1 /*! @page uhood Under the Hood
7 - Simulation Loop, LMM, sharing -> papers
8 - Context Switching, privatization -> papers
11 \section simgrid_uhood_s4u S4U
13 S4U classes are designed to be user process interfaces to Maestro resources.
14 We provide an uniform interface to them:
16 * automatic reference count with intrusive smart pointers `simgrid::s4u::FooPtr`
17 (also called `simgrid::s4u::Foo::Ptr`);
19 * manual reference count with `intrusive_ptr_add_ref(p)`,
20 `intrusive_ptr_release(p)`;
22 * delegation of the operations to a opaque `pimpl` (which is the Maestro object);
24 * the Maestro object and the corresponding S4U object have the same lifetime
25 (and share the same reference count).
27 The ability to manipulate thge objects thought pointers and have the ability
28 to use explicite reference count management is useful for creating C wrappers
29 to the S4U and should play nicely with other language bindings (such as
32 Some objects currently live for the whole duration of the simulation and do
33 not have refertence counts. We still provide dummy `intrusive_ptr_add_ref(p)`,
34 `intrusive_ptr_release(p)` and `FooPtr` for consistency.
36 In many cases, we try to have a API which is consistent with the API or
37 corresponding C++ standard classes. For example, the methods of
38 `simgrid::s4u::Mutex`. This has different benefits:
40 * we use a proven interface with a well defined and documented semantic;
42 * the interface is easy to understand and remember for people used to the C++
45 * we can use some standard C++ algorithms and helper classes with our types
46 (`simgrid::s4u::Mutex` can be used with `std::lock`, `std::unique_lock`,
49 Example of `simgris::s4u::Actor`:
53 // This is the corresponding maestro object:
54 friend simgrid::simix::Process;
55 simgrid::simix::Process* pimpl_ = nullptr;
58 Actor(simgrid::simix::Process* pimpl) : pimpl_(pimpl) {}
59 Actor(Actor const&) = delete;
60 Actor& operator=(Actor const&) = delete;
62 // Reference count is delegated to the S4u object:
63 friend void intrusive_ptr_add_ref(Actor* actor)
65 xbt_assert(actor != nullptr);
66 SIMIX_process_ref(actor->pimpl_);
68 friend void intrusive_ptr_release(Actor* actor)
70 xbt_assert(actor != nullptr);
71 SIMIX_process_unref(actor->pimpl_);
73 using Ptr = boost::intrusive_ptr<Actor>;
76 static Ptr createActor(const char* name, s4u::Host *host, double killTime, std::function<void()> code);
81 using ActorPtr = Actor::Ptr;
84 It uses the `simgrid::simix::Process` as a opaque pimple:
89 std::atomic_int_fast32_t refcount_ { 1 };
90 // The lifetime of the s4u::Actor is bound to the lifetime of the Process:
91 simgrid::s4u::Actor actor_;
93 Process() : actor_(this) {}
96 friend void intrusive_ptr_add_ref(Process* process)
98 // Atomic operation! Do not split in two instructions!
99 auto previous = (process->refcount_)++;
100 xbt_assert(previous != 0);
103 friend void intrusive_ptr_release(Process* process)
105 // Atomic operation! Do not split in two instructions!
106 auto count = --(process->refcount_);
114 smx_process_t SIMIX_process_ref(smx_process_t process)
116 if (process != nullptr)
117 intrusive_ptr_add_ref(process);
121 /** Decrease the refcount for this process */
122 void SIMIX_process_unref(smx_process_t process)
124 if (process != nullptr)
125 intrusive_ptr_release(process);
129 \section simgrid_uhood_async Asynchronous operations
131 \subsection simgrid_uhood_futures Futures
133 The `simgrid::kernel::Future` class has been added to SimGrid as an abstraction
134 to represent asynchronous operations in the SimGrid maestro. Its API is based
135 on `std::experimental::future` from the [C++ Extensions for Concurrency Technical
136 Specification](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0159r0.html):
138 - `simgrid::kernel::Future<T>` represents the result an asynchronous operations
139 in the simulation inside the SimGrid maestro/kernel;
141 - `simgrid::kernel::Promise<T>` can be used to set the value of an assocaiated
142 `simgrid::kernel::Future<T>`.
