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<div class="title">Under the Hood </div> </div>
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<div class="toc"><h3>Table of Contents</h3>
<ul><li class="level1"><a href="#simgrid_uhood_s4u">S4U</a></li>
<li class="level1"><a href="#simgrid_uhood_async">Asynchronous operations</a><ul><li class="level2"><a href="#simgrid_uhood_futures">Futures</a></li>
<li class="level2"><a href="#simgrid_uhood_timer">Timers</a></li>
</ul>
</li>
<li class="level1"><a href="#simgrid_uhood_mc">Model Checker</a><ul><li class="level2"><a href="#simgrid_uhood_mc_address_space">Address space</a></li>
<li class="level2"><a href="#simgrid_uhood_mc_address_elf_dwarf">ELF and DWARF</a></li>
</ul>
</li>
</ul>
</div>
<div class="textblock"><p>TBD</p>
<ul>
<li>Simulation Loop, LMM, sharing -> papers</li>
<li>Context Switching, privatization -> papers</li>
</ul>
<h1><a class="anchor" id="simgrid_uhood_s4u"></a>
S4U</h1>
<p>S4U classes are designed to be user process interfaces to Maestro resources. We provide an uniform interface to them:</p>
<ul>
<li>automatic reference count with intrusive smart pointers <code>simgrid::s4u::FooPtr</code> (also called <code>simgrid::s4u::Foo::Ptr</code>);</li>
<li>manual reference count with <code>intrusive_ptr_add_ref(p)</code>, <code>intrusive_ptr_release(p)</code> (which is the interface used by <a href="http://www.boost.org/doc/libs/1_61_0/libs/smart_ptr/intrusive_ptr.html"><code>boost::intrusive_ptr</code></a>);</li>
<li>delegation of the operations to a opaque <code>pimpl</code> (which is the Maestro object);</li>
<li>the Maestro object and the corresponding S4U object have the same lifetime (and share the same reference count).</li>
</ul>
<p>The ability to manipulate thge objects thought pointers and have the ability to use explicit 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).</p>
<p>Some objects currently live for the whole duration of the simulation and do not have refertence counts. We still provide dummy <code>intrusive_ptr_add_ref(p)</code>, <code>intrusive_ptr_release(p)</code> and <code>FooPtr</code> for consistency.</p>
<p>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 <code><a class="el" href="classsimgrid_1_1s4u_1_1Mutex.html" title="A classical mutex, but blocking in the simulation world. ">simgrid::s4u::Mutex</a></code> are based on <a href="http://en.cppreference.com/w/cpp/thread/mutex"><code>std::mutex</code></a>. This has several benefits:</p>
<ul>
<li>we use a proven interface with a well defined and documented semantic;</li>
<li>the interface is easy to understand and remember for people used to the C++ standard interface;</li>
<li>we can use some standard C++ algorithms and helper classes with our types (<code><a class="el" href="classsimgrid_1_1s4u_1_1Mutex.html" title="A classical mutex, but blocking in the simulation world. ">simgrid::s4u::Mutex</a></code> can be used with <a href="http://en.cppreference.com/w/cpp/thread/lock"><code>std::lock</code></a>, <a href="http://en.cppreference.com/w/cpp/thread/unique_lock"><code>std::unique_lock</code></a>, etc.).</li>
</ul>
<p>Example of <code><a class="el" href="classsimgrid_1_1s4u_1_1Actor.html" title="An actor is an independent stream of execution in your distributed application. ">simgrid::s4u::Actor</a></code>:</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Actor {</div><div class="line"> <span class="comment">// This is the corresponding maestro object:</span></div><div class="line"> <span class="keyword">friend</span> simgrid::simix::Process;</div><div class="line"> simgrid::simix::Process* pimpl_ = <span class="keyword">nullptr</span>;</div><div class="line"><span class="keyword">public</span>:</div><div class="line"></div><div class="line"> Actor(simgrid::simix::Process* pimpl) : pimpl_(pimpl) {}</div><div class="line"> Actor(Actor <span