Changeset b0f6190a


Ignore:
Timestamp:
Jan 14, 2021, 12:46:26 PM (8 months ago)
Author:
Thierry Delisle <tdelisle@…>
Branches:
arm-eh, jacob/cs343-translation, master, new-ast-unique-expr
Children:
7222630
Parents:
bb58825 (diff), ec19b21 (diff)
Note: this is a merge changeset, the changes displayed below correspond to the merge itself.
Use the (diff) links above to see all the changes relative to each parent.
Message:

Merge branch 'master' of plg.uwaterloo.ca:software/cfa/cfa-cc

Files:
9 edited

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  • doc/bibliography/pl.bib

    rbb58825 rb0f6190a  
    74357435}
    74367436
     7437@manual{POSIX17,
     7438    keywords    = {POSIX, Standard},
     7439    contributer = {pabuhr@plg},
     7440    key         = {POSIX},
     7441    title       = {1003.1 Standard for Information Technology -- Portable Operating System Interface (POSIX), Base Specifications, Issue 7},
     7442    organization= {IEEE and The Open Group},
     7443    year        = 2017,
     7444    note        = {\href{https://pubs.opengroup.org/onlinepubs/9699919799}{https://\-pubs.opengroup.org/\-onlinepubs/\-9699919799}},
     7445}
     7446
    74377447@inproceedings{ML:NJ,
    74387448    keywords    = {continuations, ML},
  • doc/theses/thierry_delisle_PhD/thesis/Makefile

    rbb58825 rb0f6190a  
    88BibTeX = BIBINPUTS=${TeXLIB} && export BIBINPUTS && bibtex
    99
    10 MAKEFLAGS = --no-print-directory --silent #
     10MAKEFLAGS = --no-print-directory --silent
    1111VPATH = ${Build} ${Figures}
    1212
     
    6666
    6767build/%.dvi : %.tex Makefile | ${Build}
     68        # Conditionally create an empty *.ind (index) file for inclusion until makeindex is run.
     69        if [ ! -r ${basename $@}.ind ] ; then touch ${basename $@}.ind ; fi
    6870        # Must have *.aux file containing citations for bibtex
    6971        if [ ! -r ${basename $@}.aux ] ; then ${LaTeX} $< ; fi
     
    7476        # Make index from *.aux entries and input index at end of document
    7577        -makeglossaries -q -s ${basename $@}.ist ${basename $@}
     78        # Make index from *.aux entries and input index at end of document
     79        -makeindex ${basename $@}.idx
    7680        # Run again to finish citations
    7781        ${LaTeX} $<
  • doc/theses/thierry_delisle_PhD/thesis/text/core.tex

