Commit 189416cb authored by Pedro Gonnet's avatar Pedro Gonnet
Browse files

latest modifications to paper.


Former-commit-id: e53686ef167cd1dc61e4fa4a50fff34963ea51e8
parent 0c82d783
......@@ -99,7 +99,7 @@ A new framework for the parallelization of Smoothed Particle Hydrodynamics (SPH)
simulations on shared-memory parallel architectures is described.
This framework relies on fast and cache-efficient cell-based neighbour-finding
algorithms, as well as task-based parallelism to achieve good scaling and
parallel efficiency on mult-core computers.
parallel efficiency on multi-core computers.
\end{abstract}
......@@ -497,7 +497,7 @@ This reduces the \oh{n\log{n}} sorting to \oh{n} for merging.
The arguably most well-known paradigm for shared-memory,
or thread-based parallelism, is OpenMP, in which
compiler annotations are used to describe if and when
specific loops or portions of the code can be execuded
specific loops or portions of the code can be executed
in parallel.
When such a parallel section, e.g.~a parallel loop, is
encountered, the sections of the loop are split statically
......@@ -517,7 +517,7 @@ is inherently parallelisable.
One such approach is {\em task-based parallelism}, in which the
computation is divided into a number of inter-dependent
computational tasks, which are then scheduled, concurrently
and aysnchronously, to a number of processors.
and asynchronously, to a number of processors.
In order to ensure that the tasks are executed in the right
order, e.g.~that data needed by one task is only used once it
has been produced by another task, and that no two tasks
......@@ -564,7 +564,7 @@ for a given cell, and, in turn, all force computations involving
that cell depend on its ghost task.
Using this mechanism, we can enforce that all density computations
for a set of particles have completed before we use this
density in the force computaitons.
density in the force computations.
The dependencies and conflicts between tasks are then given as follows:
......@@ -634,7 +634,7 @@ The dependencies and conflicts between tasks are then given as follows:
If the dependencies and conflicts are defined correctly, then
there is no risk of concurrency problems and thus each task
can be implemented without special attention to the latter,
e.g.~it can update data without using exclusinve access barriers
e.g.~it can update data without using exclusive access barriers
or atomic memory updates.
This, however, requires some care in how the individual tasks
are allocated to the computing threads, i.e.~each task should
......@@ -660,7 +660,7 @@ in the queue.
The {\tt pthread\_mutex\_t lock} is used to guarantee exclusive access
to the queue.
Task IDs are retreived from the queue as follows:
Task IDs are retrieved from the queue as follows:
\begin{center}\begin{minipage}{0.8\textwidth}
\begin{lstlisting}
......@@ -699,11 +699,11 @@ The lock on the queue is then released (line~12) and
the task ID, or {\tt -1} if no available task was found, is
returned.
The advantage of swapping the retreived task to the next
The advantage of swapping the retrieved task to the next
position in the list is that if the queue is reset, e.g.~{\tt next}
is set to zero, and used again with the same set of tasks,
they will now be traversed in the order in which they were
exectuted in the previous run.
executed in the previous run.
This provides a basic form of iterative refinement of the task
order.
The tasks can also be sorted topologically, according to their
......@@ -718,14 +718,14 @@ a large number of threads.
One way of avoiding this problem is to use several concurrent
queues, e.g.~one queue per thread, and spread the tasks over
all queues.
A fixed assignemnt of tasks to queues can, however,
A fixed assignment of tasks to queues can, however,
cause load balancing problems, e.g.~when a thread's queue is
empty before the others have finished.
In order to avoid such problems, {\em work-stealing} can be used:
If a thread cannot obtain a task from its own queue, it picks
another queue at random and tries to {\em steal} a task from it
i.e. if it can obtain a task, it removes it from the queue and
adds it to it's own queue, thus iteratively rebalancing
adds it to it's own queue, thus iteratively re-balancing
the task queues if they are used repeatedly:
\begin{center}\begin{minipage}{0.8\textwidth}
......@@ -821,7 +821,7 @@ void cell_unlocktree ( struct cell c ) {
are ``locked'' while the cells marked in yellow have a ``hold'' count
larger than zero.
The hold count is shown inside each cell and corresponds to the number
of locked cells hierarchicaly below it.
of locked cells hierarchically below it.
All cells except for those locked or with a ``hold'' count larger than
zero can still be locked without causing concurrent data access.
}
......@@ -871,13 +871,30 @@ void cell_unlocktree ( struct cell c ) {
\begin{itemize}
\item Scaling for both simulations on different parallel hardware.
\item Compare, if possible, with {\sc gadget}.
\item Results for a 1.8M particle simulation on a 32-core Intel Xeon X7550
are shown in \fig{Results}.
\item The new simulation code not only scales much better, e.g. achieving
a parallel efficiency of 63\% at 32 cores.
\end{itemize}
\begin{figure}[ht]
\centerline{\epsfig{file=figures/scaling.pdf,width=0.9\textwidth}}
\caption{Parallel scaling and efficiency for Gadget-2 and GadgetSMP
for a 1.8M particle simulation.
The numbers in the scaling plot are the average number of miliseconds
per simulation time step.
Note that not only does GadgetSMP scale better, it is also up to nine
times faster.
The timings for Gadget-2 are courtesy of Matthieu Schaller of the
Institute of Computational Cosmology at Durham University.}
\label{fig:Results}
\end{figure}
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