Running Stable Diffusion on a RPI Zero 2 (or in 260MB of RAM)

OnnxStream

The challenge is to run Stable Diffusion, which includes a large transformer model with almost 1 billion parameters, on a Raspberry Pi Zero 2, which is a microcomputer with 512MB of RAM, without adding more swap space and without offloading intermediate results on disk. The recommended minimum RAM/VRAM for Stable Diffusion is typically 8GB.

Generally major machine learning frameworks and libraries are focused on minimizing inference latency and/or maximizing throughput, all of which at the cost of RAM usage. So I decided to write a super small and hackable inference library specifically focused on minimizing memory consumption: OnnxStream.

OnnxStream is based on the idea of decoupling the inference engine from the component responsible of providing the model weights, which is a class derived from WeightsProvider. A WeightsProvider specialization can implement any type of loading, caching and prefetching of the model parameters. For example a custom WeightsProvider can decide to download its data from an HTTP server directly, without loading or writing anything to disk (hence the word "Stream" in "OnnxStream"). Two default WeightsProviders are available: DiskNoCache and DiskPrefetch.

OnnxStream can consume even 55x less memory than OnnxRuntime while being only 0.5-2x slower (on CPU, see the Performance section below).

Stable Diffusion

These images were generated by the Stable Diffusion example implementation included in this repo, using OnnxStream, at different precisions of the VAE decoder. The VAE decoder is the only model of Stable Diffusion that could not fit into the RAM of the Raspberry Pi Zero 2 in single or half precision. This is caused by the presence of residual connections and very big tensors and convolutions in the model. The only solution was static quantization (8 bit). The third image was generated by my RPI Zero 2 in about 3 hours. The first image was generated on my PC using the same latents generated by the RPI Zero 2, for comparison:

VAE decoder in W16A16 precision:

VAE decoder in W8A32 precision:

VAE decoder in W8A8 precision (generated by my RPI Zero 2 in about 3 hours):

Features of OnnxStream

• Inference engine decoupled from the WeightsProvider
• WeightsProvider can be DiskNoCache, DiskPrefetch or custom
• Attention slicing
• Dynamic quantization (8 bit unsigned, asymmetric, percentile)
• Static quantization (W8A8 unsigned, asymmetric, percentile)
• Easy calibration of a quantized model
• FP16 support (with or without FP16 arithmetic)
• 24 ONNX operators implemented (the most common)
• Operations executed sequentially but all operators are multithreaded
• Single implementation file + header file
• XNNPACK calls wrapped in the XnnPack class (for future replacement)

OnnxStream depends on XNNPACK for some (accelerated) primitives: MatMul, Convolution, element-wise Add/Sub/Mul/Div, Sigmoid and Softmax.

Performance

Stable Diffusion consists of three models: a text encoder (672 operations and 123 million parameters), the UNET model (2050 operations and 854 million parameters) and the VAE decoder (276 operations and 49 million parameters). Assuming that the batch size is equal to 1, a full image generation with 10 steps, which yields good results (with the Euler Ancestral scheduler), requires 2 runs of the text encoder, 20 (i.e. 2*10) runs of the UNET model and 1 run of the VAE decoder.

This table shows the various inference times of the three models of Stable Diffusion, together with the memory consumption (i.e. the Peak Working Set Size in Windows or the Maximum Resident Set Size in Linux).

In the case of the UNET model (when run in FP16 precision, with FP16 arithmetic enabled in OnnxStream), OnnxStream can consume even 55x less memory than OnnxRuntime while being 0.5-2x slower.

Notes:

• The first run for OnnxRuntime is a warm up inference, since its InferenceSession is created before the first run and reused for all the subsequent runs. No such thing as a warm up exists for OnnxStream since it is purely eager by design (however subsequent runs can benefit from the caching of the weights files by the OS).
• At the moment OnnxStream doesn't support inputs with a batch size != 1, unlike OnnxRuntime, which can greatly speed up the whole diffusion process using a batch size = 2 when running the UNET model.
• In my tests, changing OnnxRuntime's SessionOptions (like EnableCpuMemArena and ExecutionMode) produces no significant difference in the results.
• Performance of OnnxRuntime is very similar to that of NCNN (the other framework I evaluated), both in terms of memory consumption and inference time. I'll include NCNN benchmarks in the future, if useful.
• Tests were run on my development machine: Windows Server 2019, 16GB RAM, 8750H cpu (AVX2), 970 EVO Plus SSD, 8 virtual cores on VMWare.

Attention Slicing and Quantization

The use of "attention slicing" when running the UNET model and the use of W8A8 quantization for the VAE decoder were crucial in reducing memory consumption to a level that allowed execution on a RPI Zero 2.

While there is a lot of information on the internet about quantizing neural networks, little can be found about "attention slicing". The idea is simple: the goal is to avoid materializing the full Q @ K^T matrix when calculating the scaled dot-product attention of the various multi-head attentions in the UNET model. With an attention head count of 8 in the UNET model, Q has a shape of (8,4096,40), while K^T has a shape of (8,40,4096): so the result of the first MatMul has a final shape of (8,4096,4096), which is a 512MB tensor (in FP32 precision):

The solution is to split Q vertically and then to proceed with the attention operations normally on each chunk of Q. Q_sliced has a shape of (1,x,40), where x is 4096 (in this case) divided by onnxstream::Model::m_attention_fused_ops_parts (which has a default value of 2, but can be customized). This simple trick allows to lower the overall consumed memory of the UNET model from 1.1GB to 300MB (when the model is run in FP32 precision). A possible alternative, certainly more efficient, would be to use FlashAttention, however FlashAttention would require writing a custom kernel for each supported architecture (AVX, NEON etc), bypassing XnnPack in our case.

