AI Engine Graph Execution and Measurement - 2024.1 English - XD100

Graph and Kernel Code

1. Run the following make command to compile the design:

   make aie
   

2. Open the compile result in an AMD Vitis™ Analyzer. It is found in the following directory:

   vitis_analyzer Work/graph.aiecompile_summary
   

3. Examine the graph view in Vitis Analyzer.

   

There are two kernels, aie_dest1 and aie_dest2 in the design. These two kernels are connected through a stream connection and a buffer connection (ping-pong buffers buf1 and buf1d). The stream connection contains two stream switch FIFOs, Fifo0(24,0) and Fifo1(24,0). These hardened FIFOs are in the stream switch of AI Engine array tile 24_0. These FIFOs are inserted to prevent deadlock in the design (see AI Engine Hang Analysis).

The input from the PL is connected to the ping-pong buffers buf0 and buf0d, which are read by kernel aie_dest1. The output of kernel aie_dest2 is connected to the PL through a stream connection.

The code for aie_dest1 is as follows:

```
using namespace adf;
__attribute__ ((noinline)) void aie_dest1(input_buffer<int32,extents<32>> &in, 
    output_stream<int32> *out, output_buffer<int32,extents<32>> &outm){
	auto inIter=aie::begin_vector<4>(in);
	auto outmIter=aie::begin_vector<4>(outm);
	aie::vector<int32,4> tmp;
	for(int i=0;i<8;i++)
	chess_prepare_for_pipelining
	{
		tmp=*inIter++;
		writeincr(out,tmp);
		*outmIter++=tmp;
	}
}
```

It reads 32 int values from the input buffer and writes them to the stream and buffer output. The __attribute__ ((noinline)) command instructs the tool the keep the hierarchy of the kernel function.

The code for aie_dest2 is as follows:

```
using namespace adf;
__attribute__ ((noinline)) void aie_dest2(input_stream<int32> *in, input_buffer<int32,extents<32>> &inm, 
    output_stream<int32> *outm){
	auto inmIter=aie::begin_vector<4>(inm);
	aie::vector<int32,4> tmp;
	aie::vector<int32,4> tmp2;
	for(int i=0;i<8;i++)
	chess_prepare_for_pipelining
	{
		tmp=readincr_v<4>(in);
		tmp2=*inmIter++;
		writeincr(outm,tmp+tmp2);
	}
}
```

It reads from the stream input and the buffer, and writes to the stream output.

Graph Execution Model

1. Use the following command to run an AI Engine simulator:

   make aiesim
   

2. Open the running result by accessing the following directory:

   vitis_analyzer aiesimulator_output/default.aierun_summary
   

   Note: The --dump-vcd option is used by aiesimulator to dump the VCD file for trace.

3. Click Trace view in Vitis Analyzer. Zoom in to view the first few runs of the design:

   

   This figure lists the following events and their dependencies:

   1: Tile 24_0 DMA s2mm channel 0 (s2mm.Ch0.BD0.lock0) starts. It acquires the lock of ping of input buffer (buf0) to aie_dest1 and transfers data from the PL to buf0. Refer to Graph View for the position of the buffer in the graph.

   2: After DMA s2mm channel 0 BD 0 completes, DMA s2mm channel 0 BD 1 (s2mm.CH0.BD1.lock1) starts. It acquires the lock of pong of input buffer (buf0d) and transfers data from the PL to buf0d.

   3a: The aie_dest1 kernel (in tile 25_0) acquires the lock of buf0 (shown as read lock allocated).

   3b: aie_dest1 acquires the lock of ping of output buffer (buf1) as well.

   4a: After aie_dest1 acquires the locks of its input buffer (buf0) and output buffer (buf1), it starts. If any lock cannot be acquired, it will run into lock stall.

   4b: After tile 24_0 DMA s2mm channel 0 BD 1 (s2mm.CH0.BD1.lock1) completes, it switches back to DMA s2mm channel 0 BD 0 (s2mm.CH0.BD1.lock0). At first, buf0 is still read by aie_dest1 (read lock allocated), so it sticks at DMA lock req in red. After the read lock of the buffer is released, it acquires the lock and starts data transfer from PL.