class="keyword">const</span>&) = <span class="keyword">delete</span>;</div><div class="line"> Actor& operator=(Actor <span class="keyword">const</span>&) = <span class="keyword">delete</span>;</div><div class="line"></div><div class="line"> <span class="comment">// Reference count is delegated to the S4u object:</span></div><div class="line"> <span class="keyword">friend</span> <span class="keywordtype">void</span> <a class="code" href="namespacesimgrid_1_1s4u.html#a48c0e95542045362966c90579bcfed60">intrusive_ptr_add_ref</a>(Actor* actor)</div><div class="line"> {</div><div class="line"> <a class="code" href="group__XBT__error.html#gadd837b8bf67480aea409a8580072f0be">xbt_assert</a>(actor != <span class="keyword">nullptr</span>);</div><div class="line"> <a class="code" href="simix_8h.html#a05664a6480fe4cb1b6f34d581883deda">SIMIX_process_ref</a>(actor->pimpl_);</div><div class="line"> }</div><div class="line"> <span class="keyword">friend</span> <span class="keywordtype">void</span> <a class="code" href="namespacesimgrid_1_1s4u.html#a19a9d5168427b03b762ad5cf60f1f1c5">intrusive_ptr_release</a>(Actor* actor)</div><div class="line"> {</div><div class="line"> <a class="code" href="group__XBT__error.html#gadd837b8bf67480aea409a8580072f0be">xbt_assert</a>(actor != <span class="keyword">nullptr</span>);</div><div class="line"> <a class="code" href="simix_8h.html#a97fcd3143690eec3b596d28296819a56">SIMIX_process_unref</a>(actor->pimpl_);</div><div class="line"> }</div><div class="line"> <span class="keyword">using</span> Ptr = boost::intrusive_ptr<Actor>;</div><div class="line"></div><div class="line"> <span class="comment">// Create processes:</span></div><div class="line"> <span class="keyword">static</span> Ptr createActor(<span class="keyword">const</span> <span class="keywordtype">char</span>* <a class="code" href="smpi__mpi_8cpp.html#aded70a51d983aedc9dc071e2b2fff252">name</a>, s4u::Host *host, <span class="keywordtype">double</span> killTime, <a class="code" href="smpi__mpi_8cpp.html#a7c3edcb6f1af396c3c82a238febaf8bd">std::function</a><<span class="keywordtype">void</span>()> code);</div><div class="line"></div><div class="line"> <span class="comment">// [...]</span></div><div class="line">};</div><div class="line"></div><div class="line"><span class="keyword">using</span> <a class="code" href="namespacesimgrid_1_1s4u.html#a8facb5bdf92991f647d980cc3d09689e">ActorPtr</a> = Actor::Ptr;</div></div><!-- fragment --><p>It uses the <code>simgrid::simix::Process</code> as a opaque pimple:</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Process {</div><div class="line"><span class="keyword">private</span>:</div><div class="line"> std::atomic_int_fast32_t refcount_ { 1 };</div><div class="line"> <span class="comment">// The lifetime of the s4u::Actor is bound to the lifetime of the Process:</span></div><div class="line"> <a class="code" href="classsimgrid_1_1s4u_1_1Actor.html">simgrid::s4u::Actor</a> actor_;</div><div class="line"><span class="keyword">public</span>:</div><div class="line"> Process() : actor_(this) {}</div><div class="line"></div><div class="line"> <span class="comment">// Reference count:</span></div><div class="line"> <span class="keyword">friend</span> <span class="keywordtype">void</span> <a class="code" href="namespacesimgrid_1_1s4u.html#a48c0e95542045362966c90579bcfed60">intrusive_ptr_add_ref</a>(Process* process)</div><div class="line"> {</div><div class="line"> <span class="comment">// Atomic operation! Do not split in two instructions!