With SimDag, you express your code as a collection of interdependent
parallel tasks. So, in this model, applications can be seen as a DAG of
-tasks.
+tasks. This is the interface of choice for people wanting to port old
+code designed for SimGrid v1 or v2 to the lastest framework.
With both GRAS and MSG, your application is seen as a set of communicating
processes, exchanging data by the way of messages and performing computation
DAG is really the level of abstraction you want to work with, then you should
give a try to \ref SD_API.
-\subsubsection faq_SG Where has SG disappeared?
-
-OK, it's time to explain what's happening to the SimGrid project. Let's
-start with a little bit of history.
-
-* Historically, SimGrid was a low-level toolkit for scheduling with
-classical models such as DAGs. That was SimGrid v.1.* aka SG, written
-by Henri Casanova. I (Arnaud) had been using it in its earliest
-versions during an internship at UCSD.
-
-Then we have realized that encoding distributed algorithm in SG was a
-real pain.
-
-* So we have built MSG on top of SG and have released SimGrid v.2.*. MSG
-offered a very basic API to encode a distributed application easily.
-However encoding MSG on top of SG was not really convenient and did not
-use the DAG part since the control of the task synchronization was done
-on top of MSG and no more in SG. We have been playing a little bit with
-MSG. We have realized that:
-
- \li 1) the platform modeling was quite flexible and could be "almost"
- automated (e.g. using random generator and post-annotations);
-
- \li 2) SG was the bottleneck because of the way we were using
- it. We needed to simulate concurrent transfers, complex load
- sharing mechanisms. Many optimizations (e.g. trace integration)
- were totally inefficient when combined with MSG and made extending SG
- to implement new sharing policies, parallel tasks models, or failures
- (many people were asking for these kind of features) a real pain;
-
- \li 3) the application modeling was not really easy. Even though the
- application modeling depends on people's applications, we thought
- we could improve things here. One of our target here was realistic
-distributed applications ranging from computer sensor networks like
-the NWS to peer-to-peer applications;
-
-* So we have been planning mainly two things for SimGrid 3:
-
- \li 1) I have proposed to get rid of SG and to re-implement a new kernel
- that would be faster and more flexible. That is what I did in the
-end of 2004: SURF. SURF is based on a fast max-min linear solver
-using O(1) data-structures. I have quickly replaced SG by SURF in
-MSG and the result has been that on the MSG example, the new
-version was more than 10 times faster while we had gain a lot of
-flexibility. I think I could still easily make MSG faster but I
-have to work on MSG now (e.g. using some of the O(1)
-data-structures I've been using to build SURF) since it has become
-the bottleneck. Some MSG functions have been removed from the API
-but they were mainly intended to build the platform by hand (they
-had appeared in the earliest versions of MSG) and were therefore
-not useful anymore since we are providing a complete mechanism to
-automatically build the platform and deploy the agents on it.;
-
- \li 2) GRAS is a new project Martin and I have come up with. The idea is
-to have a programming environment that let you program real
-distributed applications while letting you the ability to run it in
-the simulator without having to change the slightest line of your
-code. From the simulation point of view, GRAS performs the
-application modeling automatically... Up until now, GRAS works on
-top MSG for historical reasons but I'm going to make it work
-directly on top of SURF so that it can use all the flex and the
-speed provided by SURF.
-
-Those two things are working, but we want to make everything as clean as
-possible before releasing SimGrid v.3.
-
-So what about those nice DAGs we used to have in SimGrid v.1.? They're
-not anymore in SimGrid v.3. At least not in their original form... Let
-me recall you the way SimGrid 3 is organized:
-
-\verbatim
-________________
-| User code |
-|______________|
-| | MSG | GRAS |
-| -------------|
-| | SURF |
-| -------------|
-| XBT |
-----------------
-\endverbatim
-
-XBT is our tool box and now, you should have an idea of what the other
-ones are. As you can see, the primitive SG is not here
-anymore. However we have written a brand new and cleaner API for this
-purpose: \ref SD_API. It is built directly on top of SURF and provides
-an API rather close to the old SG:
-
-\verbatim
-_________________________
-| User code |
-|________________________|
-| | MSG | GRAS | SimDag |
-| -----------------------|
-| | SURF |
-| -----------------------|
-| XBT |
---------------------------
-\endverbatim
-
-The nice thing is that, as it is writen on top of SURF, it seamlessly
-support DAG of parallel tasks as well as complex communications
-patterns. Some old codes using SG are currently under rewrite using
-\ref SD_API to check that all needful functions are provided.
-
\subsection faq_MIA_generic Generic features
\subsubsection faq_more_processes Increasing the amount of simulated processes
