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RSVM: a Region-based Software Virtual Memory for GPU Feng Ji*, Heshan Lin†, Xiaosong Ma*‡ * North Carolina State Univeristy † Virginia Tech ‡Oak Ridge.

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Presentation on theme: "RSVM: a Region-based Software Virtual Memory for GPU Feng Ji*, Heshan Lin†, Xiaosong Ma*‡ * North Carolina State Univeristy † Virginia Tech ‡Oak Ridge."— Presentation transcript:

1 RSVM: a Region-based Software Virtual Memory for GPU Feng Ji*, Heshan Lin†, Xiaosong Ma*‡ * North Carolina State Univeristy † Virginia Tech ‡Oak Ridge National Lab PACT 2013

2 GPU Presence Today ComputeGraphics 2

3 GPU Computing Challenge Parallel computing Memory management 3 Core 0 Core 1 Core 2 Core 3 Main memory Device memory L1/ Shared memory PCI-E CPU GPU cache L2 SM

4 Problematic Manual GPU Memory Management Malloc() – Resource limit Memcpy() – Hardcoded Working set 4 A B C Matrix Mul C = A x B Host Code Device Code

5 CPU-GPU Memory Management State of the art: host-side memory management – GPU compilers [Jablin:PLDI11, Jabin:CGO12; Pai:PACT12] – Task scheduling runtimes [Rossbach: SOSP11] – GPU ADSM [Gelado: ASPLOS10] Limitations – Memory management action before/after GPU kernel – No fine-grained control in GPU code – Cannot leverage GPU online data access GPU Runtime VMDriver DRAMCPUBUSGPU Host CodeDevice Code GPGPU API Application OS Hardware 5 CPU-GPU memory management Our solution: Enabling memory management in GPU kernel Host-side management Existing solution: host side controlling GPU memory

6 Region-based software virtual memory Match host-side virtual memory – Abstract memory domains – Automate data movement on demand – Swap out device memory Unique challenges of CPU-GPU heterogeneous memory system – No existing architecture support for VM Solution: software-based mechanisms – GPU processing massively parallel Solution: building on GPU atomic operations – GPU and drivers being black boxes Solution: implementing using standard GPGPU APIs – CPU-GPU synchronization expensive Solution: asynchronous runtimes, relaxed consistency – GPU-initiated communication difficult Solution: GPU callback Region – Repeated idea (CRL [Johnson:SOS95], ADSM [Gelado: ASPLOS10], etc.) – Finer granularity 6

7 Roadmap Introduction RSVM: region-based software virtual memory for GPU – Region API – Design: region table, transparent GPU swap Evaluation results Conclusion 7

8 Region-based Software Virtual Memory for GPU (RSVM) User specifies via Region API – Defining RSVM managed data unit (create) – Annotating data unit access code block (map/unmap) RSVM manages both CPU and GPU memory – Moving data on-demand across CPU and GPU – Intra-kernel data fetching to GPU – Transparent GPU memory swapping to host memory For GPU kernels with excessive memory requirement 8

9 RSVM Design GPU Runtime VM Driver DRAMCPUBUSGPU Host CodeDevice Code GPGPU API Application OS Hardware RSVM Region API Callback Server Region Manager Region Table Callback RPC Region Manager Region Table Callback 9

10 Region as basic data unit Decide system-managed basic data unit – Page? One size fit all? Region – User-defined data block – Linear or multi-D (CUDA supports 3D memory layout): Benefit – Abstracts CPU/GPU memory domain – Allows optimization of PCIe efficiency by varying region’s definition developers know it better than system – No false sharing 10 A B C

11 Region API 1: define region and region collection rgn_id r_A = rgn_create_cpu (size_A); rgn_coll_id rc_B = rgn_coll_create (size_B, num_rgns_in_B, B_row_length, //stride threadBlock.y, //width B_rows); //height rgn_coll_id rc_C = rgn_coll_create (size_C, num_rgns_in_C, C_row_length, threadBlock.y, C_rows); 11 A B C Region Collection: a set of regions. Rgn_coll_create: iteratively create all regions in this set.

