unnamed_ba_thesis/tex/misc/viva_slides.tex

\documentclass{beamer}

\title{Analysis of Software-Maintained Cache Coherency in ARMv8-A for Cross-Architectural DSM Systems: The Quintessential}
\author{Zhengyi Chen}
\date{\today}

\begin{document}
\frame{\titlepage}

\begin{frame}
    \frametitle{Why study this, specifically?}
    \begin{itemize}
        \item {
            Amir Noohi (Prof. Barbalace's PhD student) currently works on in-kernel distributed shared memory system implemented over RDMA.
        }
        \item {
            We agree that cache coherency implementation and overhead is an important consideration for such a system to be cross-architectural (e.g., x86 vs. ARM64).
        }
        \item {
            Unlike x86, ARM64 (as well as e.g. RISC-V) does not guarantee hardware cache coherence:
            \begin{itemize}
                \item {
                    ARM's ``SystemReady'' program requires such CPU/SoCs to be cache coherent at hardware level.
                }
                \item {
                    However, to my knowledge this is not the case across all ARM SoCs, especially as ARM64 PCs are becoming more prevalent.
                }
            \end{itemize}
        }
    \end{itemize}
\end{frame}

\begin{frame}
    \frametitle{Contributions}
    In this paper, I:
    \begin{itemize}
        \item {
            Identified and exposed the in-Linux-kernel cache coherence maintenance implementation for ARM64 architecture (as used by e.g. RDMA drivers);
        }
        \item {
            Wrote a kernel module to perform performance tests on exposed mechanism, over both virtualized and server setups.
        }
    \end{itemize}
\end{frame}

\begin{frame}
    \frametitle{Cache Coherence in ARM64 \& Linux Kernel}
    \begin{itemize}
        \item {
            ARMv8-A/R defines \textit{Point-of-Coherence} (\textit{PoC}) as the point at which all observers of memory will observe the same copy of a memory location.
        }
        \item {
            In Linux kernel (v6.7.0), this was in turn implemented as assembly macro \texttt{dcache\_[clean|inval]\_poc}, inside \textit{arch/arm64/mm/cache.S}.
            \begin{itemize}
                \item {
                    This was then called by Linux kernel's DMA API, e.g. \texttt{dma\_sync\_single\_for\_cpu}.
                }
            \end{itemize}
        }
        \item {
            For testing purposes, expose the assembly macro inside a C function symbol wrapper for dynamic ftrace support.
        }
    \end{itemize}
\end{frame}

\begin{frame}
    \frametitle{Experiment Setup: Kernel Module}
    \begin{itemize}
        \item {
            A simple kernel module for shared memory is written to test the latency of the exposed assembly macro via ftrace/bpf.
        }
        \item {
            A character device is exposed to userspace with \texttt{mmap} support.
            \begin{itemize}
                \item {
                    Allocation size on driver end can be adjusted dynamically for testing cache coherence latency over variable-sized contiguous allocation.
                }
                \item {
                    Userspace programs can then adjust size of \texttt{mmap} for testing cache coherence latency over non-contiguous allocations.
                }
            \end{itemize}
        }
        \item {
            On \texttt{.close} (e.g. on \texttt{munmap} by userspace), allocations are flushed for \textit{PoC} coherency\dots
            \begin{itemize}
                \item As is the case for DMA memory, described prior.
            \end{itemize}
        }
    \end{itemize}
\end{frame}

\begin{frame}
    \frametitle{Experiment Setup: Testbench}
    \begin{itemize}
        \item {
            Tests conducted mostly on QEMU virt-8.0 platform on x86 host.
        }
        \item {
            Some tests conducted on \textit{Ampere Altra} server system\dots
            \begin{itemize}
                \item {
                    However, \textit{Ampere Altra} is \textit{SystemReady}-certified -- i.e., supports hardware level cache coherence.
                }
                \item {
                    Latencies collected on this system hence may be non-representative of real performance.
                }
            \end{itemize}
        }
        \item {
            Ideally wider range of test setups should be explored beyond the contributions of this paper.
        }
    \end{itemize}
\end{frame}

\begin{frame}
    \frametitle{Results: Summary}
    \begin{itemize}
        \item {
            Constant allocation size, variable \texttt{mmap} size: no significant difference in per-contiguous-memory-area cache coherence latency.
        }
        \item {
            Variable allocation size:
            \begin{itemize}
                \item {
                    Latency \textbf{does not} grow linearly with increase in contiguous allocation size.
                }
                \item {
                    In general, latency remains within the same order-of-magnitude up to $2^6$ contiguous pages.
                }
                \item {
                    For larger contiguous pages, latency due to cache coherence may be amortized by less allocations and page fault handlings required (implementation-specific?).
                }
            \end{itemize}
        }
        \item {
            In general, a hypothetical DSM system should prefer using larger contiguous allocations (which seems logical).
        }
    \end{itemize}
\end{frame}

\end{document}