Describing your Simulated Platform¶
In SimGrid, platforms are usually described in XML. This formalism has some drawbacks, but using a specific format ensures that the platform is not mixed with the tested application. This separation of concern is a must for your Modeling and Simulation (M&S) work. When XML is too limiting, you may describe your platforms using the lua bindings (it is not yet possible to do so in python or directly in C++).
We understand that writing a complex platform description can be tricky, we thus included many examples in the archive. This documentation also contains some hints and howtos, as well as the full XML reference guide.
Any simulated platform must contain basic elements, such as hosts, links, storages, etc. SimGrid gives you a great liberty when defining the routing of your platform, i.e., the network path taken between each pair of hosts. Finally, you may also describe an experimental scenario, with qualitative (e.g., bandwidth variations representing an external load) and qualitative (e.g., representing how some elements fail and restart over time) changes.
Imagine you want to describe a little platform with three hosts, interconnected as follows:
This can be done with the following platform file, that considers the simulated platform as a graph of hosts and network links.
<?xml version='1.0'?> <!DOCTYPE platform SYSTEM "https://simgrid.org/simgrid.dtd"> <platform version="4.1"> <zone id="AS0" routing="Full"> <host id="host0" speed="1Gf"/> <host id="host1" speed="2Gf"/> <host id="host2" speed="40Gf"/> <link id="link0" bandwidth="125MBps" latency="100us"/> <link id="link1" bandwidth="50MBps" latency="150us"/> <link id="link2" bandwidth="250MBps" latency="50us"/> <route src="host0" dst="host1"><link_ctn id="link0"/><link_ctn id="link1"/></route> <route src="host1" dst="host2"><link_ctn id="link1"/><link_ctn id="link2"/></route> <route src="host0" dst="host2"><link_ctn id="link0"/><link_ctn id="link2"/></route> </zone> </platform>
The most important elements are the basic ones: <host>, <link>, and similar. Then come the routes between any pair of hosts, that are given explicitely with <route> (routes are symmetrical by default). Any host must be given a computational speed (in flops) while links must be given a latency (in seconds) and a bandwidth (in bytes per second). Note that you can write 1Gflops instead of 1000000000flops, and similar.
Every platform element must be located within a given networking zone . Zones are in charge of the routing, see below.
The last thing you must know on SimGrid platform files is that the
root tag must be <platform>. If the
does not match what SimGrid expects, you will be hinted to use to the
simgrid_update_xml utility to update your file.
Defining a Routing¶
Networking zones (<zone>) are used to factorize the description
to reduce the size of your platform on disk and in memory. Then, when
a host wants to communicate with another host belonging to the same
zone, it is the zone’s duty to find the list of links that are
involved in the communication. In the above examples, since we use
routing="Full", all routes must be explicitly given using the
<route> and <link_ctn> tags (this routing
model is both simple and inefficient :) It is OK to not
specify each and every route between hosts, as long as you do not try
to start a communication on any of the missing routes during your
Any zone may contain sub-zones, allowing for a hierarchical decomposition of the platform. Routing can be made more efficient (as the inter-zone routing gets factored with pf_tag_zoneroute), and allows you to have more than one routing model in your platform. For example, you can have a coordinate-based routing for the WAN parts of your platforms, a full routing within each datacenter, and a highly optimized routing within each cluster of the datacenter. In this case, determining the route between two given hosts gets routing_basics “somewhat more complex” but SimGrid still computes these routes for you in a time- and space-efficient manner. Here is an illustration of these concepts:
Circles represent processing units and squares represent network routers. Bold lines represent communication links. The zone “AS2” models the core of a national network interconnecting a small flat cluster (AS4) and a larger hierarchical cluster (AS5), a subset of a LAN (AS6), and a set of peers scattered around the world (AS7).
Add more examples, such as the cloud example described in previous paragraph