[simgrid.git] / doc / doxygen / uhood.doc

/*! @page uhood Under the Hood

\tableofcontents

TBD

 - Simulation Loop, LMM, sharing -> papers
 - Context Switching, privatization -> papers
 - @subpage inside

\section simgrid_uhood_s4u S4U

S4U classes are designed to be user process interfaces to Maestro resources.
We provide an uniform interface to them:

* automatic reference count with intrusive smart pointers `simgrid::s4u::FooPtr`
  (also called `simgrid::s4u::Foo::Ptr`);

* manual reference count with `intrusive_ptr_add_ref(p)`,
  `intrusive_ptr_release(p)` (which is the interface used by
  [`boost::intrusive_ptr`](http://www.boost.org/doc/libs/1_61_0/libs/smart_ptr/intrusive_ptr.html));

* delegation of the operations to a opaque `pimpl` (which is the Maestro object);

* the Maestro object and the corresponding S4U object have the same lifetime
  (and share the same reference count).

The ability to manipulate thge objects thought pointers and have the ability
to use explicite reference count management is useful for creating C wrappers
to the S4U and should play nicely with other language bindings (such as
SWIG-based ones).

Some objects currently live for the whole duration of the simulation and do
not have refertence counts. We still provide dummy `intrusive_ptr_add_ref(p)`,
`intrusive_ptr_release(p)` and `FooPtr` for consistency.

In many cases, we try to have a API which is consistent with the API or
corresponding C++ standard classes. For example, the methods of
`simgrid::s4u::Mutex` are based on [`std::mutex`](http://en.cppreference.com/w/cpp/thread/mutex).
This has several benefits:

 * we use a proven interface with a well defined and documented semantic;

 * the interface is easy to understand and remember for people used to the C++
   standard interface;

 * we can use some standard C++ algorithms and helper classes with our types
   (`simgrid::s4u::Mutex` can be used with
   [`std::lock`](http://en.cppreference.com/w/cpp/thread/lock),
   [`std::unique_lock`](http://en.cppreference.com/w/cpp/thread/unique_lock),
   etc.).

Example of `simgris::s4u::Actor`:

~~~
class Actor {
  // This is the corresponding maestro object:
  friend simgrid::simix::Process;
  simgrid::simix::Process* pimpl_ = nullptr;
public:

  Actor(simgrid::simix::Process* pimpl) : pimpl_(pimpl) {}
  Actor(Actor const&) = delete;
  Actor& operator=(Actor const&) = delete;

  // Reference count is delegated to the S4u object:
  friend void intrusive_ptr_add_ref(Actor* actor)
  {
    xbt_assert(actor != nullptr);
    SIMIX_process_ref(actor->pimpl_);
  }
  friend void intrusive_ptr_release(Actor* actor)
  {
    xbt_assert(actor != nullptr);
    SIMIX_process_unref(actor->pimpl_);
  }
  using Ptr = boost::intrusive_ptr<Actor>;

  // Create processes:
  static Ptr createActor(const char* name, s4u::Host *host, double killTime, std::function<void()> code);

  // [...]
};

using ActorPtr = Actor::Ptr;
~~~

It uses the `simgrid::simix::Process` as a opaque pimple:

~~~
class Process {
private:
  std::atomic_int_fast32_t refcount_ { 1 };
  // The lifetime of the s4u::Actor is bound to the lifetime of the Process:
  simgrid::s4u::Actor actor_;
public:
  Process() : actor_(this) {}

  // Reference count:
  friend void intrusive_ptr_add_ref(Process* process)
  {
    // Atomic operation! Do not split in two instructions!
    auto previous = (process->refcount_)++;
    xbt_assert(previous != 0);
    (void) previous;
  }
  friend void intrusive_ptr_release(Process* process)
  {
    // Atomic operation! Do not split in two instructions!
    auto count = --(process->refcount_);
    if (count == 0)
      delete process;
  }

  // [...]
};

smx_process_t SIMIX_process_ref(smx_process_t process)
{
  if (process != nullptr)
    intrusive_ptr_add_ref(process);
  return process;
}

/** Decrease the refcount for this process */
void SIMIX_process_unref(smx_process_t process)
{
  if (process != nullptr)
    intrusive_ptr_release(process);
}
~~~

\section simgrid_uhood_async Asynchronous operations

\subsection simgrid_uhood_futures Futures

The `simgrid::kernel::Future` class has been added to SimGrid as an abstraction
to represent asynchronous operations in the SimGrid maestro. Its API is based
on `std::experimental::future` from the [C++ Extensions for Concurrency Technical
Specification](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0159r0.html):

 - `simgrid::kernel::Future<T>` represents the result an asynchronous operations
    in the simulation inside the SimGrid maestro/kernel;

 - `simgrid::kernel::Promise<T>` can be used to set the value of an assocaiated
   `simgrid::kernel::Future<T>`.

