abstract algorithms in the domains of Cloud, P2P, HPC, IoT, and similar
settings.
-Currently (v3.21), S4U is definitely the way to go for long-term
+Since v3.20 (June 2018), S4U is definitely the way to go for long-term
projects. It is feature complete, but may still evolve slightly in the
future releases. It can already be used to do everything that can be
done in SimGrid, but you may have to adapt your code in future
.. |API_s4u_Mailbox| replace:: **Mailbox**
+.. |API_s4u_Mailboxes| replace:: **Mailboxes**
+.. _API_s4u_Mailboxes: #s4u-mailbox
+
.. |API_s4u_NetZone| replace:: **NetZone**
.. |API_s4u_Barrier| replace:: **Barrier**
.. todo:: write this section
+.. _s4u_mailbox:
+
+Mailboxes
+*********
+
+Please also refer to the :ref:`API reference for s4u::Mailbox
+<API_s4u_Mailbox>`.
+
+===================
+What are Mailboxes?
+===================
+
+|API_s4u_Mailboxes|_ are rendez-vous points for network communications,
+similar to URLs on which you could post and retrieve data. Actually,
+the mailboxes are not involved in the communication once it starts,
+but only to find the contact with which you want to communicate.
+
+They are similar to many common things: The phone number, which allows
+the caller to find the receiver. The twitter hashtag, which help
+senders and receivers to find each others. In TCP, the pair
+``{host name, host port}`` to which you can connect to find your peer.
+In HTTP, URLs through which the clients can connect to the servers.
+In ZeroMQ, the queues are used to match senders and receivers.
+
+One big difference with most of these systems is that no actor is the
+exclusive owner of a mailbox, neither in sending nor in receiving.
+Many actors can send into and/or receive from the same mailbox. TCP
+socket ports for example are shared on the sender side but exclusive
+on the receiver side (only one process can receive from a given socket
+at a given point of time).
+
+A big difference with TCP sockets or MPI communications is that
+communications do not start right away after a
+:cpp:func:`Mailbox::put() <simgrid::s4u::Mailbox::put()>`, but wait
+for the corresponding :cpp:func:`Mailbox::get() <simgrid::s4u::Mailbox::get()>`.
+You can change this by :ref:`declaring a receiving actor <s4u_receiving_actor>`.
+
+A big difference with twitter hashtags is that SimGrid does not
+offer easy support to broadcast a given message to many
+receivers. So that would be like a twitter tag where each message
+is consumed by the first receiver.
+
+A big difference with the ZeroMQ queues is that you cannot filter
+on the data you want to get from the mailbox. To model such settings
+in SimGrid, you'd have one mailbox per potential topic, and subscribe
+to each topic individually with a
+:cpp:func:`get_async() <simgrid::s4u::Mailbox::get_async()>` on each mailbox.
+Then, use :cpp:func:`Comm::wait_any() <simgrid::s4u::Comm::wait_any()>`
+to get the first message on any of the mailbox you are subscribed onto.
+
+The mailboxes are not located on the network, and you can access
+them without any latency. The network delay are only related to the
+location of the sender and receiver once the match between them is
+done on the mailbox. This is just like the phone number that you
+can use locally, and the geographical distance only comes into play
+once you start the communication by dialing this number.
+
+=====================
+How to use Mailboxes?
+=====================
+
+You can retrieve any existing mailbox from its name (which is a
+unique string, just like a twitter tag). This results in a
+versatile mechanism that can be used to build many different
+situations.
+
+To model classical socket communications, use "hostname:port" as
+mailbox names, and make sure that only one actor reads into a given
+mailbox. This does not make it easy to build a perfectly realistic
+model of the TCP sockets, but in most cases, this system is too
+cumbersome for your simulations anyway. You probably want something
+simpler, that turns our to be easy to build with the mailboxes.
+
+Many SimGrid examples use a sort of yellow page system where the
+mailbox names are the name of the service (such as "worker",
+"master" or "reducer"). That way, you don't have to know where your
+peer is located to contact it. You don't even need its name. Its
+function is enough for that. This also gives you some sort of load
+balancing for free if more than one actor pulls from the mailbox:
+the first actor that can deal with the request will handle it.
+
+=========================================
+How put() and get() Requests are Matched?
+=========================================
+
+The matching algorithm simple: first come, first serve. When a new
+send arrives, it matches the oldest enqueued receive. If no receive is
+currently enqueued, then the incoming send is enqueued. As you can
+see, the mailbox cannot contain both send and receive requests: all
+enqueued requests must be of the same sort.
+
+.. _s4u_receiving_actor:
+
+===========================
+Declaring a Receiving Actor
+===========================
+
+The last twist is that by default in the simulator, the data starts
+to be exchanged only when both the sender and the receiver are
+declared (it waits until both :cpp:func:`put() <simgrid::s4u::Mailbox::put()>`
+and :cpp:func:`get() <simgrid::s4u::Mailbox::get()>` are posted).
+In TCP, since you establish connexions beforehand, the data starts to
+flow as soon as the sender posts it, even if the receiver did not post
+its :cpp:func:`recv() <simgrid::s4u::Mailbox::recv()>` yet.
+
+To model this in SimGrid, you can declare a specific receiver to a
+given mailbox (with the function
+:cpp:func:`set_receiver() <simgrid::s4u::Mailbox::set_receiver()>`).
+That way, any :cpp:func:`put() <simgrid::s4u::Mailbox::put()>`
+posted to that mailbox will start as soon as possible, and the data
+will already be there on the receiver host when the receiver actor
+posts its :cpp:func:`get() <simgrid::s4u::Mailbox::get()>`
+
Memory Management
*****************
API Reference
*************
+.. _API_s4u_this_actor:
+
+=========================
+namespace s4u::this_actor
+=========================
+
+.. doxygennamespace:: simgrid::s4u::this_actor
.. _API_s4u_Activity:
=============
s4u::Mailbox
============
+Please also refer to the :ref:`full doc on s4u::Mailbox <s4u_mailbox>`.
+
.. doxygentypedef:: MailboxPtr
.. doxygenclass:: simgrid::s4u::Mailbox
:protected-members:
:undoc-members:
-.. _API_s4u_this_actor:
-
-=========================
-namespace s4u::this_actor
-=========================
-
-
-.. doxygennamespace:: simgrid::s4u::this_actor