12 Region API 2: use region in the host float *A = rgn_map_cpu (r_A, rgn_op_writeonly, NULL, NULL); rgn_coll_meta meta_B = rgn_coll_get_meta_cpu (rc_B); for(i: 0 to blockDim.y) { float *pB = rgn_map_cpu (meta_B->rgns[i], rgn_op_writeonly, NULL, NULL); } 12 A B C Region Collection Meta: metadata of region collection. An array of rgn_ID’s. Implemented as a region Itself. meta_B

13 Region API 3: exchange information rsvm_sync(); mmKernel >> (rc_C, r_A, rc_B); 13 A B C rsvm_sync(): host- side API. Exchange information across CPU-GPU. GPU side runtime knows r_A, rc_B, and rc_C exist.

14 Region API 4: use region in GPU kernel int complete, req; float *dA = rgn_map_gpu (r_A, rgn_op_readonly, &complete, &req); if (!complete) dA = rgn_wait_map_gpu(r_A, req); rgn_unmap_gpu(r_A); 14 A B C Asynchronously mapping in the background. useful work: e.g. map other regions

15 Region API 5: use region collection in GPU kernel rgn_coll_meta meta_B = rgn_coll_get_meta_gpu (rc_B); float *pdB = rgn_map_gpu (meta_B->rgns[blockIdx.y], rgn_op_readonly, &complete, &req); if (!complete) pdB = rgn_wait_map_gpu(meta_B- >rgns[blockIdx.y], req); rgn_unmap_gpu(meta_B- >rgns[blockIdx.y]); 15 A B C

16 Region States in RSVM Relaxed consistency – Host and Device runtimes asynchronously drive state change until rsvm_sync Protocol: MSI adapted protocol 16

17 Region Table idstatusversio n loc_ptrfar_ptr 0mod50xaaa00x8000 1shared20xaee0 2shared10xacd0 3unused0 4 0 idstatusversio n loc_ptrfar_ptr 0shared40x80000xaaa0 1sharing20x80d00xaee0 2invalid10xacd0 3 4 Table replicated on CPU and GPU – Relaxed consistency: merge at sync Challenge: local operation vs. avoid conflict Table partitioned – 4096 entries / segment, owned by one side – New region from unused entry in one’s own segment: local op. – Allocating a new segment: synchronous op. 17 Software TLB in the shared memory Consistency CPU – Owner GPU

18 GPU region fault: asynchronous map Rgn_map_GPU(rgn, op, *complete, *req) Rgn_wait_map_GPU(rgn, req) Device CodeCallback Server Callback RPC Region Manager Map_GPU() Call_async(map) Return (req) Return (complete, req) Set callback flag on GPU Wait_map_GPU(req) Call_wait(req) PCIe data transfer to GPU buffer 18 Call back: Host-side polling [Stuart:Europarw10] Avoid PCIe traffic jam Novel collective callback

19 GPU Transparent Memory swap Challenge: no specialized GPU thread Solution: embedding swap in map/unmap ops Split operations, triggered by low memory – Operation 1: Swap GPU requests CPU to fetch dirty regions – Operation 2: Reclaim GPU frees clean buffers Not-Frequent-Used (NFU) counter of each region – Updated by GPU in every map op. – Sorted by CPU during swap Swap made re-entrant: concurrent swap requester will – Back off seeing ongoing swap – Prepare candidates list from previous completed swap 19 T swap T rec T end rec

20 Evaluation Test bed – Intel x86 Xeon E5507, 6 GB main mem – Nvidia GTX480 (15 SM, 1.5 GB dev mem), PCIe 2.0 – Ubuntu 10.04 LTS, linux 2.6.32, CUDA 5.0rc Benchmark workload – Benchmark from CUDA SDK, Rodinia [Uva:rodinia] – Case study: MatrixMul, BFS [ORNL:SHOC] 20

21 Benchmarks fit in GPU MatrixMul – Computation-intensive, scale well with GPU cores – Overhead: device library code compiled into GPU kernel – Register file pressure: (# of reg / thread ): 25 -> 60 Occupancy (active threads / SM ): 1024 -> 512 21

22 Discussion: GPU register file for RSVM device library code GPU register assignment to threads – Static, equally to each thread Compiler reports max register count requirement for each thread Runtime calculates occupancy, kernel launch success/fail GPU register file not enough for RSVM Not all threads run into RSVM library code path concurrently Possible way of over-subscribing threads for register file usage? – Dynamically managing registers among threads? 22

