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glatard committed Mar 13, 2024
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shared storage. To avoid interrupting ongoing
processing with data management operations, this is accomplished via a separate thread (known as the
``flusher'') that moves data from the caches to long-term storage. Users must inform Sea of files
shared storage. To avoid interrupting ongoing
processing with data management operations, this is accomplished via a separate thread (known as the
``flusher'') that moves data from the caches to long-term storage. Users must inform Sea of files
that need to be persisted to storage within a file called
\texttt{.sea\_flushlist}, and temporary files which can be removed from
cache within a file called the \texttt{.sea\_evictlist}. Both these files
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level of the applications in our experiments. As a result, each application
process was likely attempting to utilize all cores, resulting in longer wait times
with the increase in parallelism. With this unintentional additional wait time that
process was likely attempting to utilize all cores, resulting in longer wait times
with the increase in parallelism. With this unintentional additional wait time that
is added to the applications due to compute contention, the benefits of using
Sea becomes more limited.

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\label{fig:seaneuro:beluga-wflush}
\end{figure*}

We also benchmarked Sea on a production cluster of the Digital Alliance of Canada
shared with other users,
using the same datasets and pipelines as for the controlled cluster.
We also benchmarked Sea on a production cluster of the Digital Alliance of Canada
shared with other users,
using the same datasets and pipelines as for the controlled cluster.
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resulting in Sea and Baseline performing
quite similarly to each other.

Our results with flushing enabled confirmed this theory. When running with
flushing enabled, which had the additional overhead of having to
ensure that all the data was copied to Lustre, we noticed that occasionally, we
obtained very large speedups with both the SPM and AFNI pipelines, suggesting that Lustre
Our results with flushing enabled confirmed this theory. When running with
flushing enabled, which had the additional overhead of having to
ensure that all the data was copied to Lustre, we noticed that occasionally, we
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Although Sea usage did result in performance degradation at times, the magnitude
of the degradation was less than that of possible speedups
obtained. Our controlled experiments demonstrated that using Sea with a
of the degradation was less than that of possible speedups
obtained. Our controlled experiments demonstrated that using Sea with a
degraded Lustre file system can result in average speedups of up to
2.5$\times$. Such results indicate that the risk of a slowdown with Sea are
less important that the potential speedups that can arise.

\subsection{Limitations of using Sea}

Sea's benefit is maximal when it is executed on data-intensive pipelines, that is,
when the data is written at a rate which exceeds the rate at which the
Sea's benefit is maximal when it is executed on data-intensive pipelines, that is,
when the data is written at a rate which exceeds the rate at which the
compute node's page cache can flush results to Lustre. In our experiments,
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our controlled experiments did not showcase the benefits of Sea in a highly
data-intensive scenario. Since HCP is one of the largest fMRI datasets,
the only way to augment data intensiveness of the applications is
our controlled experiments did not showcase the benefits of Sea in a highly
data-intensive scenario. Since HCP is one of the largest fMRI datasets,
the only way to augment data intensiveness of the applications is
through increased parallelism which is limited by the number of available
cores. As a result, we could only demonstrate the speedups brought by Sea when
Lustre performance had been degraded.
cores. As a result, we could only demonstrate the speedups brought by Sea when
Lustre performance had been degraded.


Sea cannot speedup applications that are very compute-intensive, as is the case with the FSL Feat pipeline.
In such cases, the performance bottleneck is the
Sea cannot speedup applications that are very compute-intensive, as is the case with the FSL Feat pipeline.
In such cases, the performance bottleneck is the
CPU time, which Sea does not address. Moreover, depending on the
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%but it's actually more complicated. When dataset increases
\subsection{Predicting speedups}

Our results expose the complexities of performance predictions on HPC clusters. We presented results on a dedicated cluster
where we had
control over the global cluster usage and were able to demonstrate that
significant speedups could be obtained when all the Lustre storage devices were
busy writing other data concurrently. However, even these experiments were
limited in demonstrating potential speedups with Sea as there were many
Our results expose the complexities of performance predictions on HPC clusters. We presented results on a dedicated cluster
where we had
control over the global cluster usage and were able to demonstrate that
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writers, the fMRI preprocessing pipelines had exclusive access to the
cluster. With busy writers, the fMRI preprocessing pipelines were executed
alongside an Apache Spark application that continuously read and wrote approximately
writers, the fMRI preprocessing pipelines had exclusive access to the
cluster. With busy writers, the fMRI preprocessing pipelines were executed
alongside an Apache Spark application that continuously read and wrote approximately
1000$\times$ \SI{617}{\mebi\byte} blocks using 64 threads, with a 5 seconds sleep between reads
and writes. In our experiments, we either had 6 nodes each executing the
Spark application or no busy writers.
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only pipeline configured to prefetch was SPM, as the input data was otherwise
read through a memmap (i.e. only loading necessary portions of the file to memory)
with Lustre. Flushing would normally be necessary with
only pipeline configured to prefetch was SPM, as the input data was otherwise
read through a memmap (i.e. only loading necessary portions of the file to memory)
with Lustre. Flushing would normally be necessary with
preprocessing as the user would always require the output data. To
investigate the impacts of flushing on these pipelines, we performed a
separate set of experiments where AFNI and SPM would flush all data
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alone (Baseline). To ensure that system performance was equivalent between
Sea and our Baseline, we executed Sea and Baseline pipelines together.

All experiments were executed using 1, 8 and 16 processes.
Each process consisted of a single application call processing a single fMRI image.
All experiments were executed using 1, 8 and 16 processes.
Each process consisted of a single application call processing a single fMRI image.

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