRAM From GK110: NVIDIA white paper
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Dynamic Parallelism The ability for device-side nested kernel launch
From GK110: NVIDIA white paper The ability for device-side nested kernel launch Eliminates host-GPU interactions Current overheads are high Matches a wider range of parallelism patterns – will cover in more depth later Examples of recursive, data dependent parallelism – AMR Can get by with a weaker CPU?
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Concurrent Kernel Launch
From GK110: NVIDIA white paper Kernels from multiple streams are now mapped to distinct hardware queues TBs from multiple kernels can share a SMX
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Warp and Instruction Dispatch
From GK110: NVIDIA white paper
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Grid Management Multiple grids launched from both CPU and GPU can be handled in Keplar Need the ability to re-prioritize and schedule new grids
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Summary Synchronous progress of a warp through the SM pipelines
Warp progress in a thread block can diverge for many reasons Barriers Control divergence Memory divergence How is the execution optimized? Next
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