1. Please do not distribute
4/24/2018
Integration for Heterogeneous SoC Modeling
Yakun Sophia Shao, Sam Xi,
Gu-Yeon Wei, David Brooks Harvard University
GYW

2. Today’s Accelerator-CPU Integration
Simple interface to accelerators: DMA
Easy to integrate lots of IP
Hard to program and share data
Core …
Core
Acc #1
Acc #n
L1 $
L1 $
SPAD
SPAD
L2 $
On-Chip System Bus
DMA
DRAM

3. Today’s Accelerator-CPU Integration
Simple interface to accelerators: DMA
Easy to integrate lots of IP
Hard to program and share data
Core …
Core
Acc #1
Acc #n
L1 $
L1 $
SPAD
SPAD
L2 $
On-Chip System Bus
DMA
DRAM

4. Typical DMA Flow Flush and invalidate input data from CPU caches.
Invalidate a region of memory to be used for receiving accelerator output.
Program a buffer descriptor describing the transfer (start, length, source, destination).
When data is large, program multiple descriptors
Initiate accelerator.
Initiate data transfer.
Wait for accelerator to complete.

5. DMA can be very expensive
Only 20% of total time!
16-way parallel md-knn accelerator

6. Co-Design vs. Isolated Design

7. Co-Design vs. Isolated Design

8. Co-Design vs. Isolated Design
No need to build such an aggressively parallel design!

9. gem5-Aladdin: An SoC Simulator

10. Features End-to-end simulation of accelerated workloads.
Models hardware-managed caches and DMA + scratchpad memory systems.
Supports multiple accelerators.
Enables system-level studies of accelerator-centric platforms.
Xenon: A powerful design sweep system.
Highly configurable and extensible.

11. DMA Engine Extend the existing DMA engine in gem5 to accelerators.
Special dmaLoad and dmaStore functions.
Insert into accelerated kernel.
Trace will capture them.
gem5-Aladdin will handle them.
Currently a timing model only.
Analytical model for cache flush and invalidation latency.
Flush throughput is 1 cache line per 56 cycles (84ns), plus 150 cycles overhead.
Invalidate throughput is 1 cache line per 47 cycles (70ns), plus 400 cycles overhead
All cycles are CPU clock cycles at 666MHz.

12. DMA Engine
/* Code representing the accelerator */ void fft1D_512(TYPE work_x[512], TYPE work_y[512]){ int tid, hi, lo, stride; /* more setup */ dmaLoad(&work_x[0], 0, 512 * sizeof(TYPE)); dmaLoad(&work_y[0], 0, 512 * sizeof(TYPE)); /* Run FFT here ... */ dmaStore(&work_x[0], 0, 512 * sizeof(TYPE)); dmaStore(&work_y[0], 0, 512 * sizeof(TYPE)); }
Maybe note that this is a bit different from how some platforms do DMA. In our case, the accelerator initiates the transfer, while on most platforms, the CPU initiates the transfer. But it doesn’t really matter for us – one way or another, the cost of DMA will be paid, and it’s just a matter of a small bit of power on the CPU side.

13. Caches and Virtual Memory
Gaining traction on multiple platforms.
Intel QuickAssist QPI-Based FPGA Accelerator Platform (QAP)
IBM POWER8’s Coherent Accelerator Processor Interface (CAPI)
System vendors provide a Host Service Layer with virtual memory and cache coherence support.
Host service layer communicates with CPUs through an agent.
Processors
FPGA
Host service layer might contain a cache and TLB.
Accelerator agent would snoop the system buses on behalf of the accelerator and service cache misses/TLB misses.
QPI/PCIe
Core …
Core
Accelerator
Acc
Agent
L1 $
L1 $
Host Service Layer
L2 $

14. Caches and Virtual Memory
Accelerator caches are connected directly to system bus.
Support for multi-level cache hierarchies.
Hybrid memory system: can use both caches and scratchpads.
Basic MOESI coherence protocol.
Special Aladdin TLB model.
Map trace address space to simulated address space.

