Merge branch 'pasc_paper' into 'master'

Pasc paper

Please let me know as soon as possible if these changes sufficiently cover the two comments, and if you are OK with the wording.

Your changes from `pasc_paper_final` (!146) have been merged into this branch because I suck at git.

See merge request !147

.gitignore

@@ -49,6 +49,7 @@ theory/latex/swift.pdf
 theory/kernel/kernels.pdf
 theory/kernel/kernel_derivatives.pdf
 theory/kernel/kernel_definitions.pdf
+theory/paper_pasc/pasc_paper.pdf
 
 m4/libtool.m4
 m4/ltoptions.m4

theory/paper_pasc/pasc_paper.tex

@@ -42,52 +42,38 @@
 \title{{\ttlit SWIFT}: Using task-based parallelism, fully asynchronous
 communication, and graph partition-based domain decomposition for
 strong scaling on more than 100\,000 cores.}
-% \title{{\ttlit SWIFT}: A task-based hybrid-parallel strongly scalable code for
-%   particle-based cosmological simulations}
 
 \numberofauthors{6}
   
 \author{
 \alignauthor
-       Main~Author\\
-       \affaddr{Institute}\\
-       \affaddr{Department}\\
-       \affaddr{University}\\
-       \affaddr{City Postal code, Country}\\
-       \email{\footnotesize \url{main.author@university.country}}
-% \alignauthor
-%        Matthieu~Schaller\\
-%        \affaddr{Institute for Computational Cosmology (ICC)}\\
-%        \affaddr{Department of Physics}\\
-%        \affaddr{Durham University}\\
-%        \affaddr{Durham DH1 3LE, UK}\\
-%        \email{\footnotesize \url{matthieu.schaller@durham.ac.uk}}
-% \alignauthor
-%        Pedro~Gonnet\\
-%        \affaddr{School of Engineering and Computing Sciences}\\
-%        \affaddr{Durham University}\\
-%        \affaddr{Durham DH1 3LE, UK}\\
-% \alignauthor
-%        Aidan~B.~G.~Chalk\\
-%        \affaddr{School of Engineering and Computing Sciences}\\
-%        \affaddr{Durham University}\\
-%        \affaddr{Durham DH1 3LE, UK}\\
-% \and
-% \alignauthor
-%        Peter~W.~Draper\\
-%        \affaddr{Institute for Computational Cosmology (ICC)}\\
-%        \affaddr{Department of Physics}\\
-%        \affaddr{Durham University}\\
-%        \affaddr{Durham DH1 3LE, UK}\\
-%        %% \alignauthor
-%        %% Tom Theuns\\
-%        %% \affaddr{Institute for Computational Cosmology}\\
-%        %% \affaddr{Department of Physics}\\
-%        %% \affaddr{Durham University}\\
-%        %% \affaddr{Durham DH1 3LE, UK}      
+       Matthieu~Schaller\\
+       \affaddr{Institute for Computational Cosmology (ICC)}\\
+       \affaddr{Department of Physics}\\
+       \affaddr{Durham University}\\
+       \affaddr{Durham DH1 3LE, UK}\\
+       \email{\footnotesize \url{matthieu.schaller@durham.ac.uk}}
+       \alignauthor
+      Pedro~Gonnet\\
+      \affaddr{School of Engineering and Computing Sciences}\\
+      \affaddr{Durham University}\\
+      \affaddr{Durham DH1 3LE, UK}\\ \vspace{1ex}
+      \affaddr{Google Switzerland GmbH}\\
+      \affaddr{8002 Z\"urich, Switzerland}\\
+\and
+\alignauthor
+       Aidan~B.~G.~Chalk\\
+       \affaddr{School of Engineering and Computing Sciences}\\
+       \affaddr{Durham University}\\
+       \affaddr{Durham DH1 3LE, UK}\\
+\alignauthor
+       Peter~W.~Draper\\
+       \affaddr{Institute for Computational Cosmology (ICC)}\\
+       \affaddr{Department of Physics}\\
+       \affaddr{Durham University}\\
+       \affaddr{Durham DH1 3LE, UK}\\
 }
 
-
 \date{\today}
 
 \maketitle
@@ -356,12 +342,20 @@ SMP~Superscalar \cite{ref:SMPSuperscalar}, OpenMP~3.0 \cite{ref:Duran2009},
 Intel's TBB \cite{ref:Reinders2007}, and, to some extent,
 Charm++ \cite{ref:Kale1993}.
 
-For convenience, and to make experimenting with different scheduling
-techniques easier, we chose to implement our own task scheduler
+Since none of these existing taks-based libraries provided the flexibility
+required to experiment with different scheduling and communication
+techniques, (\swift is an interdisciplinary effort between
+Computer Science and Astrophysics to study not only cosmological
+phenomena, but also novel simulation algorithms and parallel computing techniques)
+we chose to implement our own task scheduler
 in \swift, which has since been back-ported as the general-purpose
 \qs task scheduler \cite{gonnet2013quicksched}.
+In \qs and \swift, task dependencies are specified explicitly,
+as opposed to being implicitly derived from data dependencies,
+allowing us to more easily build complex task hierarchies.
 This also allowed us to extend the scheduler with the concept of
-task conflicts.
+task conflicts and integrate the asynchronous communication
+scheme described further on.
 
 Despite its advantages, and the variety of implementations,
 task-based parallelism is rarely used in
@@ -374,9 +368,19 @@ which is usually not an option for large and complex codebases.
 
 Since we were re-implementing \swift from scratch, this was not an issue.
 The tree-based neighbour-finding described above was replaced with a more
-task-friendly approach as described in \cite{ref:Gonnet2015}.
-Particle interactions are computed within, and between pairs, of
-hierarchical {\em cells} containing one or more particles.
+task-friendly approach as described in \cite{ref:Gonnet2015}, in which
+the domain is first decomposed into a grid of {\em cells} of edge length
+larger or equal to the largest particle radius.
+An initial set of interaction tasks is then defined over all cells and
+pairs of neighbouring cells, such that if two particles are close enough to interact,
+they are either in the same cell or they span a pair of neighbouring cells.
+These initial interaction tasks are then refined by recursively
+splitting cells that contain more than a certain number of particles
+and replacing tasks that span a pair of split cells with tasks
+spanning the neighboring sub-cells.
+The resulting refined set of tasks contains all the cells and pairs of cells
+over which particle interactions must be computed.
+
 The dependencies between the tasks are set following
 equations \eqn{rho}, \eqn{dvdt}, and \eqn{dudt}, i.e.~such that for any cell,
 all the tasks computing the particle densities therein must have
@@ -416,7 +420,10 @@ cores of a shared-memory machine \cite{ref:Gonnet2015}.
   and purple. Although the data for the yellow cell resides on
   Node~2, it is required for some tasks on Node~1, and thus needs
   to be copied over  during
-  the computation using {\tt send}/{\tt recv} tasks (diamond-shaped).}
+  the computation using {\tt send}/{\tt recv} tasks
+  (diamond-shaped). \newline 
+  Figure adapted from \cite{ref:Gonnet2015}.
+  }
 \label{tasks}
 \end{figure}