   5: After aie_dest1 completes, it releases the output buffer (buf1). The kernel aie_dest2 acquires the lock of buf1 (read lock allocated).

   6a: After the lock of buf1 is acquired, aie_dest2 starts.

   6b:. aie_dest1 acquires the lock of buf0d (read lock allocated).

   6c: aie_dest1 acquires the lock of buf1d (write lock allocated).

   7: After aie_dest1 acquires the locks of its input buffer (buf0d) and output buffer (buf1d), it starts.

   8: After aie_dest1 completes, it releases the output buffer (buf1d). Kernel aie_dest2 acquires the lock of buf1d (read lock allocated).

   9: After the lock of buf1d is acquired, aie_dest2 starts.

   Note: The stream interface does not need to acquire lock; it has inherent an backward and forward pressure for data synchronization. Every lock acquires and releases event has some cycles of overhead.

Graph Performance Measurement

There are multiple ways to measure performance:

1. The AI Engine simulator output contains a timestamp for each piece of output data. It is possible to make performance calculations both manually and by using scripts. For example, the output of the example (aiesimulator_output/data/output.txt) looks like the following:

   T 652800 ns
   2 
   ......
   T 10889600 ns
   30 
   

   The first samples come out in 652800 ps, and the last samples come out in 10889600 ps. The throughput therefore can be calculated as follows:

    Total time = 10889600 - 652800 = 10236800 ps
    Total bytes = 128 * 100 = 12800 bytes
    Throughput = 12800/(10236800*1e-6) = 1250.3 MB/s
   

   This method does not measure the latency of the first kernel execution to produce the output data. Make sure that the graph runs a number large enough that this overhead can be neglected.

2. AMD provides event APIs for performance profiling purposes. These APIs use performance counters in shim tiles to do profiling. The following enumeration usages are introduced in this tutorial:

   • event::io_stream_start_to_bytes_transferred_cycles: This records the start of a running event with a performance counter, and records the event that a specific amount of data is transferred with another performance counter. The return number with this enumeration is therefore the total cycles required to receive that amount of data. The profiled stream should be stopped after this amount of data has been transferred.

   • event::io_stream_running_event_count: This counts how many running events have occurred between start_profiling and read_profiling. It can be used to count how much data has been transferred, whether the graph is running infinitely or not.

   Take a look at aie/graph.cpp. The code to perform profiling is as follows:

   int iterations=100;
   int bytes_per_iteration=128;
   int total_bytes=bytes_per_iteration * iterations;
   event::handle handle = event::start_profiling(*dout, event::io_stream_start_to_bytes_transferred_cycles, total_bytes);
   if(handle==event::invalid_handle){
    printf("ERROR:Invalid handle. Only two performance counter in a AIE-PL interface tile\n");
    return 1;
   }
   gr.run(100);
   gr.wait();
   long long cycle_count = event::read_profiling(handle);
   std::cout<<"cycle count:"<<cycle_count<<std::endl;
   event::stop_profiling(handle);
   double throughput = (double)total_bytes / (cycle_count *0.8 * 1e-3); //Every AIE cycle is 0.8ns in production board
   printf("Throughput of the graph: %f MB/s\n",throughput);
   

   The output of AI Engine simulator looks like the following:

   cycle count:12665
   Throughput of the graph: 1263.324122 MB/s
   

   The event API can be applied in AI Engine simulator, hardware emulation, and hardware flows.

   There’re equivalent XRT event APIs for profiling in hardare and hardware emulation flows. The code can be found in sw/host.cpp.

3. The performance result can also be found in the AI Engine simulator profile report. Add the --profile option to aiesimulator, open Vitis Analyzer, and open the Profile view. The profile result can be viewed as shown in the following figure:

   

   Kernel aie_dest1 takes 8368 cycles for 100 iterations. The main function takes 4383 cycles for 100 iterations of the graph. This is around 44 cycles of overhead for each iteration of the graph. This overhead includes the lock acquires of the buffers and the overhead of API calls.