15. Demo: DMA
Exercise: change system bus width and see effect on accelerator performance.
Open up your VM.
Go to: ~gem5-aladdin/sweeps/tutorial/dma/stencil-stencil2d/0
Examine these files:
stencil-stencil2d.cfg
../inputs/dynamic_trace.gz
gem5.cfg
run.sh

16. Demo: DMA Run the accelerator with DMA simulation
Change the system bus width to 32 bits
Set xbar_width=4 in run.sh
Run again.
Compare results.
At 64-bits, cycles is 37058
At 32 bits, cycles is 45246

17. Demo: Caches
Exercise: see effect of cache size on accelerator performance.
Go to: ~gem5-aladdin/sweeps/tutorial/cache/stencil-stencil2d/0
Examine these files:
../inputs/dynamic_trace.gz
stencil-stencil2d.cfg
gem5.cfg

18. Demo: Caches Run the accelerator with caches simulation
Change the cache size to 1kB.
Set cache_size = 1kB in gem5.cfg.
Run again.
Compare results.
Play with some other parameters (associativity, line size, etc.)
Run with cache size = 1kB (92225)
Change cache size = 4kB (77738)

19. CPU – Accelerator Cosimulation
CPU can invoke an attached accelerator.
We use the ioctl system call.
Communicate status through shared memory.
Spin wait for accelerator, or do something else (e.g. start another accelerator).

20. Code example
/* Code running on the CPU. */ void run_benchmark(TYPE work_x[512], TYPE work_y[512]) { }

21. Code example
/* Code running on the CPU. */ void run_benchmark(TYPE work_x[512], TYPE work_y[512]) { /* Establish a mapping from simulated to trace * address space */ mapArrayToAccelerator(MACHSUITE_FFT_TRANSPOSE, "work_x", work_x, sizeof(work_x)); }
ioctl request code
Associate this array name with the addresses of memory accesses in the trace.
Starting address and length of one memory region that the accelerator can access.

22. Code example
/* Code running on the CPU. */ void run_benchmark(TYPE work_x[512], TYPE work_y[512]) { /* Establish a mapping from simulated to trace * address space */ mapArrayToAccelerator(MACHSUITE_FFT_TRANSPOSE, "work_x", work_x, sizeof(work_x)); mapArrayToAccelerator(MACHSUITE_FFT_TRANSPOSE, "work_y", work_y, sizeof(work_y)); }

23. Code example
/* Code running on the CPU. */ void run_benchmark(TYPE work_x[512], TYPE work_y[512]) { /* Establish a mapping from simulated to trace * address space */ mapArrayToAccelerator(MACHSUITE_FFT_TRANSPOSE, "work_x", work_x, sizeof(work_x)); mapArrayToAccelerator(MACHSUITE_FFT_TRANSPOSE, "work_y", work_y, sizeof(work_y)); // Start the accelerator and spin until it finishes. invokeAcceleratorAndBlock(MACHSUITE_FFT_TRANSPOSE); }

24. Demo: disparity You can just watch for this one.
If you want to follow along: ~/gem5-aladdin/sweeps/tutorial/cortexsuite_sweep/0
This is a multi-kernel, CPU + accelerator cosimulation.

25. How can I use gem5-Aladdin?
Investigate optimizations to the DMA flow.
Study cache-based accelerators.
Study impact of system-level effects on accelerator design.
Multi-accelerator systems.
Near-data processing.
All these will require design sweeps!

26. Xenon: Design Sweep System
A small declarative command language for generating design sweep configurations.
Implemented as a Python embedded DSL.
Highly extensible.
Not gem5-Aladdin specific.
Not limited to sweeping parameters on benchmarks.
Why “Xenon”?