23 Case study: Graph Breadth-first Search Iteration (kernel) by BFS distance Metric: traversed edges/sec (TEPS) Dynamic memory access patter – input dependent – DIMACS challenge [DIMACS] – GTgraph [Gtgraph] 23

24 BFS Input m/n – edge factor, number of edge/vector 24 (N and M in 10^6.)

25 BFS parallelism Graph: nodes + adjacent list (edges) – Warp -> each node to visit in current BFS iteration – Thread -> each neighbor of this node RSVM’s overhead – setup in each kernel (BFS iteration) – map nodes’ region, and then – map adjacent list’s region Overhead decreases with increased edge factor 25

26 Large Graphs Manual : – Partition graphs – Manual swapping between GPU buffers and host buffers in each BFS iteration – Local data access – Depend on used data in each data partition CUDA Unified Virtual Address (UVA) : – Use host-side 0-copy buffers – Access only needed data – PCIe bottleneck in traffic jam 26 RSVM Improvement due to Caching in GPU memory Batched PCIe data transfer Additional advantage Single code base UVA performs better than Manual Manual performs better than UVA

27 Conclusion Virtual memory for CPU-GPU heterogeneous system involving GPU-side runtime is possible – GPU as computation engine, rather than co- processor – Novel designs: region table, asynchronous region API, CPU assisted GPU swap, software TLB in GPU shared memory – Insight: register file pressure – Benefit dynamic memory accesses (e.g. Graph) 27

28 THANK YOU! 28

29 Reference [Augonnet:ICPADS10] C. Augonnet, J. Clet-Ortega, S. Thibault, and R. Namyst. Data-Aware Task Scheduling on Multi-accelerator Based Platforms. Parallel and Distributed Systems, International Conference on, 0:291–298, 2010. [CUDA] NVIDIA CUDA. [Diamos:HPDC08] G. F. Diamos and S. Yalamanchili. Harmony: an execution model and runtime for heterogeneous many core systems. In Proceedings of the 17th international symposium on High performance distributed computing, HPDC ’08, pages 197–200, New York, NY, USA, 2008. ACM. [DIMACS] 10th DIMACS Implementation Challenge - Graph Partitioning and Graph Clustering. [Eichenberger:PACT05] A. E. Eichenberger, K. O’Brien, K. O’Brien, P. Wu, T. Chen, P. H. Oden, D. A. Prener, J. C. Shepherd, B. So, Z. Sura, A. Wang, T. Zhang, P. Zhao, and M. Gschwind. Optimizing Compiler for the CELL Processor. In Proceedings of the 14th International Conference on Parallel Architectures and Compilation Techniques, PACT ’05, pages 161–172, Washington, DC, USA, 2005. IEEE Computer Society. [Fatahalian:SC06] K. Fatahalian, D. R. Horn, T. J. Knight, L. Leem, M. Houston, J. Y. Park, M. Erez, M. Ren, A. Aiken, W. J. Dally, and P. Hanrahan. Sequoia: programming the memory hierarchy. In Proceedings of the 2006 ACM/IEEE conference on Supercomputing, SC ’06, New York, NY, USA, 2006. ACM. [Gelado: ASPLOS10] I. Gelado, J. E. Stone, J. Cabezas, S. Patel, N. Navarro, and W.-m. W. Hwu. An asymmetric distributed shared memory model for heterogeneous parallel systems. In Proceedings of the fifteenth edition of ASPLOS on Architectural support for programming languages and operating systems, ASPLOS ’10, pages 347–358, New York, NY, USA, 2010. ACM. [GTgraph] GTgraph: A suite of synthetic random graph generators. madduri/software/GTgraph/index.html. [HSA] The HSA Foundation. 29

