\chapter{Performance Microbenchmarks}
\label{chap:perf}

The performance of \gls{dsa} has been evaluated in great detail by Reese Kuper et al. in \cite{intel:analysis}. Therefore, we will perform only a limited amount of benchmarks with the purpose of verifying the figures from \cite{intel:analysis} and analysing best practices and restrictions for applying \gls{dsa} to \gls{qdp}. \par

\section{Benchmarking Methodology}

\begin{figure}[h]
    \centering
    \includegraphics[width=0.9\textwidth]{images/nsd-benchmark.pdf}
    \caption{Benchmark Procedure Pseudocode. Timing marked with yellow background. Showing data allocation and the benchmarking loop for a single thread.}
    \label{fig:benchmark-function}
\end{figure}

Benchmarks were conducted on an Intel Xeon Max CPU, system configuration following Section \ref{sec:state:setup-and-config} with exclusive access to the system. As \gls{intel:dml} does not have support for \gls{dsa:dwq}, we ran benchmarks exclusively with access through \gls{dsa:swq}. The application written for the benchmarks can be obtained in source form under \texttt{benchmarks} in the thesis repository \cite{thesis-repo}. With the full source code available we only briefly describe a section of pseudocode, as seen in Figure \ref{fig:benchmark-function}, in the following paragraph. \par

The benchmark performs node setup as described in Section \ref{sec:state:dml} and allocates source and destination memory on the nodes passed in as parameters. To avoid page faults affecting the results, the entire memory regions are written to before the timed part of the benchmark starts. These timings are marked with yellow background in Figure \ref{fig:benchmark-function}. To get accurate results, the benchmark is repeated multiple times. At the beginning of each repetition, all threads running will synchronize by use of a barrier. The behaviour then differs depending on the submission method selected which can be a single submission or a batch of given size. This can be seen in Figure \ref{fig:benchmark-function} at the switch statement for \enquote{mode}. Single submission follows the example given in Section \ref{sec:state:dml}, and we therefore do not go into detail explaining it here. Batch submission works unlike the former. A sequence with specified size is created which tasks are then added to. This sequence is then submitted to the engine similar to the submission of a single descriptor. Further steps then follow the example from Section \ref{sec:state:dml} again. \par

\section{Benchmarks}

In this section we will describe three benchmarks. Each complete with setup information and preview, followed by plots showing the results and detailed analysis. We formulate expectations and contrast them with the observations from our measurements. Where displayed, the slim grey bars represent the standard deviation across iterations. \par

\subsection{Submission Method}
\label{subsec:perf:submitmethod}

With each submission, descriptors must be prepared and sent off to the underlying hardware. This is expected to come with a cost, affecting throughput sizes and submission methods differently. By submitting different sizes and comparing batching and single submission, we will evaluate at which submission method performs best for each tested data size. We expect single submission to perform worse consistently, with a pronounced effect on smaller transfer sizes. This is assumed, as the overhead of a single submission with the \gls{dsa:swq} is incurred for every iteration, while the batch only sees this overhead once for multiple copies. \par

\begin{figure}[h]
    \centering
    \includegraphics[width=0.7\textwidth]{images/plot-opt-submitmethod.pdf}
    \caption{Throughput for different Submission Methods and Sizes. Performing a copy with source and destination being node 0, executed by the DSA on node 0. Observable is the submission cost which affects small transfer sizes differently, as there the completion time is lower.}
    \label{fig:perf-submitmethod}
\end{figure}

In Figure \ref{fig:perf-submitmethod} we conclude that with transfers of 1 MiB and upwards, the submission method makes no noticeable difference. For smaller transfers the performance varies greatly, with batch operations leading in throughput. This is aligned with the finding that \enquote{SWQ observes lower throughput between 1-8 KB [transfer size]} \cite[p. 6 and 7]{intel:analysis} for normal submission method. \par

Another limitation may be observed in this result, namely the inherent throughput limit per \gls{dsa} chip of close to 30 GiB/s. This is apparently caused by I/O fabric limitations \cite[p. 5]{intel:analysis}. We therefore conclude, that the use of multiple \gls{dsa} is required to fully utilize the available bandwidth of \gls{hbm} which theoretically lies at 256 GB/s \cite[Table I]{hbm-arch-paper}. \par

