Merge branch 'arrays' of gitlab.inria.fr:gmevel/cosmo into arrays

papers/icfp2021/lang.tex

@@ -42,5 +42,8 @@ $\CAS\loc{\val_1}{\val_2}$ performs compare-and-set at offset~zero.
 
 % JH : TODO :
 % - ne serait-il pas pertinent de mettre un at aussi pour le CAS des tableaux ?
-% - en OCaml, c'est <- pour les tableaux et := pour les référence. Pourquoi fait-on l'inverse ici ?
+%     GLEN: le problème est qu’on ne peut pas faire l’analogue pour CAS sur une
+%           référence, "CAS e_at e e" ou "CAS !at e e e" sont trop bizarres.
+%           par contre on pourrait écrire "CAS_at e e e"…
 % - Il y a une ambiguïté dans la syntaxe des CAS pour les tableaux "CAS a[b] c d" peut à la fois parler du CAS dans un tableau mais aussi du CAS d'une référence stockée dans un tableau
+%     GLEN: cette ambiguïté existe seulement si CAS a une annotation "at". :-)

papers/icfp2021/notations.tex

@@ -204,7 +204,7 @@
 %\newcommand{\Read@op}[1]{\TextOrMath{\texttt{!#1}}{\mathop{!#1}}}
 %\newcommand{\Write@op}[1]{\TextOrMath{\texttt{:=#1}}{\mathrel{\coloneqq#1}}}
 \newcommand{\Read@op}[1]{\TextOrMath{\texttt{!#1}}{\mathop{!#1}}}
-\newcommand{\Write@op}[1]{\TextOrMath{\texttt{<-#1}}{\mathrel{\leftarrow#1}}}
+\newcommand{\Write@op}[1]{\TextOrMath{\texttt{:=#1}}{\mathrel{\coloneqq#1}}}
 \newcommand{\ReadNA}[1]{\Read@op\NA #1}
 \newcommand{\ReadAT}[1]{\Read@op\AT #1}
 \newcommand{\ReadANY}[1]{\Read@op\ACCMODEANY #1}
@@ -219,7 +219,7 @@
 %\newcommand{\ArrayWriteNA}[3]{#1[#2] \Write@op\NA #3}
 %\newcommand{\ArrayWriteAT}[3]{#1[#2] \Write@op\AT #3}
 %\newcommand{\ArrayWriteANY}[3]{#1[#2] \Write@op\ACCMODEANY #3}
-\newcommand{\ArrayWrite@op}{\TextOrMath{\texttt{:=}}{\mathrel{\coloneqq}}}
+\newcommand{\ArrayWrite@op}{\TextOrMath{\texttt{<-}}{\mathrel{\leftarrow}}}
 \newcommand{\ArrayReadNA}[2]{#1[#2]\NA}
 \newcommand{\ArrayReadAT}[2]{#1[#2]\AT}
 \newcommand{\ArrayReadANY}[2]{#1[#2]\ACCMODEANY}

papers/icfp2021/queue-impl.tex

@@ -2,16 +2,45 @@
 
 % Overview of the implementation of the bounded queue with a circular buffer.
 
-\subsection{Implementation of the Bounded Queue}
+\subsection{An Implementation of the Bounded MRMW Queue Using a Circular Buffer}
 \label{sec:queue:impl}
 
 \input{figure-queue-impl}
 
+A queue is a first-in first-out container data~structure. At any time, it holds
+an ordered list of items. It supports two main operations: \enqueue inserts
+a provided item at one extremity of the queue (the \emph{head}); \dequeue extracts
+an item ---~if there is one~--- from the other extremity (the \emph{tail}).
+
+In a concurrent setting, a legitimate question is whether several threads can
+operate the queue safely. The answer depends on the implementation.
+%
+Possible restrictions include allowing only one thread to enqueue
+(single~writer), or allowing only one thread to dequeue (single~reader),
+or both.
+% digression:
+Despite not being fully general, such implementations have use cases:
+work~stealing, for example, is commonly achieved using multiple-reader,
+single-writer queues.
+
+In this paper we study an implementation of a \emph{multiple-reader, multiple-writer}
+(MRMW) queue, that is, a queue in which any number of threads are allowed to
+enqueue and dequeue.
+% TODO: motivation for MRMW queues?
+%
+In addition, this queue is \emph{bounded}, that is, it occupies no more than a
+fixed memory size. A motivation for that trait is that items may be enqueued
+faster than they are dequeued; in this situation, a bounded queue has no risk of
+exhausting system memory; instead, if the maximum size has been reached,
+enqueuing will either block or fail.
+% TODO: ref for bounded pools/queues: Herlihy & Shavit 10.1
+
 % TODO: find where this implementation was first described
+% ref: https://github.com/rigtorp/MPMCQueue
 
-Let~$\capacity \geq 1$ be a fixed integer. \fref{fig:queue:impl} shows an
-implementation of a concurrent queue in \mocaml{} using a circular buffer of
-fixed capacity~\capacity. The buffer is represented by two arrays of
+\fref{fig:queue:impl} shows the
+implementation of the bounded MRMW queue in \mocaml{}. It uses a circular buffer of
+fixed capacity~$\capacity \geq 1$. The buffer is represented by two arrays of
 length~\capacity, \refstatuses and \refelements; at each offset (from~$0$
 included to~$\capacity$ excluded), the buffer thus stores a \emph{status} in an
 atomic field and an \emph{item} in a nonatomic field. The data~structure also
@@ -100,7 +129,7 @@ happens-before relationship from a dequeuer to an enqueuer.
 \subsubsection{Explanation of \tryenqueue and \trydequeue}
 
 The function~\tryenqueue first reads the current value of the reference~\refhead, getting
-a value~\head. We have seen that the new item should have rank~\head. To check
+a value~\head. We have seen that the item to insert should have rank~\head. To check
 that offset~$\modcap\head$ is indeed available for rank~\head, the function
 reads its status: it should be equal to~$2\head$.
 %
@@ -123,7 +152,7 @@ control of offset~$\modcap\head$, and we know that its status has not changed.
 %     et posséder un offset. Les deux vont ensemble je suppose?
 %
 % TODO: vague:
-Indeed, enqueueing an item with rank~\head\ is the only way to change it.
+Indeed, enqueuing an item with rank~\head\ is the only way to change it.
 %
 Hence the offset is still available, and we can fill it with the new item, and
 update the status accordingly. Notice that, under weak memory, the order of

papers/icfp2021/queue-spec.tex

@@ -7,11 +7,6 @@
 
 \subsubsection{Specification in a sequential setting}
 
-A queue is a first-in first-out container data~structure. At any time, it holds
-an ordered list of items. It supports two main operations: \enqueue inserts
-a provided item at one extremity of the queue (the \emph{head}); \dequeue extracts
-an item ---~if there is one~--- from the other extremity (the \emph{tail}).
-
 A queue holding $\nbelems$ items \(\elemList*\), where the left extremity
 of the list is the tail and the right extremity is the head,
 is represented in separation logic with a representation predicate:

papers/icfp2021/queue.tex

-\section{A Bounded Queue With a Circular Buffer}
+\section{A Bounded MRMW Queue}
 \label{sec:queue}
 
-\input{queue-spec}
 \input{queue-impl}
+\input{queue-spec}
 \input{queue-proof}