30 References [Jablin:CGO12] T. B. Jablin, J. A. Jablin, P. Prabhu, F. Liu, and D. I. August. Dynamically managed data for CPU-GPU architectures. In Proceedings of the Tenth International Symposium on Code Generation and Optimization, CGO ’12, pages 165–174, New York, NY, USA, 2012. ACM. [Jablin:PLDI11] T. B. Jablin, P. Prabhu, J. A. Jablin, N. P. Johnson, S. R. Beard, and D. I. August. Automatic CPU-GPU communication management and optimization. In Proceedings of the 32nd ACM SIGPLAN conference on Programming language design and implementation, PLDI ’11, pages 142–151, New York, NY, USA, 2011. ACM. [Johnson:SOS95] K. L. Johnson, M. F. Kaashoek, and D. A. Wallach. CRL: high performance all-software distributed shared memory. In Proceedings of the 15th ACM Symposium on Operating Systems Principles (SOSP ’95), pages 213–226, Copper Mountain Resort, Colorado, December 1995. [Kato:USENIX12] S. Kato, M. McThrow, C. Maltzahn, and S. Brandt. Gdev: First-class GPU resource management in the operating system. In Proceedings of the USENIX Annual Technical Conference (ATC), June 2012. [Linderman:ASPLOS08] M. D. Linderman, J. D. Collins, H. Wang, and T. H. Meng. Merge: a programming model for heterogeneous multi-core systems. In Proceedings of the 13th international conference on Architectural support for programming languages and operating systems, ASPLOS XIII, pages 287–296, New York, NY, USA, 2008. ACM. [Luk:MICRO09] C.-K. Luk, S. Hong, and H. Kim. Qilin: exploiting parallelism on heterogeneous multiprocessors with adaptive mapping. In Proceedings of the 42nd Annual IEEE/ACM International Symposium on Microarchitecture, MICRO 42, pages 45–55, New York, NY, USA, 2009. ACM. [Menon:ISCA12] J. Menon, M. De Kruijf, and K. Sankaralingam. iGPU: exception support and speculative execution on GPUs. In Proceedings of the 39 th Annual International Symposium on Computer Architecture, ISCA ’12, pages 72–83, Washington, DC, USA, 2012. IEEE Computer Society. [ORNL:SHOC] A. Danalis, G. Marin, C. McCurdy, J. S. Meredith, P. C. Roth, K. Spafford, V. Tipparaju, and J. S. Vetter. The Scalable Heterogeneous Computing (SHOC) benchmark suite. In Proceedings of the 3 rd Workshop on General- Purpose Computation on Graphics Processing Units, GPGPU ’10, pages 63–74, New York, NY, USA, 2010. ACM. 30

31 Reference [Pai:PACT’12] S. Pai, R. Govindarajan, and M. J. Thazhuthaveetil. Fast and efficient automatic memory management for GPUs using compiler-assisted runtime coherence scheme. In Proceedings of the 21st international conference on Parallel architectures and compilation techniques, PACT’12, pages 33–42, New York, NY, USA, 2012. ACM. [Rossbach: SOSP11] C. J. Rossbach, J. Currey, M. Silberstein, B. Ray, and E. Witchel. PTask: operating system abstractions to manage GPUs as compute devices. In Proceedings of the Twenty-Third ACM Symposium on Operating Systems Principles, SOSP ’11, pages 233–248, New York, NY, USA, 2011. ACM. [Saha:PLDI09] B. Saha, X. Zhou, H. Chen, Y. Gao, S. Yan, M. Rajagopalan, J. Fang, P. Zhang, R. Ronen, and A. Mendelson. Programming model for a heterogeneous x86 platform. In Proceedings of the 2009 ACM SIGPLAN conference on Programming language design and implementation, PLDI ’09, pages 431–440, New York, NY, USA, 2009. ACM. [Silberstein:ASPLOS13] M. Silberstein, B. Ford, I. Keidar, and E. Witchel. GPUfs: Integrating a File System with GPUs. In Proceedings of ASPLOS 2013, 2013. [Uva:Rodinia] Rodinia benchmark. skadron/wiki/rodinia/ index.php/Main Page. [Yan:OSR11] S. Yan, X. Zhou, Y. Gao, H. Chen, G. Wu, S. Luo, and B. Saha. Optimizing a shared virtual memory system for a heterogeneous CPU-accelerator platform. SIGOPS Oper. Syst. Rev., 45:92–100, February 2011. [Stuart:Europarw10] J. Stuart, M. Cox, and J. Owens. GPU-to-CPU Callbacks. In M. Guarracino, F. Vivien, J. Trff, M. Cannatoro, M. Danelutto, A. Hast, F. Perla, A. Knpfer, B. Di Martino, and M. Alexander, editors, Euro-Par 2010 Parallel Processing Workshops. 31