    rbb58825 rb0f6190a  
    11\chapter{Scheduling Core}\label{core}
    22
    3 Before discussing scheduling in general, where it is important to address systems that are changing states, this document discusses scheduling in a somewhat ideal scenerio, where the system has reached a steady state. For this purpose, a steady state is loosely defined as a state where there are always \glspl{thrd} ready to run and the system has the ressources necessary to accomplish the work, \eg, enough workers. In short, the system is neither overloaded nor underloaded.
     3Before discussing scheduling in general, where it is important to address systems that are changing states, this document discusses scheduling in a somewhat ideal scenario, where the system has reached a steady state. For this purpose, a steady state is loosely defined as a state where there are always \glspl{thrd} ready to run and the system has the resources necessary to accomplish the work, \eg, enough workers. In short, the system is neither overloaded nor underloaded.
    44
    5 I believe it is important to discuss the steady state first because it is the easiest case to handle and, relatedly, the case in which the best performance is to be expected. As such, when the system is either overloaded or underloaded, a common approach is to try to adapt the system to the new load and return to the steady state, \eg, adding or removing workers. Flaws in the scheduling when the system is in the steady state can therefore to be pervasive in all states.
     5I believe it is important to discuss the steady state first because it is the easiest case to handle and, relatedly, the case in which the best performance is to be expected. As such, when the system is either overloaded or underloaded, a common approach is to try to adapt the system to this new load and return to the steady state, \eg, by adding or removing workers. Therefore, flaws in scheduling the steady state can to be pervasive in all states.
    66
    77\section{Design Goals}
    8 As with most of the design decisions behind \CFA, an important goal is to match the expectation of the programmer, according to their probable mental model. To match these expectations, the design must offer the programmers sufficient guarantees so that, as long as they respect the mental model, the system will also respect this model.
     8As with most of the design decisions behind \CFA, an important goal is to match the expectation of the programmer according to their execution mental-model. To match expectations, the design must offer the programmer sufficient guarantees so that, as long as they respect the execution mental-model, the system also respects this model.
    99
    10 For threading, a simple and common mental model is the ``Ideal multi-tasking CPU'' :
     10For threading, a simple and common execution mental-model is the ``Ideal multi-tasking CPU'' :
    1111
    1212\begin{displayquote}[Linux CFS\cit{https://www.kernel.org/doc/Documentation/scheduler/sched-design-CFS.txt}]
    1313        {[The]} ``Ideal multi-tasking CPU'' is a (non-existent  :-)) CPU that has 100\% physical power and which can run each task at precise equal speed, in parallel, each at [an equal fraction of the] speed.  For example: if there are 2 tasks running, then it runs each at 50\% physical power --- i.e., actually in parallel.
     14        \label{q:LinuxCFS}
    1415\end{displayquote}
    1516
    1617Applied to threads, this model states that every ready \gls{thrd} immediately runs in parallel with all other ready \glspl{thrd}. While a strict implementation of this model is not feasible, programmers still have expectations about scheduling that come from this model.
    1718
    18 In general, the expectation at the center of this model is that ready \glspl{thrd} do not interfere with eachother but simply share the hardware. This makes it easier to reason about threading because ready \glspl{thrd} can be taken in isolation and the effect of the scheduler can be virtually ignored. This expectation of \gls{thrd} independence means the scheduler is expected to offer two guarantees:
     19In general, the expectation at the center of this model is that ready \glspl{thrd} do not interfere with each other but simply share the hardware. This assumption makes it easier to reason about threading because ready \glspl{thrd} can be thought of in isolation and the effect of the scheduler can be virtually ignored. This expectation of \gls{thrd} independence means the scheduler is expected to offer two guarantees:
    1920\begin{enumerate}
    20         \item A fairness guarantee: a \gls{thrd} that is ready to run will not be prevented to do so by another thread.
    21         \item A performance guarantee: a \gls{thrd} that wants to start or stop running will not be slowed down by other threads wanting to do the same.
     21        \item A fairness guarantee: a \gls{thrd} that is ready to run is not prevented by another thread.
     22        \item A performance guarantee: a \gls{thrd} that wants to start or stop running is not prevented by other threads wanting to do the same.
    2223\end{enumerate}
    2324
    24 It is important to note that these guarantees are expected only up to a point. \Glspl{thrd} that are ready to run should not be prevented to do so, but they still need to share a limited amount of hardware. Therefore, the guarantee is considered respected if a \gls{thrd} gets access to a \emph{fair share} of the hardware, even if that share is very small.
     25It is important to note that these guarantees are expected only up to a point. \Glspl{thrd} that are ready to run should not be prevented to do so, but they still share the limited hardware resources. Therefore, the guarantee is considered respected if a \gls{thrd} gets access to a \emph{fair share} of the hardware resources, even if that share is very small.
    2526
    26 Similarly the performance guarantee, the lack of interferance between threads, is only relevant up to a point. Ideally the cost of running and blocking would be constant regardless of contention, but the guarantee is considered satisfied if the cost is not \emph{too high} with or without contention. How much is an acceptable cost is obviously highly variable. For this document the performance experimentation will attempt to show that the cost of scheduling is at worst equivalent to existing algorithms used in popular languages. This demonstration can be made by comparing application built in \CFA to applications built with other languages or other models. Recall from a few paragraphs ago that the expectation of programmers is that the impact of the scheduler can be ignored. Therefore, if the cost of scheduling is equivalent or lower to other popular languages, I will consider the guarantee achieved.
     27Similarly the performance guarantee, the lack of interference among threads, is only relevant up to a point. Ideally, the cost of running and blocking should be constant regardless of contention, but the guarantee is considered satisfied if the cost is not \emph{too high} with or without contention. How much is an acceptable cost is obviously highly variable. For this document, the performance experimentation attempts to show the cost of scheduling is at worst equivalent to existing algorithms used in popular languages. This demonstration can be made by comparing applications built in \CFA to applications built with other languages or other models. Recall programmer expectation is that the impact of the scheduler can be ignored. Therefore, if the cost of scheduling is equivalent to or lower than other popular languages, I consider the guarantee achieved.
    2728
    2829More precisely the scheduler should be:
    2930\begin{itemize}
    3031        \item As fast as other schedulers that are less fair.
    31         \item Faster than other scheduler that have equal or better fairness.
     32        \item Faster than other schedulers that have equal or better fairness.
    3233\end{itemize}
    3334
    3435\subsection{Fairness vs Scheduler Locality}
    35 An important performance factor in modern architectures is cache locality. Waiting for data not present in the cache can have a major impact on performance, and having multiple \glspl{hthrd} writing to the same cache lines can lead to cache lines that need to be waited on again. It is therefore preferable to divide the data among each \gls{hthrd}\footnote{This can be an explicit division up front or using data structures where different \glspl{hthrd} are naturally routed to different cache lines.}.
     36An important performance factor in modern architectures is cache locality. Waiting for data at lower levels or not present in the cache can have a major impact on performance. Having multiple \glspl{hthrd} writing to the same cache lines also leads to cache lines that must be waited on. It is therefore preferable to divide data among each \gls{hthrd}\footnote{This partitioning can be an explicit division up front or using data structures where different \glspl{hthrd} are naturally routed to different cache lines.}.
    3637
    37 For a scheduler, having good locality\footnote{This section discusses \emph{internal} locality, \ie, the locality of the data used by the scheduler. \emph{External locality}, \ie, how the data used by the application is affected by scheduling, is a much more complicated subject and will be discussed in the chapters on evaluation.}, \ie, having the data be local to each \gls{hthrd}, generally conflicts with fairness. Indeed, good locality often requires avoiding the movement of cache lines, while fairness requires dynamically moving \gls{thrd}, and as a consequence cache lines, to \glspl{hthrd} that are currently more appropriate.
     38For a scheduler, having good locality\footnote{This section discusses \emph{internal locality}, \ie, the locality of the data used by the scheduler versus \emph{external locality}, \ie, how the data used by the application is affected by scheduling. External locality is a much more complicated subject and is discussed in part~\ref{Evaluation} on evaluation.}, \ie, having the data local to each \gls{hthrd}, generally conflicts with fairness. Indeed, good locality often requires avoiding the movement of cache lines, while fairness requires dynamically moving a \gls{thrd}, and as consequence cache lines, to a \gls{hthrd} that is currently available.
