Scenario

for(i=0; i<msize; i++) {
    for(j=0; j<msize; j++) {
        for(k=0; k<msize; k++) {
            c[i][j] = c[i][j] + a[i][k] * b[k][j];
       }
    }
}

Ingredients

Performance analysis tools
:
Intel® Advisor
2021.1 included in the
Intel® oneAPI Base Toolkit
The latest version is available for download at: https://software.intel.com/content/www/us/en/develop/tools/oneapi/components/advisor.html.
Application
: Standard C++ matrix multiply as indicated in the Scenario section.
Not available for download.
Compilers
:
Intel® oneAPI
DPC++/C++
Compiler
2021.1 included in the
Intel® oneAPI Base Toolkit
The latest version of the
Intel® oneAPI
DPC++/C++
Compiler
is available for download at https://software.intel.com/content/www/us/en/develop/tools/oneapi/components/dpc-compiler.html.
Operating system
: Ubuntu* 18.04
CPU
: Intel® Core™ i7-7500U processor

Identify Regions to Offload to GPU with

Offload Modeling

source <advisor-install-dir>/advisor-vars.sh

Collect application performance metrics with Survey analysis:
advisor --collect=survey --project-dir=./mmult --stackwalk-mode=online --static-instruction-mix -- /home/test/mmult
Collect Trip Counts and FLOP data:
advisor --collect=tripcounts --project-dir=./mmult --flop --target-device=gen9_gt2 -- /home/test/mmult
Model the application performance for a
gen9_gt2
configuration:
advisor --collect=projection --project-dir=./mmult --config=gen9_gt2 --no-assume-dependencies 
In the
Intel Advisor
GUI, this corresponds to a low-accuracy configuration of the
Offload Modeling
. See User Guide: Offload Modeling Accuracy Presets for details.
Go to
mmult/e000/pp000/data.0
and open the interactive HTML report
report.html
in a web browser to see the performance projection results.

In the
Summary
tab of the report, review the following:
In the
Program Metrics
pane:
Intel Advisor
predicts a 4.4x speedup if you offload the application to a GPU with the
gen9_gt2
configuration. The estimated execution time is 5.85 seconds compared to the original 25.07 seconds.
In the
Offload Bounded by
pane: The offloads are 99% bounded by the last-level cache (LLC) bandwidth.
In the
Top Offloaded
pane:
Intel Advisor
recommends to offload the loop at
multiply.c:202
. Click the loop location to go to
Offloaded Regions
tab and see more details.
In the
Top Non-Offloaded
: The time spent in other loops is too small to be modeled accurately and one of loops is outside of the code region marked for offloading. For this reason, they are not recommended for offloading.

Rewrite the Matrix Multiply Code in Data Parallel C++ (DPC++)

void multiply1(int msize, int tidx, int numt, TYPE a[][NUM], TYPE b[][NUM], TYPE c[][NUM], TYPE t[][NUM])
{ 
	int i,j,k;
 
  // Select a device
  cl::sycl::gpu_selector device;
  // Declare a deviceQueue
  cl::sycl::queue deviceQueue(device);
  // Declare a two-dimensional range
  cl::sycl::range<2> matrix_range{NUM, NUM};

  // Declare three buffers and initialize them
  cl::sycl::buffer<TYPE, 2> bufferA((TYPE*)a, matrix_range);
  cl::sycl::buffer<TYPE, 2> bufferB((TYPE*)b, matrix_range);
  cl::sycl::buffer<TYPE, 2> bufferC((TYPE*)c, matrix_range);

  // Submit our job to the queue
  deviceQueue.submit([&](cl::sycl::handler& cgh) {
    // Declare three accessors to our buffers. The first two are read, and the last one is read_write
    auto accessorA = bufferA.template get_access<sycl_read>(cgh);
    auto accessorB = bufferB.template get_access<sycl_read>(cgh);
    auto accessorC = bufferC.template get_access<sycl_read_write>(cgh);

    // Execute the matrix multiply code in parallel over our matrix_range
    // Ind is an index into this range
    cgh.parallel_for<class Matrix<TYPE>>(matrix_range,
		[=](cl::sycl::id<2> ind) {
            int k;
    	    for(k=0; k<NUM; k++) {
	       // Perform computation, where ind[0] is a row, ind[1] is a column
               accessorC[ind[0]][ind[1]]  += accessorA[ind[0]][k] * accessorB[k][ind[1]];
            }
     });
  });
}

Run GPU Roofline

Add your username to the video group. To check if you are already in the video group, run:

groups | grep video

If you are not part of the video group, add your username to it:

sudo usermod -a -G video <username>

Enable GPU metrics collection:

sudo su

echo 0 > /proc/sys/kernel/perf_stream_paranoid

Make sure that your DPC++ code runs correctly on the GPU. To check which hardware you are running on, add the following to your DPC++ code and run it:

Cl::sycl::default_selector selector;
Cl::sycl::queue queue(delector);
auto d = queue.get_device();
std::cout<<”Running on :”<<d.get_info<cl::sycl::info::device::name>()<<std::endl;

Set up the

Intel Advisor

environment variables:

source <install-dir>/setvars.sh

Run the Survey analysis with the
--profile-gpu
option:
advisor --collect=survey --project-dir=./mmult_dpcpp --profile-gpu -- /home/test/mmult_dpcpp
Run the Trip Count and FLOP analysis with
--profile-gpu
:
advisor --collect=tripcounts --project-dir=./mmult_dpcpp --flop --profile-gpu -- /home/test/mmult_dpcpp 
Generate an HTML report with a GPU Roofline chart:
advisor --report=roofline --gpu --project-dir=./mmult_dpcpp --report-output=roofline.html
Open the generated
roofline.html
in a web browser.

To get more information on different parts of memory, you can choose to display different dots based on which memory subsystem is used for the arithmetic intensity calculation. In this case, choose
GTI (Memory)
and
L3 + SLM
memory levels.

Double-click a dot to see more information about it:
The L3 dot is very close to the L3 maximum bandwidth. To get more FLOPS, you need to optimize caches further. A cache-blocking optimization strategy can make better use of memory and should increase the performance.
The GTI dot, which represents traffic between the GPU, GPU uncore (LLC), and main memory, is far from the GTI roofline. Transfer costs between CPU and GPU do not seem to be an issue.

Next Steps

Key Take-Aways

Use the
Offload Modeling
feature of the
Intel Advisor
to find the best candidates for code to offload to the GPU, estimate the outcome of porting to GPU, and identify performance bottlenecks.
Use
GPU Roofline Insights
feature of the
Intel Advisor
to identify bottlenecks in code already ported to GPU and see how close its performance is to the system maximums.

See Also