How OnnxStream works

This code can run a model defined in the path_to_model_folder/model.txt: (all the model operations are defined in the model.txt text file; OnnxStream expects to find all the weights files in that same folder, as a series of .bin files)

#include "onnxstream.h"

using namespace onnxstream;

int main()
{
    Model model;

    //
    // Optional parameters that can be set on the Model object:
    //
    // model.set_weights_provider( ... ); // specifies a different weights provider (default is DiskPrefetchWeightsProvider)
    // model.read_range_data( ... ); // reads a range data file (which contains the clipping ranges of the activations for a quantized model)
    // model.write_range_data( ... ); // writes a range data file (useful after calibration)
    // model.m_range_data_calibrate = true; // calibrates the model
    // model.m_use_fp16_arithmetic = true; // uses FP16 arithmetic during inference (useful if weights are in FP16 precision)
    // model.m_use_uint8_arithmetic = true; // uses UINT8 arithmetic during inference
    // model.m_use_uint8_qdq = true; // uses UINT8 dynamic quantization (can reduce memory consumption of some models)
    // model.m_fuse_ops_in_attention = true; // enables attention slicing
    // model.m_attention_fused_ops_parts = ... ; // see the "Attention Slicing" section above
    //

    model.read_file("path_to_model_folder/model.txt");

    tensor_vector<float> data;
    
    ... // fill the tensor_vector with the tensor data. "tensor_vector" is just an alias to a std::vector with a custom allocator.

    Tensor t;
    t.m_name = "input";
    t.m_shape = { 1, 4, 64, 64 };
    t.set_vector(std::move(data));
    model.push_tensor(std::move(t));

    model.run();
    
    auto& result = model.m_data[0].get_vector<float>();
    
    ... // process the result: "result" is a reference to the first result of the inference (a tensor_vector<float> as well).

    return 0;
}

The model.txt file contains all the model operations in ASCII format, as exported from the original ONNX file. Each line corresponds to an operation: for example this line represents a convolution in a quantized model:

Conv_4:Conv*input:input_2E_1(1,4,64,64);post_5F_quant_5F_conv_2E_weight_nchw.bin(uint8[0.0035054587850383684,134]:4,4,1,1);post_5F_quant_5F_conv_2E_bias.bin(float32:4)*output:input(1,4,64,64)*dilations:1,1;group:1;kernel_shape:1,1;pads:0,0,0,0;strides:1,1

In order to export the model.txt file and its weights (as a series of .bin files) from an ONNX file for use in OnnxStream, a notebook (with a single cell) is provided (onnx2txt.ipynb).

Some things must be considered when exporting a Pytorch nn.Module (in our case) to ONNX for use in OnnxStream:

1. When calling torch.onnx.export, dynamic_axes should be left empty, since OnnxStream doesn't support inputs with a dynamic shape.
2. It is strongly recommended to run the excellent ONNX Simplifier on the exported ONNX file before its conversion to a model.txt file.

How to Build the Stable Diffusion example on Linux/Mac/Windows

• Windows only: start the following command prompt: Visual Studio Tools > x64 Native Tools Command Prompt.
• Mac only: make sure to install cmake: brew install cmake.

First you need to build XNNPACK.

Since the function prototypes of XnnPack can change at any time, I've included a git checkout ​​that ensures correct compilation of OnnxStream with a compatible version of XnnPack at the time of writing:

git clone https://github.com/google/XNNPACK.git
cd XNNPACK
git rev-list -n 1 --before="2023-06-27 00:00" master
git checkout <COMMIT_ID_FROM_THE_PREVIOUS_COMMAND>
mkdir build
cd build
cmake -DXNNPACK_BUILD_TESTS=OFF -DXNNPACK_BUILD_BENCHMARKS=OFF ..
cmake --build . --config Release

Then you can build the Stable Diffusion example.

<DIRECTORY_WHERE_XNNPACK_WAS_CLONED> is for example /home/vito/Desktop/XNNPACK or C:\Projects\SD\XNNPACK (on Windows):

git clone https://github.com/vitoplantamura/OnnxStream.git
cd OnnxStream
cd src
mkdir build
cd build
cmake -DXNNPACK_DIR=<DIRECTORY_WHERE_XNNPACK_WAS_CLONED> ..
cmake --build . --config Release

Now you can run the Stable Diffusion example. The weights for the example can be downloaded from the Releases of this repo. These are the command line options of the Stable Diffusion example:

--models-path       Sets the folder containing the Stable Diffusion models.
--ops-printf        During inference, writes the current operation to stdout.
--output            Sets the output PNG file.
--decode-latents    Skips the diffusion, and decodes the specified latents file.
--prompt            Sets the positive prompt.
--neg-prompt        Sets the negative prompt.
--steps             Sets the number of diffusion steps.
--save-latents      After the diffusion, saves the latents in the specified file.
--decoder-calibrate Calibrates the quantized version of the VAE decoder.
--decoder-fp16      During inference, uses the FP16 version of the VAE decoder.
--rpi               Configures the models to run on a Raspberry Pi Zero 2.

Credits

• The Stable Diffusion implementation in sd.cpp is based on this project, which in turn is based on this project by @EdVince. The original code was modified in order to use OnnxStream instead of NCNN.