TensorRT/CommonFAQ

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Revision as of 18:31, 5 August 2019 by Nfeng (talk | contribs) (Can INT8 calibration table be compatible among different TRT versions or HW platforms?)
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How to check TensorRT version?

There are two methods to check TensorRT version,

  • Symbols from library
$ nm -D /usr/lib/aarch64-linux-gnu/libnvinfer.so | grep "tensorrt"
0000000007849eb0 B tensorrt_build_svc_tensorrt_20181028_25152976
0000000007849eb4 B tensorrt_version_5_0_3_2
NOTE: 20181028 is the build date and 25152976 is the top changelist and 5_0_3_2 is the version information.
  • Macros from header file
$ cat /usr/include/aarch64-linux-gnu/NvInfer.h | grep "define NV_TENSORRT"
#define NV_TENSORRT_MAJOR 5 //!< TensorRT major version.
#define NV_TENSORRT_MINOR 0 //!< TensorRT minor version.
#define NV_TENSORRT_PATCH 3 //!< TensorRT patch version.
#define NV_TENSORRT_BUILD 2 //!< TensorRT build number.
#define NV_TENSORRT_SONAME_MAJOR 5 //!< Shared object library major version number.
#define NV_TENSORRT_SONAME_MINOR 0 //!< Shared object library minor version number.
#define NV_TENSORRT_SONAME_PATCH 3 //!< Shared object library patch version number.

Whether TRT support thread-safe?

TensorRT runtime is thread-safe in the sense that parallel threads using different TRT Execution Contexts can execute in parallel without interference.


Can INT8 calibration table be compatible among different TRT versions or HW platforms?

INT8 calibration table is absolutely NOT compatible between different TRT versions. This is because the optimized network graph is probably different among various TRT versions. If you enforce to use them, TRT may not find the corresponding scaling factor for given tensor.
As long as the installed TensorRT version is identical for different HW platforms, then the INT8 calibration table can be compatible. That means you can perform INT8 calibration on a faster computation platform, like V100 or T4 and then deploy the calibration table to Tegra for INT8 inferencing as long as these platforms have the same TensorRT version installed (at least with the same major and minor version, like 5.1.5 and 5.1.6).


How to do INT8 calibration for the networks with multiple inputs

TensorRT uses bindings to denote the input and output buffer pointer and they are arranged in order. Hence, if your network has multiple input node/layer, you can pass through the input buffer pointers into bindings (void **) separately, like below network with two inputs required,

         bool getBatch(void* bindings[], const char* names[], int nbBindings) override 
         {                                                                             
             // Prepare the batch data (on GPU) for mDeviceInput and imInfoDev                                             
             ...
                                                                          
             assert(!strcmp(names[0], INPUT_BLOB_NAME0));                              
             bindings[0] = mDeviceInput;                                               
                                                                                       
             assert(!strcmp(names[1], INPUT_BLOB_NAME1));                              
             bindings[1] = imInfoDev;                                                  
                                                                                       
             return true;                                                              
         }     

NOTE: If your calibration batch is 10, then for each calibration cycle, you will need to fill each of your input buffer with 10 images accordingly.

How to check GPU utilization?

On Tegra platform, we can use tegrastats to achieve that,

$ sudo /home/nvidia/tegrastats

On Desktop platform, like Tesla, we can use nvidia-smi to achieve that,

$ nvidia-smi --format=csv -lms 500 --query-gpu=index,timestamp,utilization.gpu,clocks.current.graphics,clocks.current.sm,clocks.current.video,
clocks.current.memory,utilization.memory,memory.total,memory.free,memory.used,power.limit,power.draw,temperature.gpu,fan.speed,compute_mode,
gpu_operation_mode.current,clocks_throttle_reasons.active,pstate,clocks_throttle_reasons.hw_slowdown,clocks_throttle_reasons.gpu_idle,
clocks_throttle_reasons.applications_clocks_setting,clocks_throttle_reasons.sw_power_cap,clocks_throttle_reasons.sync_boost -i 0 | tee log.csv

What is kernel auto-tuning?

