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18 Using XML to describe the platforms is very convenient. It provides a
19 human-readable, quick way to start your experiments. Although, XML format brings
20 several drawbacks as your platforms get larger and more complex (see :ref:`platform_cpp_beyond`).
22 In this case, it may be more interesting to write your platform directly
23 in C++ code. It allows you to programmatically describe your platform and
24 remove the intermediate XML parser during simulations. Take care to follow
25 the recommendations in :ref:`Modeling Hints <howto>` to keep a clear separation
26 of concerns between your platform and your application.
31 A platform in SimGrid is composed of several resources organized in different
32 Netzones. The different resources, such as hosts, disks and links, follow the same
33 idiom: create()->set()->set()->seal().
37 NetZone* zone = s4u::create_star_zone("zone0");
38 Link* l_up = zone->create_link("link_up", "125MBps")->set_latency("24us")->seal();
39 Host* host = zone->create_host("host0", "1Gf")->seal();
42 The first NetZone created will be the root zone of your platform. You're allowed to modified
43 an object as long as you did not seal it.
45 For more details about how to describe the platforms, please give a look at :ref:`examples<platform_cpp_example>`
46 or directly at the S4U API.
51 In the XML, you are allowed to do the following description:
55 <link id="1" bandwidth="10kBps" latency="10ms" sharing_policy="SPLITDUPLEX"/>
57 <route src="S1" dst="C1" symmetrical="NO">
58 <link_ctn id="1" direction="DOWN"/>
61 It is important to notice that this is a syntactic sugar provided by the XML to ease
62 the link utilization. A split-duplex link means that upgoing communications do not
63 share the bandwidth with downgoing communications. To emulate this behavior,
64 under the hood, SimGrid creates 2 links in this case: the *1_UP*
65 link and the *1_DOWN* link. As you can see, the selection of link to use
66 in the <route> tag is done by the ``direction=`` parameter.
68 Using the C++ interface, you should describe both links separately and use them
69 in the route description.
73 Link* l_up = zone->create_link("1_UP", "125MBps")->set_latency("24us")->seal();
74 Link* l_down = zone->create_link("1_DOWN", "125MBps")->set_latency("24us")->seal();
76 zone->add_route(S1, C1, nullptr, nullptr, {link_down});
82 The C++ interface to build the platforms give you freedom to organize your code
83 as you wish, separating (or unifying) your application from your platform code.
84 However, we provide a small hack if you want to keep the same structure of the
85 old code with XML platforms. You can pass a library (.so) file to ``Engine::load_platform``
86 function, having a predefined function implemented. When loading the platform, the
87 Engine will look for a function with this signature: "**void load_platform(const sg4::Engine& e)**", and
88 execute it. It could be an easy way to make the transition between XML and C++ if necessary.
90 For more details, please refer to the cpp and CMakeLists.txt files in
91 `examples/platform <https://framagit.org/simgrid/simgrid/tree/master/examples/platforms>`_.
94 .. _platform_cpp_example:
99 The best way to build your C++ platform is starting from some examples.
100 Give a look in the examples folder in `examples/ <https://framagit.org/simgrid/simgrid/tree/master/examples/>`_.
101 For instance, the file `examples/cpp/clusters-multicpu/s4u-clusters-multicpu.cpp <https://framagit.org/simgrid/simgrid/-/blob/master/examples/cpp/clusters-multicpu/s4u-clusters-multicpu.cpp>`_ shows how to build complex platforms composed of
102 clusters of clusters.
104 Here, we present a complete example showing how to create 3 regulars clusters
105 connected through a shared link.
107 .. literalinclude:: ../../examples/platforms/griffon.cpp
111 .. _platform_cpp_beyond:
113 Beyond the XML: the power of C++ platforms
114 ******************************************
116 This section describes one of the advantages of using C++ code to write your platforms.
118 Let's see an example of the description of a Fat-Tree in XML (:ref:`platform_examples_fattree`)
120 .. literalinclude:: ../../examples/platforms/cluster_fat_tree.xml
124 Our cluster *bob* is composed of 16 hosts with the same 1Gf CPU power.
126 Imagine now that you want to simulate the same **Fat-Tree topology with** more complex **hosts**,
127 composed of **1 CPU, 1 GPU and some interconnecting bus**.
129 Unfortunately, this is not possible with the XML description since its syntax obliges
130 that the leaves in your Fat-Tree to be single Hosts. However, with the C++ API, your
131 leaves can be composed of other zones, creating a **Fat-Tree of FullZones** for example.
133 Consequently, you can describe the desired platform as follows:
137 sg4::Engine e(&argc, argv);
138 sg4::create_fatTree_zone("bob", e.get_netzone_root(), {2, {4, 4}, {1, 2}, {1, 2}}, {create_hostzone, create_loopback, {}}, 125e6,
139 50e-6, sg4::Link::SharingPolicy::SPLITDUPLEX)->seal();
141 Note that the leaves and loopback links are defined through callbacks, as follows:
145 /* create the loopback link for each leaf in the Fat-Tree */
146 static sg4::Link* create_loopback(sg4::NetZone* zone, const std::vector<unsigned int>& /*coord*/, int id)
148 // note that you could set different loopback links for each leaf
149 return zone->create_link("limiter-" + std::to_string(id), 1e6)->seal();
152 /* create each leaf in the Fat-Tree, return a pair composed of: <object (host, zone), gateway> */
153 static std::pair<simgrid::kernel::routing::NetPoint*, simgrid::kernel::routing::NetPoint*>
154 create_hostzone(const sg4::NetZone* zone, const std::vector<unsigned int>& /*coord*/, int id)
157 std::string hostname = "host" + std::to_string(id);
158 auto* host_zone = sg4::create_full_zone(hostname);
159 /* setting my parent zone */
160 host_zone->set_parent(zone);
163 std::string cpu_name = hostname + "-cpu" + std::to_string(i);
164 const sg4::Host* cpu = host_zone->create_host(cpu_name, 1e9)->seal();
166 std::string gpu_name = hostname + "-gpu" + std::to_string(i);
167 const sg4::Host* gpu = host_zone->create_host(gpu_name, 1e12)->seal();
168 /* connecting them */
169 sg4::Link* link = host_zone->create_link("link-" + cpu_name, 10e9)->set_latency(10e-9)->seal();
170 host_zone->add_route(cpu->get_netpoint(), gpu->get_netpoint(), nullptr, nullptr, std::vector<sg4::Link*>{link});
173 /* cpu is the gateway for this host */
174 return std::make_pair(host_zone->get_netpoint(), cpu->get_netpoint());
177 The code is straightforward and can be easily adapted to more complex environments thanks to the flexibility
178 provided by the C++ API.