Commit 5b574ac1 authored by Jonathan Jenkins's avatar Jonathan Jenkins

remove content now in the getting started guide

parent e8047b75
......@@ -178,7 +178,9 @@ xleftmargin=6ex
This document outlines best practices for developing models in the
CODES/ROSS framework. The reader should already be familiar with ROSS
and discrete event simulation in general; those topics are covered in the primary
ROSS documentation.
ROSS documentation. Additionally, the GETTING\_STARTED file presents a better
introduction/overview to CODES - this guide should be consulted after becoming
familiar with CODES/ROSS.
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The main purpose of this document is to help the reader produce
CODES models in a consistent, modular style so that components can be more
......@@ -270,58 +272,6 @@ headers. If the definitions are placed in a header then it makes it
possible for those event and state structs to be used as an ad-hoc interface
between LPs of different types.
\section{CODES: common utilities}
\subsection{modelnet}
Modelnet is a network abstraction layer for use in CODES models. It provides a
consistent API that can be used to send messages between nodes using a variety
of different network transport models. Note that modelnet requires the use of
the codes-mapping API, described in previous section.
modelnet can be found in the codes-net repository. See the example program for
general usage.
\subsection{lp-io}
% TODO: flesh out further
lp-io is a simple API for storing modest-sized
simulation results (not continuous traces). It handles reverse computation
and avoids doing any disk I/O until the simulation is complete. All data is
written with collective I/O into a unified output directory. lp-io is
mostly useful for cases in which you would like each LP instance to report
statistics, but for scalability and data management reasons those results
should be aggregated into a single file rather than producing a separate
file per LP. It is not recommended that lp-io be used for data intensive,
streaming output.
The API for lp-io can be found in codes/lp-io.h
% TODO: look at ross/IO code and determine how it relates to this.
\subsection{codes-workload generator}
% TODO: fill in further
codes-workload is an abstraction layer for feeding I/O / network
workloads into a simulation. It supports multiple back-ends for generating
I/O and network events; data could come from a trace file, from Darshan, or from a
synthetic description.
This component is under active development right now and not complete yet. If
you are interested in using it, a minimal example of the I/O API can be seen in
the codes-workload-dump utility and in
tests/workload/codes-workload-test-cn-lp.c
The API for the workload generator can be found in codes/codes-(nw-)workload.h.
\subsection{codes\_event\_new}
Defined in codes/codes.h, codes\_event\_new is a small convenience wrapper to
tw\_event\_new that errors out if an event exceeds the global end timestamp for
ROSS. The assumption is that CODES models will normally run to a completion
condition rather than until simulation time runs out, see later section for
more information on this approach.
\section{CODES/ROSS: general tips and tricks}
\subsection{Event magic numbers}
......@@ -375,14 +325,8 @@ steady state system). This isn't necessarily true for other models. Quite
simply, set g\_tw\_ts\_end to an arbitrary large number when running simulations
that have a well-defined end-point in terms of events processed.
\begin{comment} ROSS takes care of this
\subsection{Kicking off a simulation}
\label{sec_kickoff}
TOOD: fill this in. Each LP needs to send an event to itself at the
beginning of the simulation (explain why). We usually skew these with
random numbers to help break ties right off the bat (explain why).
\end{comment}
Within the LP finalize function, do not call tw\_now. The time returned may not
be consistent in the case of an optimistic simulation.
\subsection{Handling non-trivial event dependencies}
......@@ -464,172 +408,6 @@ section(s).
\end{enumerate}
\section{CODES Example Model}
TODO: Standardize the namings for codes configuration, mapping, and model-net.
An example model representing most of the functionality present in CODES is
available in doc/example. In
this scenario, we have a certain number of storage servers, identified
through indices $0,\ldots, n-1$ where each server has a network interface card
(NIC) associated with it. The servers exchange messages with their neighboring
server via their NIC card (i.e., server $i$ pings server $i+1$, rolling over the
index if necessary). When the neighboring server receives the message, it sends
an acknowledgement message to the sending server in response. Upon receiving the
acknowledgement, the sending server issues another message. This process continues until
some number of messages have been sent. For simplicity, it is assumed that each
server has a direct link to its neighbor, and no network congestion occurs due
to concurrent messages being sent.
The model is relatively simple to simulate through the usage of ROSS. There are
two distinct LP types in the simulation: the server and the NIC. Refer to
example.c for data structure definitions. The server LPs
are in charge of issuing/acknowledging the messages, while the NIC LPs
(implemented via CODES's model-net) transmit the data and inform their
corresponding servers upon completion. This LP decomposition strategy is
generally preferred for ROSS-based simulations: have single-purpose, simple LPs
representing logical system components.
In this program, CODES is used in the following four ways: to provide
configuration utilities for the program (example.conf), to logically separate and provide
lookup functionality for multiple LP types, to automate LP placement on KPs/PEs,
and to simplify/modularize the underlying network structure. The \codesconfig{}
API is used for the first use-case, the \codesmapping{} API is used for
the second and third use-cases, and the \codesmodelnet{} API is used for the
fourth use-case. The following sections discuss these while covering necessary
ROSS-specific information.
\subsection{\codesconfig{}}
The configuration format allows categories, and optionally subgroups within the
category, of key-value pairs for configuration. The LPGROUPS category defines
the LP configuration. The PARAMS category is currently used for
\codesmodelnet{} and ROSS-specific parameters. For instance, the
\texttt{message\_size} field defines the maximum event size used in ROSS for
memory management. Of course, user-defined categories can be used as well,
which are used in this case to define the rounds of communication and the size
of each message.