TensorRT contains various kernel implementations, including those existing in CUDNN and CUBLAS, to accommodate diverse neural network configurations (batch, input/output dims, filters, strides, pads, dilation rate and etc). During network building, TensorRT will profile all suitable kernels and find out the best one with the smallest latency, and then mark it as the final tactic to run the certain layer. We call this process as kernel auto-tuning.
Additionally, it’s not always true that INT8 kernel faster than FP16’s than FP32’s, so

  • if you run FP16 precision mode, it profiles all candidates in FP16 kernel pool and FP32 kernel pool.
  • if you run INT8 precision mode, it profiles all candidates in INT8 kernel pool and FP32 kernel pool.
  • if both FP16 and INT8 are enabled (we call it hybrid mode), it profiles all candidate in INT8 kernel pool, FP16 kernel pool and FP32 kernel pool.

If current layer chooses different mode as its bottom layer or top layer, TensorRT will insert a reformatting layer between them to do the tensor format conversion, and the time for this reformatting layer will be taken into account as the cost of current layer during auto-tuning.


How TensorRT behave when different batch size is being used?

For relatively deep network, if GPU is fully occupied, we couldn't obtain much performance gain from batching.
For relatively simper network, generally, GPU is not fully loaded, then we could obtain performance gain from batching.
In other words, inference time per frame can be improved with the bigger batch size only if GPU loading is not full.


What is maxWorkspaceSize?

maxWorkspaceSize indicates a threshold to filter the kernels/tactics of which desired workspace size is less than maxWorkspaceSize. In other words, if the workspace size a tactic requires(such as for convolution, we can use cudnnGetConvolutionForwardWorkspaceSize() to get the needed workspace size) is larger than what we specify to maxWorkspaceSize, then this tactic will be ignored during kernel auto-tuning.
NOTE: the final device memory TRT consumes has nothing to do with maxWorkspacesize.


What is the difference between enqueue() and execute()?
  • enqueue() will need user to create and synchronize cudaStream. When it's being invoked, it will submit task into GPU and return immediately on CPU basis. While with execute(), TensorRT will create and synchronize cudaStream internally, so execute() will return until everything is completed on GPU.
  • enqueue() and execute() are the APIs associated with network/execution Context, other than on layer basis (all layer implementations are in async manner, including IPlugin layer).
  • Both enqueue() and execute() support layer profiling. The time consumption of each layer will be recorded during network execution and printed out after all done, other than real-time profiling.
  • setDebugSync() is only supported along with execute(). If this flag is true, the builder or execution context will invoke cudaDeviceSynchronize() and report the status to user after each layer execution done.

How to enable VERBOSE log?

You can enable VERBOSE log for your sample through the following API provided by YOUR_TRT_PATH/sample/common/logging.h,

gLogger.setReportableSeverity(Severity::kVERBOSE)

It’s always recommended to provide VERBOSE log to NVIDIA for further assistance or help.


Can TensorRT model be compatible among different GPU platforms or TensorRT versions?

The answer is definitely NO. The reasons are as below,

  • Different GPU platforms have different SM counts, CUDA Cores or Tensor Core, or Memory efficiency, so same kernel may have different performance on these GPU platforms. That means TensorRT builder may choose different kernel/tactic implementation during network optimization even for the same input configuration.
  • Network fusion and layer kernels are evolving all the time, so when different TensorRT versions to be used, the optimized network graph or chosen best tactic might be different. It’s possible that some chosen kernel for current TensorRT engine is not existing in the other TensorRT version.

What is maximum model size TensorRT can support?

TensorRT can support 2G(INT_MAX) model at most, which is also the hard limit of Google protobuf.


Why TRT has different definition for axis parameters as compared with the other frameworks, like Caffe or TensorFlow?

TensorRT treats batch as immutable dim over the whole network, so it mostly starts indexing from channel (as axis 0). When you specify axis 1 and hope for the processing from channel, while TensorRT might consider you mean starting from H. In a word, TensorRT layer deals with CHW other than NCHW. TensorRT parser code are open on GitHub, so you could check the parsing function there and understand how it handles your layer parameter exactly and fix the discrepancy accordingly (general solution is subtract 1 from the axis to strip out batch dimension)