\subsection{Multithreaded Submission}
\label{subsec:perf:mtsubmit}

As we might encounter access to one \gls{dsa} from multiple threads through the \glsentrylong{dsa:swq} associated with it, determining the effect this type of access has is important. We benchmark multithreaded submission for one, two and twelve threads. The number twelve representing the core count of one node on the test system. Each configuration gets the same 120 copy tasks split across the available threads, all submitting to one \gls{dsa}. We perform this with sizes of 1 MiB and 1 GiB to see, if the behaviour changes with submission size. As for smaller sizes, the completion time may be faster than submission time. Therefore, smaller task sizes may see different effects of threading due to the fact that multiple threads can work to fill the queue and ensure there is never a stall due it being empty. On the other hand, we might experience lower-than-peak throughput with rising thread count, caused by the synchronization inherent with \gls{dsa:swq}. \par

\begin{figure}[h]
    \centering
    \includegraphics[width=0.7\textwidth]{images/plot-perf-mtsubmit.pdf}
    \caption{Throughput for different Thread Counts and Sizes. Multiple threads submit to the same Shared Work Queue. Performing a copy with source and destination being node 0, executed by the DSA on node 0.}
    \label{fig:perf-mtsubmit}
\end{figure}

In Figure \ref{fig:perf-mtsubmit} we see that threading has no negative impact. The synchronization therefore affects single threaded access in the same way it does for multiple. For the smaller size of 1 MiB our assumption came true and performance actually increased with threads which we attribute to queue usage. We did not find an explanation for the speed difference between sizes, which goes against the rationale that higher transfer size would lead to less impact of submission time and therefore higher throughput. \par

\subsection{Data Movement from DDR to HBM}
\label{subsec:perf:datacopy}

Moving data from DDR to \gls{hbm} is most relevant to the rest of this work, as it is the target application. As we discovered in Section \ref{subsec:perf:submitmethod}, one \gls{dsa} has a peak bandwidth limit of 30 GiB/s. We write to \gls{hbm} with its theoretical peak of 256 GB/s \cite[Table I]{hbm-arch-paper}. Our top speed is therefore limited by the slower main memory. For each node, the test system is configured with two DIMMs of DDR5-4800. We calculate the theoretical throughput as follows: \(2\ DIMMs * \frac{4800\ Megatransfers}{Second\ and\ DIMM} * \frac{64-bit width}{8 bits/byte} = 76800\ MT/s = 75\ GiB/s\)\todo{how to verify this calculation with a citation? is it even correct because we see close to 100 GiB/s throughput?}. We conclude that to achieve closer-to-peak speeds, a copy task has to be split across multiple \gls{dsa}. \par

Two methods of splitting will be evaluated. The first being a brute force approach, utilizing all available for any transfer direction. The seconds' behaviour depends on the data source and destination locations. As our system has multiple sockets, communication crossing the socket could experience a latency and bandwidth disadvantage \todo{cite this}. We argue that for intra-socket transfers, use of the \gls{dsa} from the second socket will have only marginal effect. For transfers crossing the socket, every \gls{dsa} chip will perform badly, and we choose to only use the ones with the fastest possible access, namely the ones on the destination and source node. This might result in lower performance but also uses one fourth of the engines of the brute force approach for inter-socket and half for intra-socket. This gives other threads the chance to utilize these now-free chips. \par

\begin{figure}[h]
    \centering
    \includegraphics[width=0.9\textwidth]{images/xeonmax-numa-layout.png}
    \caption{Xeon Max NUMA-Node Layout \cite[Fig. 14]{intel:maxtuning} for a 2-Socket System when configured with HBM-Flat. Showing separate Node IDs for manual HBM access and for Cores and DDR memory.}
    \label{fig:perf-xeonmaxnuma}
\end{figure}

For this benchmark, we copy 1 Gibibyte of data from node 0 to the destination node, using the previously described submission method. For each node utilized, we spawn one thread pinned to it, in charge of submission. We display data for nodes 8, 11, 12 and 15. To understand the selection, we refer to Figure \ref{fig:perf-xeonmaxnuma} which shows the configured systems' node ids and the storage technology they represent. Node 8 accesses the \gls{hbm} on node 0 and is therefore the physically closest destination possible. Node 11 is located diagonally on the chip and therefore the furthest intra-socket operation benchmarked. Node 12 and 15 lie diagonally on the second sockets CPU and should therefore be representative of inter-socket transfer operations. \par

\begin{figure}[h]
    \centering
    \includegraphics[width=0.7\textwidth]{images/plot-perf-allnodes-throughput-selectbarplot.pdf}
    \caption{Throughput for brute force copy from DDR to HBM. Using all available DSA. Copying 1 GiB from Node 0 to the Destination Node specified on the x-axis. Shows peak achievable with DSA.}
    \label{fig:perf-peak-brute}
\end{figure}

\begin{figure}[h]
    \centering
    \includegraphics[width=0.7\textwidth]{images/plot-perf-smart-throughput-selectbarplot.pdf}
    \caption{Throughput for smart copy from DDR to HBM. Using four on-socket DSA for intra-socket operation and the DSA on source and destination node for inter-socket. Copying 1 GiB from Node 0 to the Destination Node specified on the x-axis. Shows conservative performance.}
    \label{fig:perf-peak-smart}
\end{figure}

