Fall 2024
CS 259 Lab 1
Accelerating Convolutional Neural Network (CNN) on FPGAs using
Merlin Compiler
Due October 9 11:59pm
Description
Your task is to accelerate the computation of two layers in a convolutional neural network
(CNN) using a high-level synthesis (HLS) tool on an FPGA. We encourage you to start with
using the Merlin Compiler. For an input image with 228 × 228 pixels and 256 channels, you
are going to calculate the tensor after going through a 2D convolution layer and a 2D max
pooling layer. The convolution layer has 256 filters of shape 256 × 5 × 5, uses the ReLU
activation relu(x) = max{x, 0} with a bias value for each output channel. The 2D maxpooling
layer operates on 2 × 2 non-overlapping windows. You will need to implement this
function using HLS:
void CnnKernel(const float* input, const float* weight, const float* bias, float*
output)
where input is the input image of size [256][228][228], weight stores the weights of the
convolution filters of size [256][256][5][5], bias stores the offset values of size [256] that
will be added to the output channels, and output should be written to by you as defined
above to store the result of maxpool(relu(conv2d(input, weight) + bias)). The output
size is [256][112][112].
How-To
FPGA accelerator compilation typically involves three (3) stages: high-level synthesis (HLS),
bitstream generation, and onboard execution. The last two stages can take days to
complete. Therefore, in this lab, we only focus on the first stage: HLS. Your performance will
only be assessed using the estimation in the HLS reports, which is usually accurate.
However, you are welcome to try out the last two steps if you are interested.
Connecting to the Server: Method 1
In this method, you won’t be able to run Merlin directly from your /home directory, so you’ll need
to copy files back and forth.
1. Connect to the server (VPN may be required). You can find VPN details here:
https://www.it.ucla.edu/it-support-center/services/virtual-private-network-vpn-clients
ssh <username>@brimstone.cs.ucla.edu
2. Start the Docker container and share your home with –v:
docker run -v /d0/class/:/home -it vitis2021 /bin/bash
3. Source Vitis, navigate to the desired directory and clone the repository:
source /tools/Xilinx/Vitis_HLS/2021.1/settings64.sh
cd /opt
git clone https://github.com/UCLA-VAST/cs-259-f24.git
cd cs-259-f24/lab1
4. Copy the necessary file to your home directory:
cp /opt/cs-259-f24/lab1/cnn-krnl.cpp /home/<username>
Connecting to the Server: Method 2
In this method, you can run Merlin directly from your /home directory, but make sure to export your
home directory.
1. Connect to the server (VPN may be required). You can find VPN details here:
https://www.it.ucla.edu/it-support-center/services/virtual-private-network-vpn-clients
ssh <username>@brimstone.cs.ucla.edu
2. Start the Docker container and share your home with –v:
docker run --user $(id -u):100 -v /d0/class/:/home -it vitis2021 /bin/bash
3. Export your home directory:
export HOME=/home/<username>
4. Source Vitis, navigate to your home directory and clone the repository:
source /tools/Xilinx/Vitis_HLS/2021.1/settings64.sh
cd /home/<username>
git clone https://github.com/UCLA-VAST/cs-259-f24.git
cd cs-259-f24/lab1
Build and Run Baseline with Software Simulation
We have prepared the starter kit for you. Please run: make
This command will perform a software simulation of the provided starter FPGA HLS kernel. It
should show “PASS”. You need to use FPGA Developer AMI in this lab unless you are using
a computer with Xilinx Vitis HLS installation. However, you are still suggested to develop code
and run software simulation locally to test the correctness. You can move to AWS once you
enter the tuning stage.
Understand the automatic Merlin’s optimization
Before modifying the kernel and adding pragmas, synthesize the CNN kernel with Merlin and
describe in your report the automatic optimizations made by Merlin and how this reduces
latency.
Modify the HLS CNN Kernel
If you have successfully built and run the baseline HLS CNN kernel, you can now optimize
the code to design your CNN kernel. Your task is to implement a fast, parallel version of the
CNN kernel on FPGA. You should start with the provided starter kit. You should edit cnnkrnl.cpp
for this task. When editing, please use the given types input_t, weight_t, bias_t,
and output_t for the corresponding data, and compute_t for your intermediate values.
You can use them as if they are float numbers.
Parallelism should be exploited by using Merlin pragmas and tiling. You are encouraged to
focus on Merlin pragmas (#pragma ACCEL parallel, #pragma ACCEL pipeline and #pragma
ACCEL tile). You can explicitly modify the code (tiling, loop permutation, …) but make sure
the code modified is correct.
In the starter kit, we simply wrap a sequential CNN code with #pragma ACCEL kernel, and
Merlin automatically performs data caching, memory coalescing, pipelining and
parallelization, which yield about 10 GFLOPs.