The expected way to work with `simgrid::kernel::Future<T>` is to add a
completion handler/continuation:

~~~
// This code is executed in the maestro context, we cannot block for the result
// to be ready:
simgrid::kernel::Future<std::vector<char>> result = simgrid::kernel::readFile(file);

// Add a completion handler:
result.then([file](simgrid::kernel::Future<std::vector<char>> result) {
  // At this point, the operation is complete and we can safely call .get():
  xbt_assert(result.is_ready());
  try {
    std::vector<char> data = result.get();
    XBT_DEBUG("Finished reading file %s: length %zu", file.c_str(), data.size());
  }
  // If the operation failed, .get() throws an exception:
  catch (std::runtime_error& e) {
    XBT_ERROR("Could not read file %s", file.c_str());
  }
});
~~~

The SimGrid kernel cannot block so calling `.get()` or `.wait()` on a
`simgrid::kernel::Future<T>` which is not ready will deadlock. In practice, the
simulator detects this and aborts after reporting an error.

In order to generate your own future, you might need to use a
`simgrid::kernel::Promise<T>`. The promise is a one-way channel which can be
used to set the result of an associated `simgrid::kernel::Future<T>`
(with either `.set_value()` or `.set_exception()`):

~~~
simgrid::kernel::Future<void> kernel_wait_until(double date)
{
  auto promise = std::make_shared<simgrid::kernel::Promise<void>>();
  auto future = promise->get_future();
  SIMIX_timer_set(date, [promise] {
    promise->set_value();
  });
  return future;
}
~~~

Like the experimental futures, we support chaining `.then()` methods with
automatic future unwrapping.
You might want to look at some [tutorial on C++ futures](https://www.youtube.com/watch?v=mPxIegd9J3w&list=PLHTh1InhhwT75gykhs7pqcR_uSiG601oh&index=43)
for more details and examples. Some operations of the proposed experimental
futures are currently not implemented in our futures however such as
`.wait_for()`, `.wait_until()`,
[`shared_future`](http://en.cppreference.com/w/cpp/thread/shared_future),
[`when_any()`](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0159r0.html#futures.when_any).

\subsection simgrid_uhood_timer Timers

\section simgrid_uhood_mc Model Checker

The current implementation of the model-checker uses two distinct processes:

 - the SimGrid model-checker (`simgrid-mc`) itself lives in the parent process;

 - it spaws a child process for the SimGrid simulator/mastro and the simulated
   processes.

They communicate using a `AF_UNIX` `SOCK_DGRAM` socket and exchange messages
defined in `mc_protocol.h`. The `SIMGRID_MC_SOCKET_FD` environment variable it
set to the file descriptor of this socket in the child process.

The model-checker analyzes, saves and restores the state of the model-checked
process using the following techniques:

* the model-checker reads and writes in the model-checked address space;

* the model-cheker `ptrace()`s the model-checked process and is thus able to
  know the state of the model-checked process if it crashes;

* DWARF debug informations are used to unwind the stack and identify local
  variables;

* a custom heap is enabled in the model-checked process which allows the model
  checker to know which chunks are allocated and which are freed.

\subsection simgrid_uhood_mc_address_space Address space

The `AddressSpace` is a base class used for both the model-checked process
and its snapshots and has methods to read in the corresponding address space:

 - the `Process` class is a subclass representing the model-checked process;

 - the `Snapshot` class is a subclass representing a snapshot of the process.

Additional helper class include:

 - `Remote<T>` is the result of reading a `T` in a remote AddressSpace. For
    trivial types (int, etc.), it is convertible t o `T`;

 - `RemotePtr<T>` represents the address of an object of type `T` in some
    remote `AddressSpace` (it could be an alias to `Remote<T*>`).

\subsection simgrid_uhood_mc_address_elf_dwarf ELF and DWARF

[ELF](http://refspecs.linuxbase.org/elf/elf.pdf) is a standard executable file
and dynamic libraries file format.
[DWARF](http://dwarfstd.org/) is a standard for debug informations.
Both are used on GNU/Linux systems and exploited by the model-checker to
understand the model-checked process:

 - `ObjectInformation` represents the informations about a given ELF module
   (executable or shared-object);

 - `Frame` represents a subprogram scope (either a subprogram or a scope within
    the subprogram);

 - `Type` represents a type (eg. `char*`, `int`, `std::string`) and is referenced
    by variables (global, variables, parameters), functions (return type),
    and other types (type of a `struct` field, etc.);

 - `LocationList` and `DwarfExpression` are used to describe the location of
    variables.

*/