32 Backup slides Start here…. 32

33 Related Work Memory hierarchy of accelerator – Specialized programming model StarPU [Augonnet:ICPADS10], Harmony [Diamos:HPDC08], Sequoia [Fatahalian:SC06], Merge [Linderman:ASPLOS08], Qilin [Luk:MICRO09] – Transparent Software Caching CellBE: [Eichenberger:PACT05] Larrabee: [Saha:PLDI09, Yan:OSR11] 33

34 Related Work (cont’d) Compiler assisted CPU-GPU communication – ADSM [Gelado: ASPLOS10] – CGCM [Jablin:PLDI11], DyManD [Jablin: CGO12] – AMM for X10 [Pai:PACT12] OS support for GPGPU – Gdev [Kato:USENIX12] – Ptask [Rossbach:SOSP11] – GPUfs [Silberstein:ASPLOS13] 34

35 Related Work (cont’d) Distributed shared memory – ADSM [Gelado:ASPLOS10] – CRL [Johnson:SOSP95] GPU virtual memory architecture – HSA hUMA [HSA] – GPU Exception [Menon:ISCA12] 35

36 Transparent or Manual? Ideal: transparent & good performance In practice: making compromise TransparentManual Program Performance easyhard hard to reason easy to control 36

37 Region’s State Protocol 37

38 Region’s State Protocol 38

39 Region’s State Protocol 39

40 Region’s State Protocol 40

41 Software TLB for Region Table on GPU TLB state Ref. count Rgn_idstatus used24138sharing unused0 Shared memory (TLB) consistency with device memory (Region table) – Write through Shared memory (TLB) of two SMs – A safe cache line: cache hit Define: sharing/modifying Some other warp has cached it Can safely use it – Otherwise: cache miss Prepare TLB AtomInc/Dec ref. count – Fully-associative warp parallelism – Cache line reuse Shared/modified:Refcnt 0 Number configurable idstatusRef. count … 4138 sharing2 4149shared0 41 TLB in shared memory Region Table in dev memory

42 GPU callback Host-side callback server thread polling a flag [Stuart:Europarw10] – GPU code remotely sets flag (in host-side 0-copy memory) Challenge: GPU parallelism Avoid PCIe traffic jam – Novel collective callback: non-parameterized requests GPU code detects and sends one signal for all calling threads – Host-side callback server batches PCIe data transfers for multiple concurrent callback requests Both incoming parameters and returning values 42

43 GPU callbacks in RSVM Handling region fault – non-collective, asynchronous, and parameterized callback Getting new region segment – collective, synchronous, and non-parameterized callback Starting swap – collective, asynchronous, re-entrant, and non- parameterized callback 43 CallbackCollectiveSynchronousParameterizedRe-entrant Handling region faultNo YesNo Getting new region segment Yes No Starting swapYesNo Yes

44 Case 1: Matrix Multiplication Matrix A: single region Matrix B: 2-d regions 1280 MB GPU dev mem managed by RSVM RSVM: ~70% efficiency Swap: <10% overhead 44

45 Small Graph BFS TEPS – Traversed edges/ sec Iteration (kernel) by BFS distance Parallelism – Warp -> each visiting node – Thread -> each neighbor of the visiting node Overhead – RSVM mapping regions of each visit node’s adjacent list – RSVM setup each kernel 45

46 Future Work RSVM improvement – Region table merging optimization – CPU callback server optimization – Multiple GPU support – Multiple process support Compiler assisted region identification – Remove manual region creation/deletion Leverage vendor support for GPU faulting – Remove manual map/unmap 46 Johnson:SOS95

47 GPU Transparent Memory swap Callback Server Callback RPC Region Manager Call_async_reentrant (swap) Set callback flag on GPU PCIe data transfer from GPU buffer to host mem Call_async_reentrant (swap) Available dev mem resource low Call_async_reentrant (swap) Return (swapped regions) Rgn states to shared, Form a candidate list Available dev mem resource keep decreasing. Reclaim candidate rgn’s buffers. 47

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