</span></div><div class="line"> <span class="keyword">auto</span> previous = (process->refcount_)++;</div><div class="line"> <a class="code" href="group__XBT__error.html#gadd837b8bf67480aea409a8580072f0be">xbt_assert</a>(previous != 0);</div><div class="line"> (<a class="code" href="smpi__mpi_8cpp.html#ac9c84fa68bbad002983e35ce3663c686">void</a>) previous;</div><div class="line"> }</div><div class="line"> <span class="keyword">friend</span> <span class="keywordtype">void</span> <a class="code" href="namespacesimgrid_1_1s4u.html#a19a9d5168427b03b762ad5cf60f1f1c5">intrusive_ptr_release</a>(Process* process)</div><div class="line"> {</div><div class="line"> <span class="comment">// Atomic operation! Do not split in two instructions!</span></div><div class="line"> <span class="keyword">auto</span> <a class="code" href="smpi__mpi_8cpp.html#a5fd414b1e1d8b4d4f94cd964258c4a53">count</a> = --(process->refcount_);</div><div class="line"> <span class="keywordflow">if</span> (<a class="code" href="smpi__mpi_8cpp.html#a5fd414b1e1d8b4d4f94cd964258c4a53">count</a> == 0)</div><div class="line"> <span class="keyword">delete</span> process;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// [...]</span></div><div class="line">};</div><div class="line"></div><div class="line">smx_process_t <a class="code" href="simix_8h.html#a05664a6480fe4cb1b6f34d581883deda">SIMIX_process_ref</a>(smx_process_t process)</div><div class="line">{</div><div class="line"> <span class="keywordflow">if</span> (process != <span class="keyword">nullptr</span>)</div><div class="line"> <a class="code" href="namespacesimgrid_1_1s4u.html#a48c0e95542045362966c90579bcfed60">intrusive_ptr_add_ref</a>(process);</div><div class="line"> <span class="keywordflow">return</span> process;</div><div class="line">}</div><div class="line"></div><div class="line"><span class="keywordtype">void</span> <a class="code" href="simix_8h.html#a97fcd3143690eec3b596d28296819a56">SIMIX_process_unref</a>(smx_process_t process)</div><div class="line">{</div><div class="line"> <span class="keywordflow">if</span> (process != <span class="keyword">nullptr</span>)</div><div class="line"> <a class="code" href="namespacesimgrid_1_1s4u.html#a19a9d5168427b03b762ad5cf60f1f1c5">intrusive_ptr_release</a>(process);</div><div class="line">}</div></div><!-- fragment --><h1><a class="anchor" id="simgrid_uhood_async"></a>
Asynchronous operations</h1>
<h2><a class="anchor" id="simgrid_uhood_futures"></a>
Futures</h2>
<p>The <code><a class="el" href="classsimgrid_1_1kernel_1_1Future.html" title="Result of some (probably) asynchronous operation in the SimGrid kernel. ">simgrid::kernel::Future</a></code> class has been added to SimGrid as an abstraction to represent asynchronous operations in the SimGrid maestro. Its API is based on <code>std::experimental::future</code> from the <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0159r0.html">C++ Extensions for Concurrency Technical Specification</a>:</p>
<ul>
<li><code><a class="el" href="classsimgrid_1_1kernel_1_1Future.html" title="Result of some (probably) asynchronous operation in the SimGrid kernel. ">simgrid::kernel::Future</a><T></code> represents the result an asynchronous operations in the simulation inside the SimGrid maestro/kernel;</li>
<li><code><a class="el" href="classsimgrid_1_1kernel_1_1Promise.html" title="Producer side of a simgrid::kernel::Future. ">simgrid::kernel::Promise</a><T></code> can be used to set the value of an assocaiated <code><a class="el" href="classsimgrid_1_1kernel_1_1Future.html" title="Result of some (probably) asynchronous operation in the SimGrid kernel. ">simgrid::kernel::Future</a><T></code>.</li>
</ul>
<p>The expected way to work with <code><a class="el" href="classsimgrid_1_1kernel_1_1Future.html" title="Result of some (probably) asynchronous operation in the SimGrid kernel. ">simgrid::kernel::Future</a><T></code> is to add a completion handler/continuation:</p>