Although the skeleton kernel is provided, you are also free to create your own by removing
the header file inclusion of “lib/cnn-krnl.h” and implement the basic kernel from scratch.
However, this would require specific expertise in Xilinx FPGA architecture and is not
recommended for this course.
Test Your HLS CNN Kernel with Software Simulation
To perform software emulation of your FPGA implementation of CNN kernel:
make
If you see something similar to the following message, your implementation is incorrect.
Found 21201** errors
FAIL Since the software simulation step uses the CPU to emulate the hardware behavior, it only
serves as correctness test and its execution time doesn’t reflect that of actual hardware. Your
estimated execution time should be retrieved using the command below:
make estimate
This command will print out the estimated latency and resource usage of your kernel:
+---------------------------+------------------------+----------+----------+---------+--------+-------+------+
| Kernel | Cycles | LUT | FF | BRAM | DSP | URAM |Detail|
+---------------------------+------------------------+----------+----------+---------+--------+-------+------+
|CnnKernel (cnn-krnl.cpp:12)|4179564052 (16718.256ms)|49558 (4%)|49381 (2%)|810 (18%)|202 (2%)|25 (2%)|- |
+---------------------------+------------------------+----------+----------+---------+--------+-------+------+
The time highlighted in yellow is the estimated execution time of your FPGA kernel. You can
get the performance by “kNum*kNum*kImSize*kImSize*kKernel*kKernel*2/latency”, or
164.4/latency (in s) to get the performance in GFLOPS.
IMPORTANT: Please make sure that all your loops have fixed loop bounds. If any of the loop
bounds are variable, a performance estimation will not be shown and you will receive no
performance grade.
IMPORTANT: The “make estimate” command should finish in 30 minutes, or in two hours
with highly-complex optimizations. Our recommendation is to halt your estimation using
Ctrl-C when the time exceeds 30 minutes, except for your last step (after you reach ~100
GOPS). More than 12 hours in the estimation will result in zero for the performance score.
As your kernel design becomes more complex, the software simulation and the estimation
will start to take a significantly longer time.
IMPORTANT: As you apply more optimizations, your resource usage will also increase.
Ideally, you should keep applying optimization until your kernel occupies about 80% of these
resources. The remaining 20% should be reserved for the interfaces (DRAM/PCI-e controller)
and the downstream flows. Please make sure that resource utilization is less than 80% for all
FPGA resources. If any of the resources are over this limit, you will receive no performance
grade.
IMPORTANT: You can check the HLS report by opening merlin.rpt with a text editor. This
file will be generated with the command make estimate. You must submit this file with your
final submission. You should not modify this file in your submission, and it will be all verified
after submission due. Any modification to this file in your submission constitutes academic
misconduct and will be reported.
Advanced Tips for HLS
Kernel Profiling: If you want to “profile” your kernel, you can open merlin.rpt with a text
editor and scroll down to Performance Estimate. You can see the trip count, accumulated
cycles and cycles per call, as well as pipeline initiation interval and parallel factor for each
loop in the table. For resource usage, you can go to Resource Estimate. No loop level
information is available, though. If you want to check the resource usage of a code region,
you can wrap it with a function then run again.
Kernel after transformation: If you want to see the kernel after being transformed by Merlin,
you can look for that in .merlin_prj/run/implement/exec/hls/kernel. Annotation for Profiling: If you find the loops in your report hard to read, you can name the
loops you are interested in using a goto label. For example, this_loop: for (int i = 0;
i < n; i++);
Debugging Pipelining: If you are not sure about why you cannot achieve a specific initiation
interval as you expected, you can open the file below and read the logs. HLS usually gives out
a reason.
.merlin_prj/run/implement/exec/hls/_x/logs/CnnKernel/CnnKernel/vitis_hls.log
Long Synthesis Time In Pipelining: You will experience long HLS synthesis time (for
generating the estimation) if you pipeline a loop with a large loop body. Besides, please note
that as all loops inside a pipeline will be unrolled, it may be automatically a large loop body.
In this case, you may want to exchange the order of pipelining and unrolling and see if the time
can get improved.
Use Functions for Shorter Synthesis Time: If you experience long synthesis time, you may try
wrapping some loops into a function and specify #pragma HLS inline off inside the
function body. However, this may lead to inaccurate dependency analysis or memory port
analysis and cause lower performance sometimes. There might be some workarounds, or
not. For example, if you have access to A[k + i][j] inside the function, passing A + k to
the function and accessing A’[i][j] can allow HLS to understand the array partitioning
better than passing A. You need to do experiments.
