diff --git a/doc/design/scope.md b/doc/design/scope.md new file mode 100644 index 0000000000000..2ff416f06e8ad --- /dev/null +++ b/doc/design/scope.md @@ -0,0 +1,124 @@ +# Design of Scope in Paddle + +## Overview + +Scope is an important concept in programming languages, which defines a program region that a set of bindings between names and entities applies. In a specific scope, a valid name is uniquely associated with an entity, such as a variable. And in another scope, this name may refer to other entity or nothing at all. It clearly restricts the visibility and validity of names in a program. Hence **Scope** is introduced to PaddlePaddle to manage variables in context. But different from the original abstract concept, Scope now becomes an object with two important attributes: + +- Scope is an association of a name to variable. +- Variables in a parent scope can be retrieved from local scope. + +A detailed explanation of these two attributes goes as following. + + +## Scope is an association of a name to variable. + +Scope is an association of a name to variable. All variables belong to `Scope`. You need to specify a scope to run a Net, i.e., `net.Run(&scope)`. One net can run in different scopes and update different variable in the scope. + + +1. Scope only contains a map of a name to variable. + + All parameters, data, states in a Net should be variables and stored inside a scope. Each op should get inputs and outputs to do computation from a scope, such as data buffer, state(momentum) etc. + +1. Variable can only be created by Scope and a variable can only be got from Scope. User cannot create or get a variable outside a scope. This is a constraints of our framework, and will keep our framework simple and clear. + +1. Scope only contains methods that are used to Create and Get Variables. Scope do not contain Operators and have no information to run them. + `Net` is designed to drive the computation and Scope only contains a map of variables. There is no computation logic inside a `Scope`. Scope just handles the lifetime management of variables. + - `Create` is used to create a Variable by its name and add the mapping relation. + - `Get` is used to find a Variable by name. + +1. Every variable only belongs to one certain Scope. + + Variable can not belong to many scopes. If you want to use variables from parent scope, you can use `parent scope`. + +1. Scope should destruct all Variables inside it when itself is destructed. User can never store `Variable` pointer somewhere else. + + Because Variable can only be got from Scope. When destroying Scope, we also need to destroy all the Variables in it. If user store `Variable` pointer to private data member or some global variable, the pointer will be a invalid pointer when associated `Scope` is destroyed. + +```cpp +class Scope { + public: + Variable* CreateVariable(const std::string& name); + const Variable* GetVariable(const std::string& name) const; + + private: + std::unordered_map> vars_; +}; +``` + + +## Parent scope and local scope + +Just like [scope](https://en.wikipedia.org/wiki/Scope_(computer_science)) in programming languages, `Scope` in the neural network can also be a local scope. There are two attributes about local scope. + +1. We can create local variables in a local scope. When that local scope are destroyed, all local variables should also be destroyed. +2. Variables in a parent scope can be retrieved from local scopes of that parent scope, i.e., when user get a variable from a scope, it will try to search this variable in current scope. If there is no such variable in the local scope, `scope` will keep searching from its parent, until the variable is found or there is no parent. + +```cpp +class Scope { + public: + Scope(const std::shared_ptr& scope): parent_(scope) {} + + Variable* GetVariable(const std::string& name) const { + Variable* var = GetVarLocally(name); + if (var != nullptr) { + return var; + } else if (parent_ != nullptr) { + return parent_->GetVariable(name); + } else { + return nullptr; + } + } + + private: + std::shared_ptr parent_ {nullptr}; +}; +``` + +In `Scope` class, there is a private data member called `parent_`. `parent_` is a smart pointer to its parent scope. When user `Get` a variable by its `name`, the `name` will be searched inside the current scope. If the variable cannot be found locally and parent scope is not a `nullptr`, the variable will be searched inside that parent scope. `parent_` pointer's default value is `nullptr`. It means that the scope is a global scope when `parent_` is nullptr. + +A local scope is very useful when we implement Recurrent Neural Network. Each timestep of an RNN should be a `Net`. Each `Net` of timestep (`StepNet` for short) should use an independent local scope. Just like variables in a while loop is inside a local scope in programming languages. By using a single `StepNet` and changing local scope, we can implement an RNN easily. + +# Interface Design + +```cpp +class Variable { + private: + Variable() = default; + friend class Scope; +}; + +class Scope { + private: + Scope(const std::shared_ptr& parent = nullptr); + + public: + static std::shared_ptr Create(const std::shared_ptr& parent = nullptr); + + // return nullptr if not found. + Variable* GetVariable(const std::string& name) const; + + // return Error if already contains same name variable. + Error CreateVariable(const std::string& name); + + private: + std::shared_ptr parent_; + std::unordered_map> vars_; +}; +``` +## Only scope can create a variable + +To ensure `only scope can create a variable`, we should mark `Variable`'s constructor as a private member function, and Scope is a friend class of Variable. And then only `CreateVariable` can construct `Variable`. + +## When scope destroyed, all variables inside this scope should be destroyed together + +The scope hold unique pointers for all variables. User can `GetVariable` from scope, but he should not hold this pointer as a member variable. Because when scope is destroyed, all variables inside this scope will be destroyed together. + +## Sharing a parent scope + +Local scope contains a `parent_` pointer. It is a linked-list for scopes. Using a `shared_ptr` because when a local scope is using, its parents cannot be destroyed. + +Also, as the parent scope is a `shared_ptr`, we can only `Create()` a scope shared pointer. We cannot construct a scope variable, because it cannot be passed to other scope as `parent` pointer. + +## Orthogonal interface + +`GetVariable` will return `nullptr` when `name` is not found. It can be used as `Contains` method. `CreateVariable` will return a `Error` when there is a name conflict locally. Combine `GetVariable` and `CreateVariable`, we can implement `CreateOrGetVariable` easily.