<div class="fragment"><div class="line"><span class="comment">// This code is executed in the maestro context, we cannot block for the result</span></div><div class="line"><span class="comment">// to be ready:</span></div><div class="line"><a class="code" href="classsimgrid_1_1kernel_1_1Future.html">simgrid::kernel::Future<std::vector<char></a>> <a class="code" href="smpi__mpi_8cpp.html#a6579216cffb6f565d98798aece8e0d2c">result</a> = simgrid::kernel::readFile(<a class="code" href="smpi__mpi_8cpp.html#afab32e5255798738d086a51432feb177">file</a>);</div><div class="line"></div><div class="line"><span class="comment">// Add a completion handler:</span></div><div class="line">result.<a class="code" href="classsimgrid_1_1kernel_1_1Future.html#af9cb00dba33320c7d80c2f455696397b">then</a>([<a class="code" href="smpi__mpi_8cpp.html#afab32e5255798738d086a51432feb177">file</a>](<a class="code" href="classsimgrid_1_1kernel_1_1Future.html">simgrid::kernel::Future</a><std::vector<char>> result) {</div><div class="line"> <span class="comment">// At this point, the operation is complete and we can safely call .get():</span></div><div class="line"> <a class="code" href="group__XBT__error.html#gadd837b8bf67480aea409a8580072f0be">xbt_assert</a>(result.<a class="code" href="classsimgrid_1_1kernel_1_1Future.html#a0f63fc1cc897d4ecb0fbd157d04d796a">is_ready</a>());</div><div class="line"> <span class="keywordflow">try</span> {</div><div class="line"> std::vector<char> data = result.<a class="code" href="classsimgrid_1_1kernel_1_1Future.html#a45e6e7568ad65d6b4101b0f316b61c4d">get</a>();</div><div class="line"> <a class="code" href="group__XBT__log.html#gaec7da836fa1482fcda89227dfb1284d9">XBT_DEBUG</a>(<span class="stringliteral">"Finished reading file %s: length %zu"</span>, <a class="code" href="smpi__mpi_8cpp.html#afab32e5255798738d086a51432feb177">file</a>.c_str(), data.<a class="code" href="structs__smpi__mpi__datatype.html#a0d55be7f4df6cdba3900d442fb071761">size</a>());</div><div class="line"> }</div><div class="line"> <span class="comment">// If the operation failed, .get() throws an exception:</span></div><div class="line"> <span class="keywordflow">catch</span> (std::runtime_error& e) {</div><div class="line"> <a class="code" href="group__XBT__log.html#ga46c64033e6f2d9f309f498bbb2c475f5">XBT_ERROR</a>(<span class="stringliteral">"Could not read file %s"</span>, <a class="code" href="smpi__mpi_8cpp.html#afab32e5255798738d086a51432feb177">file</a>.c_str());</div><div class="line"> }</div><div class="line">});</div></div><!-- fragment --><p>The SimGrid kernel cannot block so calling <code>.get()</code> or <code>.wait()</code> on a <code><a class="el" href="classsimgrid_1_1kernel_1_1Future.html" title="Result of some (probably) asynchronous operation in the SimGrid kernel. ">simgrid::kernel::Future</a><T></code> which is not ready will deadlock. In practice, the simulator detects this and aborts after reporting an error.</p>
<p>In order to generate your own future, you might need to use a <code><a class="el" href="classsimgrid_1_1kernel_1_1Promise.html" title="Producer side of a simgrid::kernel::Future. ">simgrid::kernel::Promise</a><T></code>. The promise is a one-way channel which can be used to set the result of an associated <code><a class="el" href="classsimgrid_1_1kernel_1_1Future.html" title="Result of some (probably) asynchronous operation in the SimGrid kernel. ">simgrid::kernel::Future</a><T></code> (with either <code>.set_value()</code> or <code>.set_exception()</code>):</p>
<div class="fragment"><div class="line"><a class="code" href="classsimgrid_1_1kernel_1_1Future.html">simgrid::kernel::Future<void></a> kernel_wait_until(<span class="keywordtype">double</span> date)</div><div class="line">{</div><div class="line"> <span class="keyword">auto</span> promise = std::make_shared<simgrid::kernel::Promise<void>>();</div><div class="line"> <span class="keyword">auto</span> future = promise->get_future();</div><div class="line"> <a class="code" href="simix_8h.html#abb3c5e7d0f7d1b1f2cc5e0c6023e5e27">SIMIX_timer_set</a>(date, [promise] {</div><div class="line"> promise->set_value();</div><div class="line"> });</div><div class="line"> <span class="keywordflow">return</span> future;</div><div class="line">}</div></div><!