General Tips
● When you develop on AWS, to resume a session in case you lose your connection, you
can run screen after login. You can recover your session with screen -DRR. You should
stop your AWS instance if you are going to come back and resume your work in a few
hours or days. Your data will be preserved but you will be charged for the EBS storage
for $0.10 per GB per month (with default settings). You should terminate your instance
if you are not going to come back and resume your work. Data on the instance will be
lost.
● You are recommended to use private repositories provided by GitHub to backup your
code. Never put your code in a public repo to avoid potential plagiarism. To check in
your code to a private GitHub repo, create a repo first.
git branch -m upstream
git checkout -b main # skip these two lines if you are reusing the folder in Lab 1
... // your modifications
git add cnn-krnl.cpp merlin.rpt
git commit -m "lab1: first version" # change commit message accordingly
# please replace the URL with your own URL
git remote add origin git@github.com:YourGitHubUserName/your-repo-name.git
git push -u origin main
● You are recommended to git add and git commit often so that you can keep track of
the history and revert whenever necessary.
● Make sure your code produces correct results!
(Optional) Modify the HLS CNN Kernel using Vitis Pragmas
You are encouraged to use mainly Merlin pragmas. If needed, you can use Vitis pragmas for
finer-grained control and optimization. The list of pragmas in Vitis can be found here. You can simply write Vitis pragmas and Merlin pragmas in the same file (cnn-krnl.cpp), but note
that, to apply an HLS pragma to a loop, you need to put the pragma inside the loop body
instead of before it.
Submission
You need to report the estimated performance results of your FPGA-based implementation on
a Xilinx Ultrascale+ VU9P FPGA (the FPGA we are using, specified in the makefile). Please
express your performance in GFLOPS and the speedup compared with the starter-kit version.
Your report should also include:
● Please run the input C file through the Merlin Compiler, identify the code
transformation and HLS pragmas that Merlin added, and discuss why.
● Please explain the parallelization and optimization strategies you have applied for
each loop in the CNN program (convolution, max pooling, etc) in this lab. Include the
pragmas (if any) or code segments you have added to achieve your strategy.
● Please incrementally evaluate each parallelization/optimization that you have applied
and explain why it improves the performance.
● Please report the FPGA resources (LUT/FF/DSP/BRAM) usages, in terms of resource
count and percentage of the total. Which resource has been used most, in terms of
percentage?
● Optional: The challenges you faced, and how you overcame them.
● (Bonus +5pts): Analyze your code and check if the DSP/BRAM resource usage
matches your expectation. Only the adders, multipliers, and size of arrays need to be
considered. Please attach related code segments to your report and show how you
computed the expected number. Provide a discussion on possible reasons if they
differ significantly.
You also need to submit your optimized kernel code. Do not modify code in the lib directory.
Please submit on Gradescope. Your final submission should contain and only contain these
files individually:
├ cnn-krnl.cpp
├ merlin.rpt
└ lab**report.pdf
File lab**report.pdf must be in PDF format.
Grading Policy
Your submission will only be graded if it complies with the formatting requirements.
Missing reports/code or compilation errors will result in 0 for the corresponding
category(ies).
Correctness (40%)
Please check the correctness using the command “make”. Performance (40%)
Your performance will be evaluated based on the estimation report generated using the
command “make estimate”. The performance point will be added only if you have the
correct result, so please prioritize the correctness over performance. Your performance will
be evaluated based on the ranges of throughput (GOPS). Ranges A+ and A++ will be defined
after all the submissions are graded:
● Range A++, better than Range A+ performance: 40 points + 20 points (bonus)
● Range A+, better than Range A performance: 40 points + 10 points (bonus)
● Range A GFLOPS [200, 280]: 40 points
● Range B GFLOPS [120, 200): 30 points
● Range C GFLOPS [60, 120): 20 points
● Range D GFLOPS [30, 60): 10 points
● Lower than range F [0, 30): 0 points
Report (20%)
Points may be deducted if your report misses any of the sections described above.
Academic Integrity
All work is to be done individually, and any sources of help are to be explicitly cited. You must
not modify the HLS report merlin.rpt in your submission. Any instance of academic
dishonesty will be promptly reported to the Office of the Dean of Students. Academic
dishonesty includes, but is not limited to, cheating, fabrication, plagiarism, copying code from
other students or from the internet, modifying the software-generated report, or facilitating
academic misconduct. We’ll use automated software to identify similar sections between
different student programming assignments, against previous students’ code, or against
Internet sources. We’ll run HLS on all submissions and compare the reproduced HLS
report with the submitted report. Students are not allowed to post the lab solutions on public
websites (including GitHub). Please note that any version of your submission must be your
own work and will be compared with sources for plagiarism detection.
Late policy: Late submission will be accepted for 24 hours with a 10% penalty. No late
submission will be accepted after that (you lost all points after the late submission time).
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