   The performance of aie_dest1 is bounded by the stream interface throughput. The theoretical limit is up to 4 bytes a cycle (5 GB/s), and there are 128 bytes of input for one run. This means that it at least has 32 cycles for the main loop, although it takes 80 cycles. This indicates that the loop is not well-pipelined.

4. Check performance-related events to see if they match expectations. For example, check the event PL_TO_SHIM to see if the PL can send data at the best achievable performance for a single stream interface. Look for the PL_TO_SHIM event in Trace view in Vitis Analyzer:

   

   Search it for multiple times. When it is stable, it can be seen that every eight cycles, it happens once. This is because the frequency of the PL has been set at 312.5 MHz by the option pl-freq=312.5 for the AI Engine compiler and AI Engine-PL interface is 32-bit width, which is one-fourth of the best achievable performance.

   From a best performance and best resource perspective, you might select a 64-bit interface at 625 MHz if timing allows. If not, it is possible to have the PL running at 312.5 MHz with a 128-bit width interface.

5. The following methods are introduced in AI Engine Performance Profile:

   • Profiling by C++ class API

   • Profiling by AI Engine cycles from AI Engine kernels

   • Profiling by event API

Design Optimization Considerations

In this section, the reference design is in testcase_dmafifo_opt. From performing the above analysis, it can be seen that the bottleneck of this design contains the following issues:

• The interface bandwidth is not optimal. The design uses a PLIO width of 32 bits running at 312.5 MHz. Change it to 128 bits running at 312.5 MHz. The relevant code is in aie/graph.h:

  in=input_plio::create("Datain0", plio_128_bits,  "data/input.txt");
  dataout=output_plio::create("Dataout0", plio_128_bits,  "data/output.txt");
  

• The overhead of the graph iterations is too large. The hierarchy of the design should not be touched. Increase the buffer size from 128 bits to 4096 bits. To avoid deadlock, the FIFO size also needs to be increased. The relevant code is in aie/graph.h:

  connect< >net0(in, k[0].in[0]);
  connect< stream >net1(k[0].out[0], k[1].in[0]);
  connect< >net2(k[0].out[1], k[1].in[1]);
  connect< stream >net3(k[1].out[0], dataout);
  fifo_depth(net1)=1024;
  

  Note: When the FIFO depth is large, the DMA FIFO is used. Do not set the FIFO depth to larger than (or equal to) 8192 for a single DMA FIFO.

• The kernel is not well-pipelined. As well as increasing the loop count to deal with more data, more instructions should be added in the loop body and a __restrict keyword should be added to the ports to make the tool schedule instructions more freely. The optimized code for aie_dest1 is as follows:

  using namespace adf;
  __attribute__ ((noinline)) void aie_dest1(input_buffer<int32,extents<1024>> & __restrict in, 
      output_stream<int32> * __restrict out, output_buffer<int32,extents<1024>> & __restrict outm){
  	auto inIter=aie::begin_vector<4>(in);
  	auto outmIter=aie::begin_vector<4>(outm);
  	aie::vector<int32,4> tmp;
  	for(int i=0;i<128;i++)
  	chess_prepare_for_pipelining
  	{
  		tmp=*inIter++;
  		writeincr(out,tmp);
  		*outmIter++=tmp;
  		tmp=*inIter++;
  		writeincr(out,tmp);
  		*outmIter++=tmp;
  	}
  }
  

  Similar optimization is done for aie_dest2. For more information about loop analysis and optimization, refer to the AI Engine Kernel Coding Best Practices Guide.

After making these optimizations, run the following command:

```
make aiesim
```

It can be seen that the design performance can be increased from around 828 MBps to around 3748 MBps. This is approaching the theoretical limit of the design (4 GBps).

Next, run the design in hardware emulation:

```
make run_hw_emu
```

In QEMU, run the following commands:

```
mount /dev/mmcblk0p1 /mnt
cd /mnt
./host.exe a.xclbin
```

Build the design for hardware:

```
make package TARGET=hw
```

The performance in hardware is similar:

```
cycle count:110610
Throughput of the graph: 4628.88 MB/s
```