    3839
    39 However, I claim that in practice it is possible to strike a balance between fairness and performance because the need for these do not necessarily overlap temporaly. Figure~\ref{fig:fair} shows an visual representation of this behaviour. As mentionned, a little bit of unfairness can be acceptable, therefore it can be desirable to have an algorithm that prioritizes cache locality as long as no threads is left behind for too long.
     40However, I claim that in practice it is possible to strike a balance between fairness and performance because these goals do not necessarily overlap temporally, where Figure~\ref{fig:fair} shows a visual representation of this behaviour. As mentioned, some unfairness is acceptable; therefore it is desirable to have an algorithm that prioritizes cache locality as long as thread delay does not exceed the execution mental-model.
    4041
    4142\begin{figure}
    42         \begin{center}
    43                 \input{fairness.pstex_t}
    44         \end{center}
    45         \caption{Fairness vs Locality}
     43        \centering
     44        \input{fairness.pstex_t}
     45        \vspace*{-10pt}
     46        \caption[Fairness vs Locality graph]{Rule of thumb Fairness vs Locality graph \smallskip\newline The importance of Fairness and Locality while a ready \gls{thrd} awaits running is shown as the time the ready \gls{thrd} waits increases, Ready Time, the chances that its data is still in cache, Locality, decreases. At the same time, the need for fairness increases since other \glspl{thrd} may have the chance to run many times, breaking the fairness model. Since the actual values and curves of this graph can be highly variable, the graph is an idealized representation of the two opposing goals.}
    4647        \label{fig:fair}
    47         Rule of thumb graph: Importance of Fairness and Locality while a ready \gls{thrd} waits run.
    48         As the time a ready \gls{thrd} waits increases, ``Ready Time'', the chances that its data is still in cache decreases. At the same time, the need for fairness increases since other \glspl{thrd} may have the chance to run many times, breaking the fairness model mentionned above. Since the actual values and curves of this graph can be highly variable, the graph is left intentionally fuzzy and innacurate.
    4948\end{figure}
    5049
    5150\section{Design}
    52 A naive strictly \glsxtrshort{fifo} ready-queue does not offer sufficient performance. As shown in the evaluation sections, most production schedulers scale when adding multiple \glspl{hthrd} and that is not possible with a single point of contention. Therefore it is vital to shard the ready-queue so that multiple \glspl{hthrd} can access the ready-queue without performance degradation.
     51In general, a na\"{i}ve \glsxtrshort{fifo} ready-queue does not scale with increased parallelism from \glspl{hthrd}, resulting in decreased performance. The problem is adding/removing \glspl{thrd} is a single point of contention. As shown in the evaluation sections, most production schedulers do scale when adding \glspl{hthrd}. The common solution to the single point of contention is to shard the ready-queue so each \gls{hthrd} can access the ready-queue without contention, increasing performance though lack of contention.
    5352
    5453\subsection{Sharding} \label{sec:sharding}
    55 An interesting approach to sharding a queue is presented in \cit{Trevors paper}. This algorithm represents a queue with relaxed \glsxtrshort{fifo} guarantee using an array of strictly \glsxtrshort{fifo} sublists as shown in Figure~\ref{fig:base}. Each cell of the array contains a linked-list with a lock and each node in these list is marked with a timestamp indicating when they were added to the list. Push operations are done by picking a random cell and attempting to push to its list. If the cell is already locked, the operation is simply retried on a new cell until a lock is acquired. Pop operations are done in a similar fashion except two random cells are picked. If both cells are not already locked and both cells contain non-empty lists, the operation pops the node with the oldest timestamp. If only one of the cell is unlocked and non-empty, the operation pops from that cell. If both cells are either locked or empty, the operation picks two new cells and tries again.
     54An interesting approach to sharding a queue is presented in \cit{Trevors paper}. This algorithm presents a queue with a relaxed \glsxtrshort{fifo} guarantee using an array of strictly \glsxtrshort{fifo} sublists as shown in Figure~\ref{fig:base}. Each \emph{cell} of the array has a timestamp for the last operation and a pointer to a linked-list with a lock and each node in the list is marked with a timestamp indicating when it is added to the list. A push operation is done by picking a random cell, acquiring the list lock, and pushing to the list. If the cell is locked, the operation is simply retried on another random cell until a lock is acquired. A pop operation is done in a similar fashion except two random cells are picked. If both cells are unlocked with non-empty lists, the operation pops the node with the oldest cell timestamp. If one of the cells is unlocked and non-empty, the operation pops from that cell. If both cells are either locked or empty, the operation picks two new random cells and tries again.
    5655
    5756\begin{figure}
    58         \begin{center}
    59                 \input{base.pstex_t}
    60         \end{center}
    61         \caption{Relaxed FIFO list}
     57        \centering
     58        \input{base.pstex_t}
     59        \caption[Relaxed FIFO list]{Relaxed FIFO list \smallskip\newline List at the base of the scheduler: an array of strictly FIFO lists. The timestamp is in all nodes and cell arrays.}
    6260        \label{fig:base}
    63         List at the base of the scheduler: an array of strictly FIFO lists.
    64         The timestamp is in all nodes and cell arrays.
    6561\end{figure}
    6662
    6763\subsection{Finding threads}
    68 Once threads have been distributed onto multiple queues, indentifying which queues are empty and which are not can become a problem. Indeed, if the number of \glspl{thrd} does not far exceed the number of queues, it is probable that several of these queues are empty. Figure~\ref{fig:empty} shows an example with 2 \glspl{thrd} running on 8 queues, where the chances of getting an empty queue is 75\% per pick, meaning two random picks yield a \gls{thrd} only half the time.
    69 
     64Once threads have been distributed onto multiple queues, identifying empty queues becomes a problem. Indeed, if the number of \glspl{thrd} does not far exceed the number of queues, it is probable that several of the cell queues are empty. Figure~\ref{fig:empty} shows an example with 2 \glspl{thrd} running on 8 queues, where the chances of getting an empty queue is 75\% per pick, meaning two random picks yield a \gls{thrd} only half the time. This scenario leads to performance problems since picks that do not yield a \gls{thrd} are not useful and do not necessarily help make more informed guesses.
    7065
    7166\begin{figure}
    72         \begin{center}
    73                 \input{empty.pstex_t}
    74         \end{center}
    75         \caption{``More empty'' Relaxed FIFO list}
     67        \centering
     68        \input{empty.pstex_t}
     69        \caption[``More empty'' Relaxed FIFO list]{``More empty'' Relaxed FIFO list \smallskip\newline Emptier state of the queue: the array contains many empty cells, that is strictly FIFO lists containing no elements.}
    7670        \label{fig:empty}
    77         Emptier state of the queue: the array contains many empty cells, that is strictly FIFO lists containing no elements.
    7871\end{figure}
    7972
    80 This can lead to performance problems since picks that do not yield a \gls{thrd} are not useful and do not necessarily help make more informed guesses.
     73There are several solutions to this problem, but they ultimately all have to encode if a cell has an empty list. My results show the density and locality of this encoding is generally the dominating factor in these scheme. Classic solutions to this problem use one of three techniques to encode the information:
    8174
    82 Solutions to this problem can take many forms, but they ultimately all have to encode where the threads are in some form. My results show that the density and locality of this encoding is generally the dominating factor in these scheme. Classic solutions to this problem use one of three techniques to encode the information:
     75\paragraph{Dense Information} Figure~\ref{fig:emptybit} shows a dense bitmask to identify the cell queues currently in use. This approach means processors can often find \glspl{thrd} in constant time, regardless of how many underlying queues are empty. Furthermore, modern x86 CPUs have extended bit manipulation instructions (BMI2) that allow searching the bitmask with very little overhead compared to the randomized selection approach for a filled ready queue, offering good performance even in cases with many empty inner queues. However, this technique has its limits: with a single word\footnote{Word refers here to however many bits can be written atomically.} bitmask, the total amount of ready-queue sharding is limited to the number of bits in the word. With a multi-word bitmask, this maximum limit can be increased arbitrarily, but the look-up time increases. Finally, a dense bitmap, either single or multi-word, causes additional contention problems that reduces performance because of cache misses after updates. This central update bottleneck also means the information in the bitmask is more often stale before a processor can use it to find an item, \ie mask read says there are available \glspl{thrd} but none on queue when the subsequent atomic check is done.
    8376
    8477\begin{figure}
    85         \begin{center}
    86                 {\resizebox{0.73\textwidth}{!}{\input{emptybit.pstex_t}}}
