Post Training Quantization Tools

To support int8 model deployment on mobile devices,we provide the universal post training quantization tools which can convert the float32 model to int8 model.

User Guide

Example with mobilenet, just need three steps.

1. Optimize model

NOTE: If your model is converted via pnnx, skip this step.

./ncnnoptimize mobilenet.param mobilenet.bin mobilenet-opt.param mobilenet-opt.bin 0

2. Create the calibration table file

2.1 From image

We suggest that using the verification dataset for calibration, which is more than 5000 images.

Some imagenet sample images here https://github.com/nihui/imagenet-sample-images

find images/ -type f > imagelist.txt
./ncnn2table mobilenet-opt.param mobilenet-opt.bin imagelist.txt mobilenet.table mean=[104,117,123] norm=[0.017,0.017,0.017] shape=[224,224,3] pixel=BGR thread=8 method=kl

• mean and norm are the values you passed to Mat::substract_mean_normalize()
• shape is the blob shape of your model, [w,h] or [w,h,c]

* if w and h both are given, image will be resized to exactly size.

* if w and h both are zero or negative, image will not be resized.
* if only h is zero or negative, image's width will scaled resize to w, keeping aspect ratio.
* if only w is zero or negative, image's height will scaled resize to h

• pixel is the pixel format of your model, image pixels will be converted to this type before Extractor::input()
• thread is the CPU thread count that could be used for parallel inference
• method is the post training quantization algorithm, kl and aciq are currently supported

If your model has multiple input nodes, you can use multiple list files and other parameters

./ncnn2table mobilenet-opt.param mobilenet-opt.bin imagelist-bgr.txt,imagelist-depth.txt mobilenet.table mean=[104,117,123],[128] norm=[0.017,0.017,0.017],[0.0078125] shape=[224,224,3],[224,224,1] pixel=BGR,GRAY thread=8 method=kl

2.2 From npy

We suggest that using the validation(development) set for calibration.

Use the same preprocessing as the training set to get the input vectors, in the case of batchsize=1, store each input vector as an npy file, n inputs correspond to n npy files, the actual stored vectors to remove the batch dimension.

test net, shape is in NCHW format, but there's no N.

in0, shape=[512]
in1, shape=[2, 1, 64]
in2, shape=[2, 1, 64]

filelist_in0.txt

0_in0.npy
1_in0.npy
2_in0.npy
...

filelist_in1.txt

0_in1.npy
1_in1.npy
2_in1.npy
...

filelist_in2.txt

0_in2.npy
1_in2.npy
2_in2.npy
...

./ncnn2table test.param test.bin filelist_in0.txt,filelist_in1.txt,filelist_in2.txt test.table shape=[512],[64,1,2],[64,1,2] thread=8 method=kl type=1

Here shape is WHC, because the order of the arguments to ncnn::Mat.

ncnn2table can generate static weight scales without a calibration dataset for RNN,GRU,LSTM,MultiHeadAttention and Embed layers

./ncnn2table rnn.param rnn.bin rnn.table method=kl

3. Quantize model

./ncnn2int8 mobilenet-opt.param mobilenet-opt.bin mobilenet-int8.param mobilenet-int8.bin mobilenet.table

Weight-only block quantized Gemm and MultiHeadAttention

LLM-oriented Gemm and MultiHeadAttention weight-only block quantization is separate from the post training int8 flow above. It stores weight as signed int4/int6/int8 blocks and keeps activation/output in fp32.

The workflow is similar to ncnn2table and ncnn2int8:

./ncnnllm2table in.param in.bin model.llm.table method=minmax bits=6 block=64
./ncnnllm2int in.param in.bin out.param out.bin model.llm.table

method can be minmax,mseclip,awq,gptq. bits can be 4,6,8. block can be 32,64,128. thread is the CPU thread count.

awq and gptq need calibration data, same as npy calibration in ncnn2table.

./ncnnllm2table in.param in.bin calib.list awq.llm.table method=awq bits=4 block=64 type=1 shape=[...]
./ncnnllm2int in.param in.bin awq.param awq.bin awq.llm.table

./ncnnllm2table in.param in.bin calib.list gptq.llm.table method=gptq bits=4 block=128 type=1 shape=[...]
./ncnnllm2int in.param in.bin gptq.param gptq.bin gptq.llm.table

The calibration list format follows ncnn2table.

gemm_name_param_1 bits=4 block=64 method=mseclip scale0 scale1 ...
mha_name_param_0  bits=4 block=64 method=mseclip scale0 scale1 ...
mha_name_param_1  bits=4 block=64 method=mseclip scale0 scale1 ...
mha_name_param_2  bits=4 block=64 method=mseclip scale0 scale1 ...
mha_name_param_3  bits=4 block=64 method=mseclip scale0 scale1 ...

For MultiHeadAttention, _param_0/_param_1/_param_2/_param_3 are q/k/v/out weights.

gemm_name_param_1_input_scale method=awq scale0 scale1 ...
mha_name_param_0_input_scale  method=awq scale0 scale1 ...

method=gptq uses fixed symmetric block scales and standard GPTQ error compensation. It writes packed qweight files and records them in the table.

gemm_name_param_1 bits=4 block=128 method=gptq qweight=gemm.qweight scale0 scale1 ...

The table is text and may be edited before conversion. Missing Gemm rows are skipped. MultiHeadAttention q/k/v/out rows must exist together. Unused rows are rejected and at least one layer must be quantized.

The generated layer quantize_term is bits * 100 + input_scale * 10 + block_code, and block_code 0/1/2 means block 32/64/128.

For quick conversion without saving a table, ncnnllm2int can still compute scales directly:

./ncnnllm2int in.param in.bin out.param out.bin method=minmax bits=6 block=64

This format is signed symmetric scale-only. Zero point is not used.

use ncnn int8 inference

the ncnn library would use int8 inference automatically, nothing changed in your code

ncnn::Net mobilenet;
mobilenet.load_param("mobilenet-int8.param");
mobilenet.load_model("mobilenet-int8.bin");

mixed precision inference

Before quantize your model, comment the layer weight scale line in table file, then the layer will do the float32 inference

conv1_param_0 156.639840536

#conv1_param_0 156.639840536

Source: docs/how-to-use-and-FAQ/quantized-int8-inference.md