27. Xenon: Generation Procedure
Read sweep configuration file
Execute sweep commands
Generate all configurations
Export configurations to JSON
Backend: read JSON, rewrite into desired format.
Backend: Generate any additional outputs

28. Xenon: Data Structures
md-knn
md_kernel
loop_i
loop_j
force_x
force_y
force_z
cycle_time
pipelining
partition_type
partition_factor
memory_type
Example of a Python data structure that Xenon will operate on.
unrolling
A benchmark suite would contain many of these.

29. Xenon: Commands
set unrolling 4 set partition_type “cyclic” set unrolling for md_knn.* 8 set partition_type for md_knn.force_x “block” sweep cycle_time from 1 to 5 sweep partition_factor from 1 to 8 expstep 2 set partition_factor for md_knn.force_x 8 generate configs generate trace

30. Xenon: Execute Every configuration in a JSON file.
"Benchmark(\"md-knn\")": { "Array(\"NL\")": { "memory_type": "cache", "name": "NL", "partition_factor": 1, "partition_type": "cyclic", "size": 4096, "type": "Array", "word_length": 8 }, "Array(\"force_x\")": { "name": "force_x", "size": 256, "Array(\"force_y\")": { "name": "force_y", } ...
Every configuration in a JSON file.
A backend is then invoked to load this JSON object and write application specific config files.

31. Enough talking…let’s do a demo

32. Demo: Design Sweeps with Xenon
Exercise: sweep some parameters.
Go to: ~gem5-aladdin/sweeps/tutorial/
Examine these files:
../inputs/dynamic_trace.gz
stencil-stencil2d.cfg
gem5.cfg

33. Tutorial References
Y.S. Shao, S. Xi, V. Srinivasan, G.-Y. Wei, D. Brooks, “Co-Designing Accelerators and SoC Interfaces using gem5-Aladdin”, MICRO, 2016.
Y.S. Shao, S. Xi, V. Srinivasan, G.-Y. Wei, D. Brooks, “Toward Cache-Friendly Hardware Accelerators”, SCAW, 2015.
Y.S. Shao and D. Brooks, “ISA-Independent Workload Characterization and its Implications for Specialized Architectures,” ISPASS’13.
B. Reagen, Y.S. Shao, G.-Y. Wei, D. Brooks, “Quantifying Acceleration: Power/Performance Trade-Offs of Application Kernels in Hardware,” ISLPED’13.
Y.S. Shao, B. Reagen, G.-Y. Wei, D. Brooks, “Aladdin: A Pre-RTL, Power-Performance Accelerator Simulator Enabling Large Design Space Exploration of Customized Architectures,” ISCA’14.
B. Reagen, B. Adolf, Y.S. Shao, G.-Y. Wei, D. Brooks, “MachSuite: Benchmarks for Accelerator Design and Customized Architectures,” IISWC’14.

34. Backup slides

35. Validation Implemented accelerators in Vivado HLS
Designed complete system in Vivado Design Suite

36. Reducing DMA Overhead

37. Reducing DMA Overhead

38. Reducing DMA Overhead

39. DMA Optimization Results
Overlap of flush and data transfer

40. DMA Optimization Results
Overlap of data transfer and compute

41. DMA Optimization Results
md-knn is able to completely overlap compute with data transfer.

42. DMA vs Caches DMA Push-based data access Bulk transfer efficiency
Simple hardware, lower power
Manual coherence management
Manual optimizations
Coarse grained communication
Cache
On-demand data access
Automatic coherence handling
Fine grained communication
Automatic eviction
Larger, higher power cost
Less efficient at bulk data movement.

43. DMA vs Caches DMA is faster and lower power than caches.
Regular access patterns
Small input data size
Caches will have cold misses.

44. DMA vs Caches DMA and caches are approximately equal.
Power is dominated by floating point units.

45. DMA vs Caches DMA is faster and lower power than caches.
Regular access patterns
Small input data size
Caches will have cold misses.