    87         \end{center}
     78        \centering
    8879        \vspace*{-5pt}
    89         \caption{Underloaded queue with added bitmask to indicate which array cells have items.}
     80        {\resizebox{0.75\textwidth}{!}{\input{emptybit.pstex_t}}}
     81        \vspace*{-5pt}
     82        \caption[Underloaded queue with bitmask]{Underloaded queue with bitmask indicating array cells with items.}
    9083        \label{fig:emptybit}
    91         \begin{center}
    92                 {\resizebox{0.73\textwidth}{!}{\input{emptytree.pstex_t}}}
    93         \end{center}
     84
     85        \vspace*{10pt}
     86        {\resizebox{0.75\textwidth}{!}{\input{emptytree.pstex_t}}}
    9487        \vspace*{-5pt}
    95         \caption{Underloaded queue with added binary search tree indicate which array cells have items.}
     88        \caption[Underloaded queue with binary search-tree]{Underloaded queue with binary search-tree indicating array cells with items.}
    9689        \label{fig:emptytree}
    97         \begin{center}
    98                 {\resizebox{0.9\textwidth}{!}{\input{emptytls.pstex_t}}}
    99         \end{center}
     90
     91        \vspace*{10pt}
     92        {\resizebox{0.95\textwidth}{!}{\input{emptytls.pstex_t}}}
    10093        \vspace*{-5pt}
    101         \caption{Underloaded queue with added per processor bitmask to indicate which array cells have items.}
     94        \caption[Underloaded queue with per processor bitmask]{Underloaded queue with per processor bitmask indicating array cells with items.}
    10295        \label{fig:emptytls}
    10396\end{figure}
    10497
    105 \paragraph{Dense Information} Figure~\ref{fig:emptybit} shows a dense bitmask to identify which inner queues are currently in use. This approach means processors can often find \glspl{thrd} in constant time, regardless of how many underlying queues are empty. Furthermore, modern x86 CPUs have extended bit manipulation instructions (BMI2) that allow using the bitmask with very little overhead compared to the randomized selection approach for a filled ready queue, offering good performance even in cases with many empty inner queues. However, this technique has its limits: with a single word\footnote{Word refers here to however many bits can be written atomically.} bitmask, the total number of underlying queues in the ready queue is limited to the number of bits in the word. With a multi-word bitmask, this maximum limit can be increased arbitrarily, but the look-up will nolonger be constant time. Finally, a dense bitmap, either single or multi-word, causes additional contention problems which reduces performance because of cache misses after updates. This central update bottleneck also means the information in the bitmask is more often stale before a processor can use it to find an item, \ie mask read says there are available \glspl{thrd} but none on queue.
     98\paragraph{Sparse Information} Figure~\ref{fig:emptytree} shows an approach using a hierarchical tree data-structure to reduce contention and has been shown to work in similar cases~\cite{ellen2007snzi}. However, this approach may lead to poorer performance due to the inherent pointer chasing cost while still allowing significant contention on the nodes of the tree if the tree is shallow.
    10699
    107 \paragraph{Sparse Information} Figure~\ref{fig:emptytree} shows an approach using a hierarchical tree data-structure to reduce contention and has been shown to work in similar cases~\cite{ellen2007snzi}. However, this approach may lead to poorer performance due to the inherent pointer chasing cost while still allowing more contention on the nodes of the tree if the tree is not deep enough.
     100\paragraph{Local Information} Figure~\ref{fig:emptytls} shows an approach using dense information, similar to the bitmap, but each \gls{hthrd} keeps its own independent copy. While this approach can offer good scalability \emph{and} low latency, the liveliness and discovery of the information can become a problem. This case is made worst in systems with few processors where even blind random picks can find \glspl{thrd} in a few tries.
    108101
    109 \paragraph{Local Information} Figure~\ref{fig:emptytls} shows an approach using dense information, similar to the bitmap, but have each thread keep its own independent copy of it. While this approach can offer good scalability \emph{and} low latency, the liveliness and discovery of the information can become a problem. This case is made worst in systems with few processors where even blind random picks can find \glspl{thrd} in few tries.
    110 
    111 I built a prototype of these approach and none of these techniques offer satisfying performance when few threads are present. All of these approach hit the same 2 problems. First, blindly picking two sub-queues is very fast which means that any improvement to the hit rate can easily be countered by a slow-down in look-up speed. Second, the array is already as sharded as is needed to avoid contention bottlenecks, so any denser data structure will tend to become a bottleneck. In all cases, these factors meant that the best cases scenerio, many threads, would get worst throughput and the worst case scenario, few threads, would get a better hit rate, but an equivalent throughput. As a result I tried an entirely different approach.
     102I built a prototype of these approaches and none of these techniques offer satisfying performance when few threads are present. All of these approach hit the same 2 problems. First, randomly picking sub-queues is very fast but means any improvement to the hit rate can easily be countered by a slow-down in look-up speed when there are empty lists. Second, the array is already as sharded to avoid contention bottlenecks, so any denser data structure tends to become a bottleneck. In all cases, these factors meant the best cases scenario, \ie many threads, would get worst throughput, and the worst-case scenario, few threads, would get a better hit rate, but an equivalent poor throughput. As a result I tried an entirely different approach.
    112103
    113104\subsection{Dynamic Entropy}\cit{https://xkcd.com/2318/}
    114 In the worst case scenario there are few \glspl{thrd} ready to run, or more accuratly given $P$ \glspl{proc}, $T$ \glspl{thrd} and $\epsilon$, as usual, a very small number, in this case $\epsilon \ll P$, we have $T = P + \epsilon$. An important thing to note is that in this case, fairness is effectively irrelevant. Indeed, this case is close to \emph{actually matching} the model of the ``Ideal multi-tasking CPU'' presented in this chapter\footnote{For simplicity, this assumes there is a one-to-one match between \glspl{proc} and \glspl{hthrd}.}. Therefore, in this context it is possible to use a purely internal locality based approach and still meet the fairness requirements. This approach would simply have each \gls{proc} running a single \gls{thrd} repeatedly. Or from the shared ready-queue viewpoint, each \gls{proc} would push to a given sub-queue and then pop from the \emph{same} subqueue. Ideally, the scheduler would achieve this without affecting the fairness guarantees in cases where $T \gg P$.
     105In the worst-case scenario there are only few \glspl{thrd} ready to run, or more precisely given $P$ \glspl{proc}\footnote{For simplicity, this assumes there is a one-to-one match between \glspl{proc} and \glspl{hthrd}.}, $T$ \glspl{thrd} and $\epsilon$ a very small number, than the worst case scenario can be represented by $\epsilon \ll P$, than $T = P + \epsilon$. It is important to note in this case that fairness is effectively irrelevant. Indeed, this case is close to \emph{actually matching} the model of the ``Ideal multi-tasking CPU'' on page \pageref{q:LinuxCFS}. In this context, it is possible to use a purely internal-locality based approach and still meet the fairness requirements. This approach simply has each \gls{proc} running a single \gls{thrd} repeatedly. Or from the shared ready-queue viewpoint, each \gls{proc} pushes to a given sub-queue and then popes from the \emph{same} subqueue. In cases where $T \gg P$, the scheduler should also achieves similar performance without affecting the fairness guarantees.
    115106
    116 To achieve this, I use a communication channel I have not mentionned yet and which I believe I use in a novel way : the \glsxtrshort{prng}. If the scheduler has a \glsxtrshort{prng} instance per \gls{proc} exclusively used for scheduling, its seed effectively encodes a list of all the accessed subqueues, from the latest to the oldest. The only requirement to achieve this is to be able to ``replay'' the \glsxtrshort{prng} backwards. As it turns out, this is an entirely reasonnable requirement and there already exist \glsxtrshort{prng}s that are fast, compact \emph{and} can be run forward and backwards. Linear congruential generators\cite{wiki:lcg} are an example of \glsxtrshort{prng}s that match these requirements.
     107To handle this case, I use a pseudo random-number generator, \glsxtrshort{prng} in a novel way. When the scheduler uses a \glsxtrshort{prng} instance per \gls{proc} exclusively, the random-number seed effectively starts an encoding that produces a list of all accessed subqueues, from latest to oldest. The novel approach is to be able to ``replay'' the \glsxtrshort{prng} backwards and there exist \glsxtrshort{prng}s that are fast, compact \emph{and} can be run forward and backwards. Linear congruential generators~\cite{wiki:lcg} are an example of \glsxtrshort{prng}s that match these requirements.
    117108
    118 The algorithm works as follows :
     109The algorithm works as follows:
    119110\begin{itemize}
    120111        \item Each \gls{proc} has two \glsxtrshort{prng} instances, $F$ and $B$.
    121         \item Push and Pop operations happen as mentionned in Section~\ref{sec:sharding} with the following exceptions:
     112        \item Push and Pop operations occur as discussed in Section~\ref{sec:sharding} with the following exceptions:
    122113        \begin{itemize}
    123114                \item Push operations use $F$ going forward on each try and on success $F$ is copied into $B$.
     
