OneFlow源码解析:Tensor类型体系与Local Tensor

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撰文|郑建华

更新|赵露阳

tensor和op是神经网络模型最基本的组件:op是模型的节点,tensor是连接节点的边。然而,构建一个tensor并不仅仅是构造一个对象那么简单,至少要考虑以下问题:

  • 要支持节点本地的local tensor,以及分布式的global tensor;

  • 要支持eager和lazy执行模式;

  • 要支持不同的数据类型,包括float、double、int等;

  • 要支持不同设备。

创建tensor的方法

与PyTorch类似,在OneFlow中也可以通过两种主要的方式来创建tensor:Tensor和tensor。这两种方式最终都会创建出OneFlow内部的C++ Tensor对象,即对应Python层的flow.Tensor类型。

1.1 Tensor

Python层的Tensor是在tensor.py(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/python/oneflow/framework/tensor.py#L23)中引入的,通过python c api注册的Tensor类型对象,此对象在MakeTensorType

https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/framework/tensor.cpp#L623)中被定义和返回。

在MakeTensorType中主要通过PyTensorObject_init创建了Tensor对象:

static int PyTensorObject_init(PyObject* self, PyObject* args, PyObject* kwargs) 
  HANDLE_ERRORS
  auto* temp = functional::_legacy_tensor_ctor(NULL, args, kwargs);
  if (PyErr_Occurred())  throw py::error_already_set(); 
  auto* _self = (PyTensorObject*)self;
  _self->data = PyTensor_Unpack(temp);
  _self->data->set_pyobject(self);




  // reset temp data to prevent clearing the pyobject
  // when the temp is deallocated
  ((PyTensorObject*)temp)->data.reset();
  Py_XDECREF(temp);
  return 0;
  END_HANDLE_ERRORS_RET(-1)

通过functional::_legacy_tensor_ctor函数创建了OneFlow内部的c++ Tensor对象:oneflow::one::Tensor,并作为data绑定至Python的Tensor类型。在MakeTensorType中,还通过PyMethodDef(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/framework/tensor.cpp#L639-L641)为Tensor注册了很多C++方法,如:

static PyMethodDef PyTensorObject_methods[] = 
    "storage_offset", PyTensorObject_storage_offset, METH_NOARGS, NULL,
    "stride", PyTensorObject_stride, METH_NOARGS, NULL,
    "is_contiguous", PyTensorObject_is_contiguous, METH_NOARGS, NULL,
    "contiguous", PyTensorObject_contiguous, METH_NOARGS, NULL,
    "contiguous_", PyTensorObject_contiguous_, METH_NOARGS, NULL,
    "pin_memory", PyTensorObject_pin_memory, METH_NOARGS, NULL,
    "is_pinned", PyTensorObject_is_pinned, METH_NOARGS, NULL,
    "requires_grad_", (PyCFunction)PyTensorObject_requires_grad_, METH_VARARGS | METH_KEYWORDS,
     NULL,
    "retain_grad", PyTensorObject_retain_grad, METH_NOARGS, NULL,
    "detach", PyTensorObject_detach, METH_NOARGS, NULL,
    "clone", PyTensorObject_clone, METH_NOARGS, NULL,
    "zero_", PyTensorObject_zero_, METH_NOARGS, NULL,
    "register_hook", PyTensorObject_register_hook, METH_O, NULL,
    "_register_post_grad_accumulation_hook", PyTensorObject__register_post_grad_accumulation_hook,
     METH_O, NULL,
    "global_id", PyTensorObject_global_id, METH_NOARGS, NULL,
    "check_meta_consistency", PyTensorObject_check_meta_consistency, METH_NOARGS, NULL,
    "to_numpy", PyTensorObject_to_numpy, METH_NOARGS, NULL,
    "type", (PyCFunction)PyTensorObject_type, METH_VARARGS | METH_KEYWORDS, NULL,

此外,在Python层通过RegisterMethods(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/python/oneflow/framework/tensor.py#L502)也为Tensor注册了一些Python实现的Tensor方法或属性(如tensor.numpy),在OneFlow包初始化时会通过RegisterMethod4Class

https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/python/oneflow/framework/register_class_method_util.py#L23)完成这些Python方法和属性的注册。RegisterMethod4Class的调用流程如下:

 

相比于Python实现来说,Tensor的++实现的方法/属性通常具有较高的性能。

1.2 tensor函数

Tensor是类型,而tensor则是函数,flow.tensor函数在oneflow/api/python/functional/tensor_api.yaml中被定义:

- name: "tensor"
  signature: [
      "Tensor (PyObject* data, *, DataType dtype=None, Device device=None,
      Bool requires_grad=False, Bool pin_memory=False) => TensorWithData",
      "Tensor (PyObject* data, *, DataType dtype=None, Placement placement,
      SbpList sbp, Bool requires_grad=False) => GlobalTensorWithData",
    ]
  bind_python: True

其C++实现位于tensor_api.yaml.pybind.cpp中,这是构建阶段自动生成的文件。

通过函数签名可以看到,flow.tensor()有两种重载的方法:

  • TensorWithData

  • GlobalTensorWithData

它们分别用于构造local tensor和global tensor的构造。和上面的Tensor类似,flow.tensor返回的也是OneFlow内部的oneflow::one::Tensor对象(绑定至Python的Tensor对象)。

1.3 手动构建tensor的两种方式

和PyTorch类似,在OneFlow中常用创建tensor的方式也分为两种:

  • flow.Tensor

  • flow.tensor

创建方式示例:

import oneflow
import numpy as np


oneflow.tensor([[1., -1.], [1., -1.]])
# tensor([[ 1., -1.],
#         [ 1., -1.]], dtype=oneflow.float32)
oneflow.tensor(np.array([[1, 2, 3], [4, 5, 6]]))
# tensor([[ 1, 2, 3],
#         [ 4, 5, 6]], dtype=oneflow.int64)
flow.Tensor([[1,2,3],[4,5,6]])

大多数情况下(和PyTorch类似的eager模式),可以通过指定device、dtype、shape等参数创建普通tensor(local tensor);

少数情况下(如OneFlow特有的eager global、lazy模式),需要global tensor时,可以通过指定sbp和placement的方式直接创建global tensor,也可通过tensor.to_global的方式将普通tensor转换为global tensor,可参考:

  • oneflow.tensor

https://oneflow.readthedocs.io/en/master/generated/oneflow.tensor.html#

  • global tensor

    https://docs.oneflow.org/master/parallelism/03_consistent_tensor.html

2

OneFlow的tensor类型体系

上述内容中介绍的oneflow内部的C++ Tensor对象,实际上其定义位于:oneflow/core/framework/tensor.h,是一个抽象的Tensor类型。

 

其中LocalTensor即为普通的单卡视角下的Tensor(和PyTorch的Tensor类似);GlobalTensor则为OneFlow所特有的全局视角下的Tensor(通常用于eager global模式或lazy模式下)。Tensor使用了Bridge模式,每个Tensor子类内部有一个TensorImpl字段,负责抽象Tensor的实际实现:

 

3

local tensor的构造

我们以flow.tensor([[1,2,3],[4,5,6]])为例,看一下tensor构造的过程。主要的流程如下:

在这个例子中,由于使用的是flow.tensor方法创建tensor(且为普通的local tensor)所以会用到在oneflow/api/python/functional/tensor_api.yaml中定义的TensorWithData方法,其实现,是位于oneflow/api/python/functional/tensor_api.cpp的TensorWithDataFunctor:

class TensorWithDataFunctor 
 public:
  Maybe<Tensor> operator()(PyObject* data, const Optional<Symbol<DType>>& dtype,
                           const Optional<Symbol<Device>>& device, const bool requires_grad,
                           const bool pin_memory) const 
    ...
    if (PyTensor_Check(data)) 
      // Throw warnings like pytorch.
      auto ret = PyErr_WarnEx(
          PyExc_UserWarning,
          "To copy construct from a tensor, it is recommended to use sourceTensor.clone().detach() "
          "or sourceTensor.clone().detach().requires_grad_(True), rather than "
          "oneflow.tensor(sourceTensor).",
          1);
      if (ret != 0)  return Error::RuntimeError(); 


      const auto& other = PyTensor_Unpack(data);
      return MakeTensorFromOtherTensor(other, dtype, device, requires_grad, pin_memory);
     else 
      // Make tensor from python sequence or numpy array.
      return MakeLocalTensorFromData(data, dtype, device, requires_grad, pin_memory);
    
