Article¶
- The project originally started during GSoC project 2012
- “DCE cradle: simulate network protocols with real stacks for better realism”, WNS3 2013, [PDF]
Main classes and main data structures¶
DceManager¶
The DceManager is somewhat the entry point of DCE. It will create virtual processes and manages their execution. There is an instance of DceManager which is associated to each node which need to virtualize the execution of a process. In reality, the developer uses the classes DceManagerHelper and DceApplicationHelper.
I invite you to look at the source code dce-manager.cc and dce-manager.h and particularly the public methods Start, Stop, Exit, Wakeup, Wait and Yield; the following private methods are also important: CreateProcess, PrepareDoStartProcess, DoStartProcess, AllocatePid, TaskSwitch, CleanupThread and LoadMain. The Start method is called when starting the executable, if you look at, it begins by initializing an object of type struct Process. struct Process is very important, it contains information about the virtual processes that DCE creates, this type is described below. Start then also initializes a structure of type struct thread, it represents the principal thread in which the main entry of the executable will run. Finally Start asks the TaskManager to create a new Task and to start this one using the method DceManager::DoStartProcess as the entry point.
Class TaskManager is a major class of DCE, it is described below.
Process¶
struct process contains everything you need to describe a process, I invite you to study the source code in the file process.h.
This structure contains references to standard objects for example a list of open files via a vector of FILE *, but it contains also the references to objects useful to manage DCE threads, the memory allocated by the process...
Field openFiles represents the open file descriptors of the process, the key is the fd and the value a pointer to an object of type DCE FileUsage. The field threads contains all the threads in the process see description below. Field loader is a pointer to the Loader used to load the corresponding code.
The alloc field is important it is a pointer to the memory allocator used to allocate the memory used by this process, at the end of the process it will liberate all the memory allocated so as simple and efficient.
Thread¶
struct thread represents a thread, It contains various fields including a pointer to the process to which it belongs, and also a pointer to Task object described later.
Taskmanager and Task¶
The TaskManager manages the Tasks, ie the threads of virtualized processes by DCE. It allows you to create new task. It controls the activity of the task by the following methods: Stop, Wakeup, Sleep and Yield. A Task possesses a stack which contains the call stack functions. There is one instance of TaskManager per node. The implementation of TaskManager is based on a class of type FiberManager described below.
FiberManager¶
FiberManager actually implements the creation of the execution contexts. These contexts are called fiber. FiberManager offers the following:
- Create a context, it returns a fiber
- Delete a fiber
- Yield hand to another fiber
DCE provides two implementations:
- PthreadFiberManager, which is based on the pthread library,
- UcontextFiberManager which is based on the POSIX API functions offered by ucontext.h: makecontext, getcontext and setcontext.
I invite you to watch the corresponding man.
LoaderFactory and Loader¶
The Loader is a very important object of DCE. A DCE Loader loads the executable code in memory of a special way, load several times the same executable, while isolating each of the other executable. The Loader must link the executable loaded with the 3 emulated libraries, i.e., lib C, lib pthread and lib rt. The same way the libraries used by the executable must also be linked with the emulated libraries.
DCE offers several actually Loader:
- CoojaLoader: it has the following characteristics: it loads into memory only a copy of the code, by cons it duplicates data (i.e., global variables and static). For each change of context there are 2 memory copies: backup data of the current context then restoration of context memory that will take control. Comment: it is rather reliable, the size of the copied memory size depends on the total static and global variables, and in general there is little, in a well-designed executable.
- DlmLoader: Uses a specialized loader to not duplicate the code but only the data but without special operations to do when changing context. Comment: offers the best performance in memory and cpu, but not very reliable especially during the unloading phase.