    126117\end{itemize}
    127118
    128 The main benefit of this technique is that it basically repects the desired properties of Figure~\ref{fig:fair}. When looking for work, \glspl{proc} will look first at the last cells they pushed to, if any, and then move backwards through the cells. As the \glspl{proc} continue looking for work, $F$ moves back and $B$ stays in place. As a result the relation between the two becomes weaker, which means that the probablisitic fairness of the algorithm reverts to normal. Chapter~\ref{proofs} discusses more formally the fairness guarantees of this algorithm.
     119The main benefit of this technique is that it basically respects the desired properties of Figure~\ref{fig:fair}. When looking for work, a \gls{proc} first looks at the last cell they pushed to, if any, and then move backwards through its accessed cells. As the \gls{proc} continues looking for work, $F$ moves backwards and $B$ stays in place. As a result, the relation between the two becomes weaker, which means that the probablisitic fairness of the algorithm reverts to normal. Chapter~\ref{proofs} discusses more formally the fairness guarantees of this algorithm.
    129120
    130121\section{Details}
  • doc/theses/thierry_delisle_PhD/thesis/text/io.tex

    rbb58825 rb0f6190a  
    55Since \glsxtrshort{io} operations are generally handled by the
    66
    7 \subsection{\texttt{epoll}, \texttt{poll} and \texttt{select}}
     7\subsection{\lstinline|epoll|, \lstinline|poll| and \lstinline|select|}
    88
    99\subsection{Linux's AIO}
     
