mnn推理时间异常

[jamesdod]

两个现象：
1.fp32和fp16的推理时间一样
2.int8的推理时间大于fp16/fp32

模型	fp32/ms	fp16/ms	int8/ms
较大	313	312	339
较小	41	40	47

armv8，linux aarch64

推理代码上，唯一的区别就是fp32是Precision_High，fp16和int8是Precision_Low。

可能的原因是什么呢？谢谢

[zhenjing]

测试用哪个模型？有参考的开源模型吗？

[jxt1234]

1. ARMv8 不支持 fp16 计算，可以换用 bf16 试一下：打开 MNN_SUPPORT_BF16 ，precision 设成 low_bf16
2. 关于 int8 慢的问题，mnn 版本是多少？更到最新再量化试试

[jamesdod]

测试用哪个模型？有参考的开源模型吗？

这是用benchmark跑的结果

## fp32推理
./benchmark.out models/ 50 0 0 1 1 0 1 0
MNN benchmark
Forward type: CPU thread=1 precision=1 sparsity=0 sparseBlockOC=1 testQuantizedModel=0
--------> Benchmarking... loop = 50, warmup = 0
[-INFO-]: precision!=2, use fp32 inference.
The device support i8sdot:0, support fp16:0, support i8mm: 0
[ - ] squeezenetv1.1.mnn          max =   60.000 ms  min =   24.000 ms  avg =   38.203 ms
[ - ] mobilenet-v1-1.0.mnn        max =  112.000 ms  min =   43.000 ms  avg =   69.666 ms
[ - ] SqueezeNetV1.0.mnn          max =  111.000 ms  min =   44.000 ms  avg =   70.897 ms
[ - ] resnet-v2-50.mnn            max =  375.593 ms  min =  250.000 ms  avg =  315.703 ms
[ - ] inception-v3.mnn            max =  544.302 ms  min =  411.683 ms  avg =  481.361 ms
[ - ] nasnet.mnn                  max =  141.000 ms  min =   67.000 ms  avg =   97.284 ms
[ - ] MobileNetV2_224.mnn         max =   65.000 ms  min =   26.994 ms  avg =   41.303 ms


## fp16推理
./benchmark.out models/ 50 0 0 1 2 0 1 0
MNN benchmark
Forward type: CPU thread=1 precision=2 sparsity=0 sparseBlockOC=1 testQuantizedModel=0
--------> Benchmarking... loop = 50, warmup = 0
[-INFO-]: precision=2, use fp16 inference if your device supports and open MNN_ARM82=ON.
The device support i8sdot:0, support fp16:0, support i8mm: 0
[ - ] squeezenetv1.1.mnn          max =   59.000 ms  min =   22.000 ms  avg =   37.575 ms
[ - ] mobilenet-v1-1.0.mnn        max =   98.000 ms  min =   44.000 ms  avg =   68.641 ms
[ - ] SqueezeNetV1.0.mnn          max =  114.000 ms  min =   47.000 ms  avg =   71.006 ms
[ - ] resnet-v2-50.mnn            max =  397.000 ms  min =  234.340 ms  avg =  303.782 ms
[ - ] inception-v3.mnn            max =  533.531 ms  min =  433.063 ms  avg =  482.815 ms
[ - ] nasnet.mnn                  max =  134.000 ms  min =   81.866 ms  avg =  102.169 ms
[ - ] MobileNetV2_224.mnn         max =   63.000 ms  min =   27.000 ms  avg =   39.890 ms

[jamesdod]

1. ARMv8 不支持 fp16 计算，可以换用 bf16 试一下：打开 MNN_SUPPORT_BF16 ，precision 设成 low_bf16
2. 关于 int8 慢的问题，mnn 版本是多少？更到最新再量化试试

1.好的，我这样编译和推理试一下
2.目前用的是2.8.2，应该算比较新了吧。下面是int8跑benchmark的结果

## int8推理
./benchmark.out models/ 50 0 0 1 2 0 1 1
MNN benchmark
Forward type: CPU thread=1 precision=2 sparsity=0 sparseBlockOC=1 testQuantizedModel=1
--------> Benchmarking... loop = 50, warmup = 0
[-INFO-]: precision=2, use fp16 inference if your device supports and open MNN_ARM82=ON.
[-INFO-]: Auto set sparsity=0 when test quantized model in benchmark...
Auto set sparsity=0 when test quantized model in benchmark...
The device support i8sdot:0, support fp16:0, support i8mm: 0
[ - ] squeezenetv1.1.mnn          max =   68.000 ms  min =   25.000 ms  avg =   40.229 ms
[ - ] quant-squeezenetv1.1.mnn    max =   64.000 ms  min =   34.000 ms  avg =   48.659 ms
[ - ] mobilenet-v1-1.0.mnn        max =  108.000 ms  min =   39.999 ms  avg =   69.633 ms
[ - ] quant-mobilenet-v1-1.0.mnn    max =   93.000 ms  min =   47.994 ms  avg =   63.897 ms
[ - ] SqueezeNetV1.0.mnn          max =  104.000 ms  min =   53.000 ms  avg =   72.103 ms
[ - ] quant-SqueezeNetV1.0.mnn    max =  123.000 ms  min =   78.000 ms  avg =   97.089 ms
[ - ] resnet-v2-50.mnn            max =  370.000 ms  min =  208.000 ms  avg =  307.182 ms
[ - ] quant-resnet-v2-50.mnn      max =  403.509 ms  min =  305.362 ms  avg =  354.695 ms
[ - ] inception-v3.mnn            max =  579.816 ms  min =  427.000 ms  avg =  480.296 ms
[ - ] quant-inception-v3.mnn      max =    0.000 ms  min =    0.000 ms  avg =    0.000 ms
[ - ] nasnet.mnn                  max =  132.000 ms  min =    0.000 ms  avg =   94.448 ms
[ - ] quant-nasnet.mnn            max =    0.000 ms  min =    0.000 ms  avg =    0.000 ms
[ - ] MobileNetV2_224.mnn         max =   68.000 ms  min =   -0.000 ms  avg =   41.312 ms
[ - ] quant-MobileNetV2_224.mnn    max =   75.000 ms  min =   34.000 ms  avg =   44.936 ms

[jamesdod]

1. ARMv8 不支持 fp16 计算，可以换用 bf16 试一下：打开 MNN_SUPPORT_BF16 ，precision 设成 low_bf16
2. 关于 int8 慢的问题，mnn 版本是多少？更到最新再量化试试

1.关于1打开bf16解了一些编译问题，测试发现：
运行50次，使用low_bf16(precision=3)的运行时间为46ms，使用low(precision=2)的运行时间为41ms。
模型完全一样，推理代码只有precision的区别。

bf16更慢也是能感受到的，不是单次误差，测过多次loop=50，都是bf16的推理时间更长。