  
;

由于这里传入的data是一个Python的list对象,所以最终会调用MakeLocalTensorFromData方法,创建tensor主要的逻辑都在这个函数中。其中大量调用Python和Numpy的接口,检查PyObject的数据类型,获取Shape

https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.cpp#L184)和DataType(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.cpp#L185),如果用户没有制定device,默认会设置为CPU设备(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.cpp#L191)。

后面主要是调用EmptyFunctor

https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.cpp#L194)和SwitchCopyLocalTensorFromUntypedArray(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.cpp#L195)。前者为tensor分配内存,后者进行数据拷贝,两个步骤都会通过虚拟机指令完成。其中EmptyFunctor会走普通的OpCall指令、而CopyLocalTensorFromUntypedArray会根据是否需要同步copy走到AccessBlobByCallback/SyncAccessBlobByCallback指令。

为什么要通过虚拟机指令完成呢?无论是内存资源的分配,还是数据拷贝,CPU和CUDA等不同设备上的操作都不一样。之前讨论Op/Kernel时已经看到,在OneFlow中所有动静态图任务执行、eager模式下op/kernel执行、内存/显存的分配和释放、device、stream等统一由虚拟机进行管理。

3.1 分配内存:EmptyFunctor

matmul和relu(inplace=false时)等操作在执行过程中也会创建output tensor。之前讨论relu时重点关注了op和kernel的计算逻辑,而忽略了tensor相关的内容。

而这里只需要先构造一个空tensor对象,不需要其它计算,所以是一个Empty操作,Empty op对应的kernel——EmptyKernel(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/user/kernels/empty_kernel.cpp#L30)没有实质性的计算逻辑,只是先根据shape、dtype、device信息创建一个空tensor,等待后续将实际的数据从内存中copy至此空tensor,从而完成整个tensor的创建过程。

EmptyFunctor同样和其他functor一样,最终会被Dispacth至对应的interpreter被解释执行,这里由于是eager模式下的local tensor,EmptyFunctor最终会进入eager local interpreter,交给NaiveInterpret(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/core/framework/op_interpreter/eager_local_op_interpreter.cpp#L74)方法处理。流程如下:

1. 在构造EagerLocalTensorImpl(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/core/framework/op_interpreter/eager_local_op_interpreter.cpp#L110)对象,用于存放tensor结果。但这只是一个壳子,还没有为tensor的数据分配存储空间。

2. 之后会初始化EagerBlobObject(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/core/framework/op_interpreter/eager_local_op_interpreter.cpp#L114)、TensorStorage(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/core/framework/tensor_impl.cpp#L120),这样tensor主要的字段基本构建完毕

3. 然后构造OpCall指令、提交虚拟机PhysicalRun(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/core/framework/op_interpreter/eager_local_op_interpreter.cpp#L134-L136),等待vm的调度执行。

OpCall对应的指令策略最终会进入oneflow/core/vm/op_call_instruction_policy.cpp,并在Prepare方法中通过AllocateOutputBlobsMemory方法对TensorStorage完成实际的内存分配;在Compute方法中启动(empty op对应的)实际的kernel执行。

3.2 拷贝数据:SwitchCopyLocalTensorFromUntypedArray

SwitchCopyMirroredTensorFromUntypedArray其实是MAKE_SWITCH_ENTRY(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.cpp#L150)宏展开后的函数名。宏展开后的代码如下。实际会调用CopyLocalTensorFromUntypedArray(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.cpp#L68)。

template<typename... Args>
static Maybe<void> SwitchCopyLocalTensorFromUntypedArray(
    const std::tuple<DataType>& switch_tuple, Args&& ... args) 
  static const std::map<std::tuple<DataType>, std::function<Maybe<void>(Args && ...)>>
      case_handlers 
          SwitchCase(DataType::kFloat),
           [](Args&&... args) 
             return CopyLocalTensorFromUntypedArray<float>(std::forward<Args>(args)...);
           ,
           // ...
      ;
  return case_handlers.at(switch_tuple)(std::forward<Args>(args)...);
;