    3333
    3434\subsection{\texttt{io\_uring}}
    35 A very recent addition to Linux, \texttt{io\_uring}\cit{io\_uring} is a framework that aims to solve many of the problems listed with the above mentioned solutions.
     35A very recent addition to Linux, @io_uring@\cit{io\_uring} is a framework that aims to solve many of the problems listed with the above mentioned solutions.
    3636
    3737\subsection{Extra Kernel Threads}\label{io:morethreads}
  • doc/theses/thierry_delisle_PhD/thesis/text/runtime.tex

    rbb58825 rb0f6190a  
    11\chapter{\CFA Runtime}
    2 This chapter offers an overview of the capabilities of the \CFA runtime prior to this work.
     2This chapter presents an overview of the capabilities of the \CFA runtime prior to this thesis work.
    33
    4 Threading in \CFA offers is based on \Gls{uthrding}, where \glspl{thrd} are the representation of a unit of work. As such, \CFA programmers should expect these units to be fairly inexpensive, that is: programmers should be able to create a large number of \glspl{thrd} and switch between \glspl{thrd} liberally without many concerns for performance.
     4\Celeven introduced threading features, such the @_Thread_local@ storage class, and libraries @stdatomic.h@ and @threads.h@. Interestingly, almost a decade after the \Celeven standard, the most recent versions of gcc, clang, and msvc do not support the \Celeven include @threads.h@, indicating no interest in the C11 concurrency approach (possibly because of the recent effort to add concurrency to \CC). While the \Celeven standard does not state a threading model, the historical association with pthreads suggests implementations would adopt kernel-level threading (1:1)~\cite{ThreadModel}, as for \CC. This model uses \glspl{kthrd} to achieve parallelism and concurrency. In this model, every thread of computation maps to an object in the kernel. The kernel then has the responsibility of managing these threads, \eg creating, scheduling, blocking. This also entails that the kernel has a perfect view of every thread executing in the system\footnote{This is not completely true due to primitives like \lstinline|futex|es, which have a significant portion of their logic in user space.}.
    55
    66\section{M:N Threading}\label{prev:model}
    77
    8 C traditionnally uses a 1:1 threading model. This model uses \glspl{kthrd} to achive parallelism and concurrency. In this model, every thread of computation maps to an object in the kernel. The kernel then has the responsibility of managing these threads, \eg creating, scheduling, blocking. This also entails that the kernel has a perfect view of every thread executing in the system\footnote{This is not completly true due to primitives like \texttt{futex}es, which have a significant portion of their logic in user space.}.
     8Threading in \CFA is based on \Gls{uthrding}, where \glspl{thrd} are the representation of a unit of work. As such, \CFA programmers should expect these units to be fairly inexpensive, \ie programmers should be able to create a large number of \glspl{thrd} and switch among \glspl{thrd} liberally without many concerns for performance.
    99
    10 By contrast \CFA uses an M:N threading models, where concurrency is achieved using many user-level threads mapped onto fewer \glspl{kthrd}. The user-level threads have the same semantic meaning as a \glspl{kthrd} in the 1:1 model, they represent an independant thread of execution with it's on stack. The difference is that user-level threads do not have a corresponding object in the kernel, they are handled by the runtime in user space and scheduled onto \glspl{kthrd}, referred to as \glspl{proc} in this document. \Glspl{proc} run a \gls{thrd} until it context switches out, it then choses a different \gls{thrd} to run.
     10The \CFA M:N threading models is implemented using many user-level threads mapped onto fewer \glspl{kthrd}. The user-level threads have the same semantic meaning as a \glspl{kthrd} in the 1:1 model: they represent an independent thread of execution with its own stack. The difference is that user-level threads do not have a corresponding object in the kernel, they are handled by the runtime in user space and scheduled onto \glspl{kthrd}, referred to as \glspl{proc} in this document. \Glspl{proc} run a \gls{thrd} until it context switches out, it then chooses a different \gls{thrd} to run.
    1111
    1212\section{Clusters}
     13\CFA allows the option to group user-level threading, in the form of clusters. Both \glspl{thrd} and \glspl{proc} belong to a specific cluster. \Glspl{thrd} are only be scheduled onto \glspl{proc} in the same cluster and scheduling is done independently of other clusters. Figure~\ref{fig:system} shows an overview of the \CFA runtime, which allows programmers to tightly control parallelism. It also opens the door to handling effects like NUMA, by pining clusters to a specific NUMA node\footnote{This is not currently implemented in \CFA, but the only hurdle left is creating a generic interface for cpu masks.}.
     14
    1315\begin{figure}
    1416        \begin{center}
    1517                \input{system.pstex_t}
    1618        \end{center}
    17         \caption{Overview of the \CFA runtime}
     19        \caption[Overview of the \CFA runtime]{Overview of the \CFA runtime \newline \Glspl{thrd} are scheduled inside a particular cluster, where it only runs on the \glspl{proc} which belong to the cluster. The discrete-event manager, which handles preemption and timeout, is a \gls{kthrd} which lives outside any cluster and does not run \glspl{thrd}.}
    1820        \label{fig:system}
    19         \Glspl{thrd} are scheduled inside a particular cluster, where it only runs on the \glspl{proc} which belong to the cluster. The discrete-event manager, which handles preemption and timeout, is a \gls{kthrd} which lives outside any cluster and does not run \glspl{thrd}.
    2021\end{figure}
    21 \CFA allows the option to group user-level threading, in the form of clusters. Both \glspl{thrd} and \glspl{proc} belong to a specific cluster. \Glspl{thrd} will only be scheduled onto \glspl{proc} in the same cluster and scheduling is done independantly of other clusters. Figure~\ref{fig:system} shows an overview if this system. This allows programmers to control more tightly parallelism. It also opens the door to handling effects like NUMA, by pining clusters to specific NUMA node\footnote{This is not currently implemented in \CFA, but the only hurdle left is creating a generic interface for cpu masks.}.
    2222
    2323\section{Scheduling}
    24 The \CFA runtime was previously using a strictly \glsxtrshort{fifo} ready queue with a single lock. This setup offers perfect fairness in terms of opportunities to run/ However, it offers poor scalability, since the performance of the ready queue can never be improved by adding more \glspl{hthrd}, but the contention can cause significant performance degradation.
     24The \CFA runtime previously used a \glsxtrshort{fifo} ready-queue with a single lock. This setup offers perfect fairness in terms of opportunities to run. However, it offers poor scalability, since the performance of the ready queue can never be improved by adding more \glspl{hthrd} and contention can cause significant performance degradation.
    2525
    2626\section{\glsxtrshort{io}}\label{prev:io}
    27 Prior to this work, the \CFA runtime did not add any particular support for \glsxtrshort{io} operations. \CFA being built on C, this means that, while all the operations available in C are available in \CFA, \glsxtrshort{io} operations are designed for the POSIX threading model\cit{pthreads}. Using these operations in a M:N threading model, when they are built for 1:1 threading, means that operations block \glspl{proc} instead of \glspl{thrd}. While this can work in certain cases, it limits the number of concurrent operations to the number of \glspl{proc} rather than \glspl{thrd}. This also means that deadlocks can occur because all \glspl{proc} are blocked even if at least one \gls{thrd} is ready to run. A simple example of this type of deadlock would be as follows:
     27Prior to this work, the \CFA runtime did not add any particular support for \glsxtrshort{io} operations. %\CFA being built on C, this means that,