144 The expected way to work with `simgrid::kernel::Future<T>` is to add a
145 completion handler/continuation:
148 // This code is executed in the maestro context, we cannot block for the result
150 simgrid::kernel::Future<std::vector<char>> result = simgrid::kernel::readFile(file);
152 // Add a completion handler:
153 result.then([file](simgrid::kernel::Future<std::vector<char>> result) {
154 // At this point, the operation is complete and we can safely call .get():
155 xbt_assert(result.is_ready());
157 std::vector<char> data = result.get();
158 XBT_DEBUG("Finished reading file %s: length %zu", file.c_str(), data.size());
160 // If the operation failed, .get() throws an exception:
161 catch (std::runtime_error& e) {
162 XBT_ERROR("Could not read file %s", file.c_str());
167 The SimGrid kernel cannot block so calling `.get()` or `.wait()` on a
168 `simgrid::kernel::Future<T>` which is not ready will deadlock. In practice, the
169 simulator detects this and aborts after reporting an error.
171 In order to generate your own future, you might need to use a
172 `simgrid::kernel::Promise<T>`. The promise is a one-way channel which can be
173 used to set the result of an associated `simgrid::kernel::Future<T>`
174 (with either `.set_value()` or `.set_exception()`):
177 simgrid::kernel::Future<void> kernel_wait_until(double date)
179 auto promise = std::make_shared<simgrid::kernel::Promise<void>>();
180 auto future = promise->get_future();
181 SIMIX_timer_set(date, [promise] {
182 promise->set_value();
188 Like the experimental futures, we support chaining `.then()` methods with
189 automatic future unwrapping.
190 You might want to look at some [C++ tutorial on futures](https://www.youtube.com/watch?v=mPxIegd9J3w&list=PLHTh1InhhwT75gykhs7pqcR_uSiG601oh&index=43)
191 for more details and examples. Some operations of the proposed experimental
192 futures are currently not implemented in our futures however such as
193 `.wait_for()`, `.wait_until()`, `shared_future`, `when_any()`.
195 \subsection simgrid_uhood_timer Timers
197 \section simgrid_uhood_mc Model Checker
199 The current implementation of the model-checker uses two distinct processes:
201 - the SimGrid model-checker (`simgrid-mc`) itself lives in the parent process;
203 - it spaws a child process for the SimGrid simulator/mastro and the simulated
206 They communicate using a `AF_UNIX` `SOCK_DGRAM` socket and exchange messages
207 defined in `mc_protocol.h`. The `SIMGRID_MC_SOCKET_FD` environment variable it
208 set to the file descriptor of this socket in the child process.
210 The model-checker analyzes, saves and restores the state of the model-checked
211 process using the following techniques:
213 * the model-checker reads and writes in the model-checked address space;
215 * the model-cheker `ptrace()`s the model-checked process and is thus able to
216 know the state of the model-checked process if it crashes;
218 * DWARF debug informations are used to unwind the stack and identify local
221 * a custom heap is enabled in the model-checked process which allows the model
222 checker to know which chunks are allocated and which are freed.
224 \subsection simgrid_uhood_mc_address_space Address space
226 The `AddressSpace` is a base class used for both the model-checked process
227 and its snapshots and has methods to read in the corresponding address space:
229 - the `Process` class is a subclass representing the model-checked process;
231 - the `Snapshot` class is a subclass representing a snapshot of the process.
233 Additional helper class include:
235 - `Remote<T>` is the result of reading a `T` in a remote AddressSpace. For
236 trivial types (int, etc.), it is convertible t o `T`.
238 - `RemotePtr<T>` represents the address of an object of type `T` in some
239 remote `AddressSpace` (it could be an alias to `Remote<T*>`).
241 \subsection simgrid_uhood_mc_address_elf_dwarf ELF and DWARF
243 ELF is a standard executable file and dynamic libraries file format.
244 DWARF is a standard for debug informations. Both are used on GNU/Linux systems
245 and exploited by the model-checker to understand the model-checked process:
247 - `ObjectInformation` represents the informations about a given ELF module
248 (executable or shared-object);
250 - `Frame` represents a subprogram scope (either a subprogram or a scope within
253 - `Type` represents a type (`char*`, `int`, `std::string`) and is referenced
254 by variables (global, variables, parameters), functions (return type),
255 and other types (type of a `struct` field, etc.);
257 - `LocationList` and `DwarfExpression` are used to describe the location of