-- fragment --><p>Like the experimental futures, we support chaining <code>.then()</code> methods with automatic future unwrapping. You might want to look at some <a href="https://www.youtube.com/watch?v=mPxIegd9J3w&list=PLHTh1InhhwT75gykhs7pqcR_uSiG601oh&index=43">tutorial on C++ futures</a> for more details and examples. Some operations of the proposed experimental futures are currently not implemented in our futures however such as <code>.wait_for()</code>, <code>.wait_until()</code>, <a href="http://en.cppreference.com/w/cpp/thread/shared_future"><code>shared_future</code></a>, <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0159r0.html#futures.when_any"><code>when_any()</code></a>.</p>
<h2><a class="anchor" id="simgrid_uhood_timer"></a>
Timers</h2>
<h1><a class="anchor" id="simgrid_uhood_mc"></a>
Model Checker</h1>
<p>The current implementation of the model-checker uses two distinct processes:</p>
<ul>
<li>the SimGrid model-checker (<code>simgrid-mc</code>) itself lives in the parent process;</li>
<li>it spaws a child process for the SimGrid simulator/maestro and the simulated processes.</li>
</ul>
<p>They communicate using a <code>AF_UNIX</code> <code>SOCK_DGRAM</code> socket and exchange messages defined in <code>mc_protocol.h</code>. The <code>SIMGRID_MC_SOCKET_FD</code> environment variable it set to the file descriptor of this socket in the child process.</p>
<p>The model-checker analyzes, saves and restores the state of the model-checked process using the following techniques:</p>
<ul>
<li>the model-checker reads and writes in the model-checked address space;</li>
<li>the model-cheker <code>ptrace()</code>s the model-checked process and is thus able to know the state of the model-checked process if it crashes;</li>
<li>DWARF debug informations are used to unwind the stack and identify local variables;</li>
<li>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.</li>
</ul>
<h2><a class="anchor" id="simgrid_uhood_mc_address_space"></a>
Address space</h2>
<p>The <code>AddressSpace</code> is a base class used for both the model-checked process and its snapshots and has methods to read in the corresponding address space:</p>
<ul>
<li>the <code>Process</code> class is a subclass representing the model-checked process;</li>
<li>the <code>Snapshot</code> class is a subclass representing a snapshot of the process.</li>
</ul>
<p>Additional helper class include:</p>
<ul>
<li><code>Remote<T></code> is the result of reading a <code>T</code> in a remote AddressSpace. For trivial types (int, etc.), it is convertible t o <code>T</code>;</li>
<li><code>RemotePtr<T></code> represents the address of an object of type <code>T</code> in some remote <code>AddressSpace</code> (it could be an alias to <code>Remote<T*></code>).</li>
</ul>
<h2><a class="anchor" id="simgrid_uhood_mc_address_elf_dwarf"></a>
ELF and DWARF</h2>
<p><a href="http://refspecs.linuxbase.org/elf/elf.pdf">ELF</a> is a standard executable file and dynamic libraries file format. <a href="http://dwarfstd.org/">DWARF</a> 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:</p>
<ul>
<li><code>ObjectInformation</code> represents the informations about a given ELF module (executable or shared-object);</li>
<li><code>Frame</code> represents a subprogram scope (either a subprogram or a scope within the subprogram);</li>
<li><code>Type</code> represents a type (eg. <code>char*</code>, <code>int</code>, <code>std::string</code>) and is referenced by variables (global, variables, parameters), functions (return type), and other types (type of a <code>struct</code> field, etc.);</li>
<li><code>LocationList</code> and <code>DwarfExpression</code> are used to describe the location of variables. </li>
</ul>
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