     28While all I/O operations available in C are available in \CFA, \glsxtrshort{io} operations are designed for the POSIX threading model~\cite{pthreads}. Using these 1:1 threading operations in an M:N threading model means I/O operations block \glspl{proc} instead of \glspl{thrd}. While this can work in certain cases, it limits the number of concurrent operations to the number of \glspl{proc} rather than \glspl{thrd}. It also means deadlock can occur because all \glspl{proc} are blocked even if at least one \gls{thrd} is ready to run. A simple example of this type of deadlock would be as follows:
     29\begin{quote}
     30Given a simple network program with 2 \glspl{thrd} and a single \gls{proc}, one \gls{thrd} sends network requests to a server and the other \gls{thrd} waits for a response from the server. If the second \gls{thrd} races ahead, it may wait for responses to requests that have not been sent yet. In theory, this should not be a problem, even if the second \gls{thrd} waits, because the first \gls{thrd} is still ready to run and should be able to get CPU time to send the request. With M:N threading, while the first \gls{thrd} is ready, the lone \gls{proc} \emph{cannot} run the first \gls{thrd} if it is blocked in the \glsxtrshort{io} operation of the second \gls{thrd}. If this happen, the system is in a synchronization deadlock\footnote{In this example, the deadlocked could be resolved if the server sends unprompted messages to the client. However, this solution is not general and may not be appropriate even in this simple case.}.
     31\end{quote}
     32Therefore, one of the objective of this work is to introduce \emph{User-Level \glsxtrshort{io}}, like \glslink{uthrding}{User-Level \emph{Threading}} blocks \glspl{thrd} rather than \glspl{proc} when doing \glsxtrshort{io} operations, which entails multiplexing the \glsxtrshort{io} operations of many \glspl{thrd} onto fewer \glspl{proc}. This multiplexing requires that a single \gls{proc} be able to execute multiple I/O operations in parallel. This requirement cannot be done with operations that block \glspl{proc}, \ie \glspl{kthrd}, since the first operation would prevent starting new operations for its blocking duration. Executing I/O operations in parallel requires \emph{asynchronous} \glsxtrshort{io}, sometimes referred to as \emph{non-blocking}, since the \gls{kthrd} does not block.
    2833
    29 Given a simple network program with 2 \glspl{thrd} and a single \gls{proc}, one \gls{thrd} sends network requests to a server and the other \gls{thrd} waits for response from the server. If the second \gls{thrd} races ahead, it may wait for responses to requests that have not been sent yet. In theory, this should not be a problem, even if the second \gls{thrd} waits, the first \gls{thrd} is still ready to run and should just be able to get CPU time and send the request. In practice with M:N threading, while the first \gls{thrd} is ready, the lone \gls{proc} in this example will \emph{not} try to run the first \gls{thrd} if it is blocked in the \glsxtrshort{io} operation of the second \gls{thrd}. If this happen, the system is effectively deadlocked\footnote{In this example, the deadlocked could be resolved if the server sends unprompted messages to the client. However, this solution is not general and may not be appropriate even in this simple case.}.
     34\section{Interoperating with C}
     35While \glsxtrshort{io} operations are the classical example of operations that block \glspl{kthrd}, the non-blocking challenge extends to all blocking system-calls. The POSIX standard states~\cite[\S~2.9.1]{POSIX17}:
     36\begin{quote}
     37All functions defined by this volume of POSIX.1-2017 shall be thread-safe, except that the following functions1 need not be thread-safe. ... (list of 70+ potentially excluded functions)
     38\end{quote}
     39Only UNIX @man@ pages identify whether or not a library function is thread safe, and hence, may block on a pthread lock or system call; hence interoperability with UNIX library functions is a challenge for an M:N threading model.
    3040
    31 One of the objective of this work, is to introduce \emph{User-Level \glsxtrshort{io}} which, as a parallel to \glslink{uthrding}{User-Level \emph{Threading}}, blocks \glspl{thrd} rather than \glspl{proc} when doing \glsxtrshort{io} operations. This entails multiplexing the \glsxtrshort{io} operations of many \glspl{thrd} onto fewer \glspl{proc}. This multiplexing requires that a single \gls{proc} be able to execute multiple operations in parallel. This cannot be done with operations that block \glspl{proc}, \ie \glspl{kthrd}, since the first operation would prevent starting new operations for its duration. Executing operations in parallel requires \emph{asynchronous} \glsxtrshort{io}, sometimes referred to as \emph{non-blocking}, since the \gls{kthrd} is not blocked.
     41Languages like Go and Java, which have strict interoperability with C\cit{JNI, GoLang with C}, can control operations in C by ``sandboxing'' them, \eg a blocking function may be delegated to a \gls{kthrd}. Sandboxing may help towards guaranteeing that the kind of deadlock mentioned above does not occur.
    3242
    33 \section{Interoperating with \texttt{C}}
    34 While \glsxtrshort{io} operations are the classical example of operations that block \glspl{kthrd}, the challenges mentioned in the previous section do not require \glsxtrshort{io} to be involved. These challenges are a product of blocking system calls rather than \glsxtrshort{io}. C offers no tools to identify whether or not a librairy function will lead to a blocking system call. This fact means interoperatability with C becomes a challenge in a M:N threading model.
    35 
    36 Languages like Go and Java, which have strict interoperatability with C\cit{JNI, GoLang with C}, can control operations in C by ``sandboxing'' them. They can, for example, delegate C operations to \glspl{kthrd} that are not \glspl{proc}. Sandboxing may help towards guaranteeing that the deadlocks mentioned in the previous section do not occur.
    37 
    38 As mentioned in Section~\cit{\CFA intro}, \CFA is binary compatible with C and, as such, trivially supports calls to and from C librairies. Furthermore, interoperatability can happen within a single library, through inline code or simply C and \CFA translation units archived together. The fine-grained interoperatability between C and \CFA has two consequences:
     43As mentioned in Section~\ref{intro}, \CFA is binary compatible with C and, as such, must support all C library functions. Furthermore, interoperability can happen at the function-call level, inline code, or C and \CFA translation units linked together. This fine-grained interoperability between C and \CFA has two consequences:
    3944\begin{enumerate}
    40         \item Precisely identifying C calls that could block is difficult.
    41         \item Introducing code where interoperatability occurs could have a significant impact on general performance.
     45        \item Precisely identifying blocking C calls is difficult.
     46        \item Introducing new code can have a significant impact on general performance.
    4247\end{enumerate}
    43 
    44 Because of these consequences, this work does not attempt to ``sandbox'' calls to C. It is possible that conflicting calls to C could lead to deadlocks on \CFA's M:N threading model where they would not in the traditionnal 1:1 threading model. However, I judge that solving this problem in general, in a way that is composable and flexible, is too complex in itself and would add too much work to this thesis. Therefore it is outside the scope of this thesis.
     48Because of these consequences, this work does not attempt to ``sandbox'' calls to C. Therefore, it is possible for an unidentified library calls to block a \gls{kthrd} leading to deadlocks in \CFA's M:N threading model, which would not occur in a traditional 1:1 threading model. Currently, all M:N thread systems interacting with UNIX without sandboxing suffer from this problem but manage to work very well in the majority of applications. Therefore, a complete solution to this problem is outside the scope of this thesis.
  • doc/theses/thierry_delisle_PhD/thesis/thesis.tex