CopyLocalTensorFromUntypedArray方法如下:

template<typename T>
Maybe<void> CopyLocalTensorFromUntypedArray(const std::shared_ptr<Tensor>& tensor,
                                            PyObject* array) 
  return CopyBetweenLocalTensorAndNumpy<T>(tensor, array, CopyFromNumpyArray, "mut",
                                           /*block_host_until_done=*/false);

其内部实际调用了CopyBetweenLocalTensorAndNumpy方法。

CopyBetweenLocalTensorAndNumpy

顾名思义,这个方法主要是用在numpy和tensor之间进行数据copy的。其中第3个参数:CopyFromNumpyArray实际是一个函数回调的callback方法,其主要通过SyncAutoMemcpy进行array和tensor(blob)之间的内存拷贝:

void CopyFromNumpyArray(ep::Stream* stream,
                        const std::shared_ptr<vm::EagerBlobObject>& eager_blob_object,
                        const NumPyArrayPtr& array_ptr) 
  SyncAutoMemcpy(stream, eager_blob_object->mut_dptr(), array_ptr.data(),
                 eager_blob_object->ByteSizeOfBlobBody(), eager_blob_object->mem_case(),
                 memory::MakeHostMemCase());

继续看CopyBetweenLocalTensorAndNumpy(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/api/python/utils/tensor_utils.h#L93)方法,其中最关键的是:

JUST(PhysicalRun([&](InstructionsBuilder* builder) -> Maybe<void> 
      return builder->AccessBlobByCallback(
          tensor,
          [array_ptr, Copy](ep::Stream* stream,
                            const std::shared_ptr<vm::EagerBlobObject>& eager_blob_object) 
            Copy(stream, eager_blob_object, array_ptr);
          ,
          modifier);
    ));

通过InstructionsBuilder构建了AccessBlobByCallback指令,参数为上面通过EmptyFuncor创建的空tensor、callback的函数指针及参数、以及modifier(string "mut"表示可动态修改)。

AccessBlobByCallback

和OpCall类似,InstructionsBuilder调用AccessBlobByCallback时,也会实际构造对应的vm指令策略——AccessBlobArgCbInstructionPolicy并派发至vm,等待被调度和实际执行:

template<typename T>
Maybe<void> InstructionsBuilder::AccessBlobByCallback(
    const T tensor,
    const std::function<void(ep::Stream*, const std::shared_ptr<vm::EagerBlobObject>&)>& callback,
    const std::string& modifier) 
  const std::shared_ptr<vm::EagerBlobObject>& eager_blob_object = JUST(tensor->eager_blob_object());
  Symbol<Device> device = JUST(GetDevice(tensor));
  ...
  Symbol<Stream> stream = JUST(GetDefaultStreamByDevice(device));
  JUST(SoftSyncStream(eager_blob_object, stream));
  auto instruction = intrusive::make_shared<vm::Instruction>(
      // Never replace `stream` with producer_stream or last_used_stream.
      JUST(Singleton<VirtualMachine>::Get()->GetVmStream(stream)),
      std::make_shared<vm::AccessBlobArgCbInstructionPolicy>(eager_blob_object, callback,
                                                             modifier));
  instruction_list_->EmplaceBack(std::move(instruction));
  return Maybe<void>::Ok();

等该条AccessBlobArgCbInstructionPolicy指令实际执行时,会在指令的Compute(https://github.com/Oneflow-Inc/oneflow/blob/2e6a72c8734b9929191306df35b4284e9caa8126/oneflow/core/vm/access_blob_arg_cb_instruction_policy.h#L79)方法中调用callback完成从tensor的blob <-> numpy的ndarray之间的数据copy,至此拷贝过程结束,flow.tensor的创建全部完成。

(本文经授权后发布。原文:

https://segmentfault.com/a/1190000041989895)

参考资料

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