From the brute force approach in Figure \ref{fig:perf-peak-brute}, we observe peak speeds of 96 GiB/s when copying across the socket from Node 0 to Node 15. This invalidates our assumption, that peak bandwidth would be achieved in the intra-socket scenario \todo{find out why maybe?} and goes against the calculated peak throughput of the memory on our system \todo{calculation wrong? other factors?}. The results however, align with findings in \cite[Fig. 10]{intel:analysis}. \par

While using significantly more resources, the brute force copy shown in Figure \ref{fig:perf-peak-brute} outperforms the smart approach from Figure \ref{fig:perf-peak-smart}. We observe an increase in transfer speed by utilizing all available \gls{dsa} of 2 GiB/s for copy to Node 8, 18 GiB/s for Node 11 and 12 and 30 GiB/s for Node 15. The smart approach could accommodate another intra-socket copy on the second socket, we assume without seeing negative impact. From this we conclude that the smart copy assignment is worth to use, as it provides better scalability. \par

\subsection{Data Movement using CPU}

For evaluating CPU copy performance two approaches were selected. For Figure \ref{fig:perf-cpu-peak-smart} we used the smart copy procedure as in \ref{subsec:perf:datacopy}, just selecting the software path, with no other changes to the test. In Figure \ref{fig:perf-cpu-peak-brute} the test was adapted to spawn 12 threads on Node 0. \par

\begin{figure}[h]
    \centering
    \includegraphics[width=0.7\textwidth]{images/plot-perf-brute-cpu-throughput-selectbarplot.pdf}
    \caption{Throughput from DDR to HBM on CPU. Using 12 Threads spawned on Node 0 for the task. Copying 1 GiB from Node 0 to the Destination Node specified on the x-axis.}
    \label{fig:perf-cpu-peak-brute}
\end{figure}

The benchmark resulting in Figure \ref{fig:perf-cpu-peak-brute} fully utilizes node 0 of the test system through spawning 12 threads. We attribute the low performance of the inter-socket copy operation to overhead of the interconnect. The slight difference between node 12 and 15 may be explained using Figure \ref{fig:perf-xeonmaxnuma}, where we observe that physically node 12 is closer to node 0 than node 15. \par

\begin{figure}[h]
    \centering
    \includegraphics[width=0.7\textwidth]{images/plot-perf-smart-cpu-throughput-selectbarplot.pdf}
    \caption{Throughput from DDR to HBM on CPU. Using the exact same code as smart copy, therefore spawning one thread on Node 0, 1, 2 and 3. Copying 1 GiB from Node 0 to the Destination Node specified on the x-axis. This shows the low performance of software path, when not adapting the code.}
    \label{fig:perf-cpu-peak-smart}
\end{figure}

Comparing the results in Figure \ref{fig:perf-cpu-peak-smart} with the data from the adapted test displayed in Figure \ref{fig:perf-cpu-peak-brute} we conclude that the software path just serves compatibility. The throughput is comparatively low, requiring code changes to increase performance. This leads us not to recommend using it. \par

\section{Analysis}

In this section we summarize the conclusions drawn from the three benchmarks performed in the sections above and outline a utilization guideline. We also compare CPU and \gls{dsa} for the task of copying data from DDR to \gls{hbm}. \par

\begin{enumerate}
    \item From \ref{subsec:perf:submitmethod} we conclude that small copies under 1 MiB in size require batching and still do not reach peak performance. Datum size should therefore be at or above 1 MiB if possible.
    \item Subsection \ref{subsec:perf:mtsubmit} assures that access from multiple threads does not negatively affect the performance when using \glsentrylong{dsa:swq} for work submission.
    \item In \ref{subsec:perf:datacopy}, we chose to use the presented smart copy methodology to split copy tasks across multiple \gls{dsa} chips to achieve low utilization with acceptable performance.
\end{enumerate}

We again refer to Figures \ref{fig:perf-peak-brute} and \ref{fig:perf-cpu-peak-brute} which both represent the maximum throughput achieved with utilization of either \gls{dsa} for the former and CPU for the latter. Noticeably, the \gls{dsa} does not seem to suffer from the inter-socket overhead like the CPU. To the contrary, we do see the highest throughput when copying across sockets \todo{explanation?}. In any case, \gls{dsa} outperforms the CPU for transfer sizes over 1 MiB, demonstrating its potential to increase throughput while at the same time freeing CPU cycles. \par

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