    rbb58825 rb0f6190a  
    120120% although it's supposed to be in both the TeX Live and MikTeX distributions. There are also documentation and
    121121% installation instructions there.
     122\renewcommand*{\glstextformat}[1]{\textsf{#1}}
    122123
    123124\usepackage{csquotes}
     
    125126
    126127% Setting up the page margins...
     128\setlength{\textheight}{9in}\setlength{\topmargin}{-0.45in}\setlength{\headsep}{0.25in}
    127129% uWaterloo thesis requirements specify a minimum of 1 inch (72pt) margin at the
    128130% top, bottom, and outside page edges and a 1.125 in. (81pt) gutter
     
    191193% cfa macros used in the document
    192194\input{common}
     195\CFAStyle                                               % CFA code-style for all languages
     196\lstset{basicstyle=\linespread{0.9}\tt}
    193197
    194198% glossary of terms to use
    195199\input{glossary}
     200\makeindex
    196201
    197202%======================================================================
     
    230235\input{text/io.tex}
    231236\part{Evaluation}
     237\label{Evaluation}
    232238\chapter{Theoretical and Existance Proofs}
    233239\chapter{Micro-Benchmarks}
     
    256262\addcontentsline{toc}{chapter}{\textbf{References}}
    257263
    258 \bibliography{local}
     264\bibliography{local,pl}
    259265% Tip 5: You can create multiple .bib files to organize your references.
    260266% Just list them all in the \bibliogaphy command, separated by commas (no spaces).
     
    301307\printglossary
    302308\cleardoublepage
     309
     310% Index
     311% -----------------------------
     312%\input{thesis.ind}                             % index
     313
    303314\phantomsection
    304315
  • libcfa/src/concurrency/io.cfa

    rbb58825 rb0f6190a  
    749749                __cfaabi_dbg_debug_do(
    750750                        memset(sqe, 0xde, sizeof(*sqe));
    751                         sqe->opcode = IORING_OP_LAST;
     751                        sqe->opcode = (sizeof(opcodes) / sizeof(const char *)) - 1;
    752752                );
    753753
  • libcfa/src/concurrency/io/call.cfa.in

    rbb58825 rb0f6190a  
    231231                sqe->addr = 0;
    232232                sqe->len = 0;
    233                 sqe->accept_flags = 0;
     233                sqe->fsync_flags = 0;
    234234                sqe->__pad2[0] = 0;
    235235                sqe->__pad2[1] = 0;
  • libcfa/src/concurrency/io/setup.cfa

    rbb58825 rb0f6190a  
    401401                        const __u32 num = *sq.num;
    402402                        for( i; num ) {
    403                                 sq.sqes[i].opcode = IORING_OP_LAST;
    404                                 sq.sqes[i].user_data = 3ul64;
     403                                __sqe_clean( &sq.sqes[i] );
    405404                        }
    406405                }
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