\subsection{\codesmapping{}}
\label{subsec:codes_mapping}
The \codesmapping{} API transparently maps user LPs to global LP IDs and MPI
ranks (Aka ROSS PE's). The LP type and count can be specified through
\codesconfig{}. In this section, we focus on the \codesmapping{} API as well as
configuration. Multiple LP types are specified in a single LP group (there can
also be multiple LP groups in a config file).
In Listing~\ref{snippet2}, there is 1 server LP and 1
\texttt{modelnet\_simplenet} LP type in a group and this combination is repeated
16 time (repetitions="16"). ROSS will assign the LPs to the PEs (PEs is an
abstraction for MPI rank in ROSS) by placing 1 server LP then 1
\texttt{modelnet\_simplenet} LP a total of 16 times. This configuration is
useful if there is heavy communication involved between the server and
\texttt{modelnet\_simplenet} LP types, in which case ROSS will place them on the
same PE so that the communication between server and
\texttt{modelnet\_simplenet} LPs will not involve remote messages.
An important consideration when defining the configuration file is the way
\codesmodelnet{} maps the network-layer LPs (the NICs in this example) and the upper
level LPs (e.g., the servers). Specifically, each NIC is mapped in a one-to-one
manner with the calling LP through the calling LP's group name, repetition
number, and number within the repetition.
After the initialization function calls of ROSS (\texttt{tw\_init}), the
configuration file can be loaded in the example program (see the main function
in example.c). Each LP type must register itself using
\texttt{lp\_type\_register} before setting up the mapping.
The \codesmapping{} API provides ways to query information like number of LPs of
a particular LP types, group to which a LP type belongs, repetitions in the
group (For details see codes-base/codes/codes-mapping.h file). Figure
\ref{snippet3} shows how to setup the \codesmapping{} API with our CODES example
and computes basic information by querying the number of servers in a particular
group.
\subsection{Event Handlers}
In this example, we have two LP types i.e. a server LP and a model-net LP.
Since the servers only send and receive messages to each other, the server LP state
maintains a count of the number of remote messages it has sent and received as
well as the number of local completion messages.
For the server event message, we have four message types KICKOFF, REQ, ACK and
LOCAL. With a KICKOFF event, each LP sends a message to itself to begin the
simulation proper. To avoid event ties, we add a small noise using
codes\_local\_latency. The ``REQ'' message is sent by a server to its
neighboring server and when received, neighboring server sends back a message
of type ``ACK''.
\subsection{\codesmodelnet{}}
\codesmodelnet{} is an abstraction layer that allow models to send messages
across components using different network transports. This is a consistent API
that can send messages across both simple and complex network models without
changing the higher level model code.
In the CODES example, we use \emph{simple-net} as the underlying plug-in for
\codesmodelnet{}. The simple-net parameters are specified by the user in the
example.conf config file and loaded via model\_net\_configure.
\codesmodelnet{} assumes that the caller already knows what LP it wants to
deliver the message to (e.g.\ by using the codes-mapping API) and how large the
simulated message is. It carries two types of events (1) a remote event to be
delivered to a higher level model LP (In the example, the \codesmodelnet{} LPs
carry the remote event to the server LPs) and (2) a local event to be delivered
to the caller once the message has been transmitted from the node (In the
example, a local completion message is delivered to the server LP once the
\codesmodelnet{} LP sends the message).
\subsection{Reverse computation}
ROSS has the capability for optimistic parallel simulation, but instead of
saving the state of each LP, they instead require users to perform \emph{reverse
computation}. That is, while the event messages are themselves preserved (until
the Global Virtual Time (GVT) algorithm renders the messages unneeded), the LP
state is not preserved. Hence, it is up to the simulation developer to provide
functionality to reverse the LP state, given the event to be reversed. ROSS
makes this simpler in that events will always be rolled back in exactly the
order they were applied. Note that ROSS also has both serial and parallel
conservative modes, so reverse computation may not be necessary if the
simulation is not compute- or memory-intensive.
For our example program, recall the ``forward'' event handlers. They perform the
following:
\begin{enumerate}
\item Kickoff: send a message to the peer server, and increment sender LP's
count of sent messages.
\item Request (received from peer server): increment receiver count of
received messages, and send an acknowledgement to the sender.
\item Acknowledgement (received from message receiver): send the next
message to the receiver and increment messages sent count. Set a flag
indicating whether a message has been sent.
\item Local \codesmodelnet{} callback: increment the local model-net
received messages count.
\end{enumerate}
In terms of LP state, the four operations are simply modifying counts. Hence,
the ``reverse'' event handlers need to merely roll back those changes:
\begin{enumerate}
\item Kickoff: decrement sender LP's count of sent messages.
\item Request (received from peer server): decrement receiver count of
received messages.
\item Acknowledgement (received from message receiver): decrement messages
sent count if flag indicating a message has been sent has not been
set.
\item Local \codesmodelnet{} callback: decrement the local model-net
received messages count.
\end{enumerate}
For more complex LP states (such as maintaining queues), reverse event
processing becomes similarly more complex. Other sections of this document
highlight strategies of dealing with those.
Note that ROSS maintains the ``lineage'' of events currently stored, which
enables ROSS to roll back the messages in the order they were originally
processed. This greatly simplifies the reverse computation process: the LP state
when reversing the effects of a particular event is exactly the state that
resulted from processing the event in the first place (of course, unless the
event handlers are buggy).
\section{TODO}
\begin{itemize}
......@@ -686,7 +464,6 @@ event handlers are buggy).
differ only in timestamp (e.g., event to remote -> roll back
-> event to remote)
\end{itemize}
\item don't use tw\_now at finalize - gives inconsistent results
\end{itemize}
\begin{comment} ==== SCRATCH MATERIAL ====
......
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