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---++ Concurrent Systems (prof. Gorla) - AA 2021-2022


| *Teacher* | Daniele Gorla |||
| *Phone* | 06-49255431 |||
| *Office* | Viale Regina Elena 295, 2nd floor, room 205 |||
| *Email* | gorla@di.uniroma1.it  |||

The class is focused on the foundational aspects and on the formal/mathematical semantics of concurrent systems. The class is structured in two main parts. The first part describes the main characteristics and the basilar problems of every concurrent system (mutual exclusion, synchronization, atomicity, deadlock/livelock/starvation, ...) and the relative solutions at the implementation level (semaphores, monitors, system primitives, ...). Furthermore, more evolute notions are shown, like: failure detectors, their implementation and their use to obtain wait-free implementations; universal object, consensus object and consensus number; transactional memory, ... The second part of the course describes the preliminary notions of a minimal concurrent language called CCS (execution of parallel processes through labelled transition systems, interleaving semantics, syntonization, non-determinism, process simulability) and presents a mathematical model, with different features for the specification and the analysis of systems written in such a language. 

In the time left, we shall have lectures in the form of seminars where more advanced programming mechanisms (like name creation and exchange, type systems for the verification of properties, cryptography, distribution, truly concurrent semantics) will be presented. 

The course integrates didactic parts to recent research problems.

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---++ Timetable

2nd semester (end of February --> end of May).

<center>
| *Timetable*    | *Lecture room* |
| wed 8:15 - 11:00 | Aula T1 (Viale Regina Elena, building E, ground floor) |
| fri 8:15 - 10:00 | Aula  T1 (Viale Regina Elena, building E, ground floor) |
</center>


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---++ Text books

First part:
   * M. Raynal: <I>Concurrent Programming: Algorithms, Principles and Foundations</I>. Springer, 2013. (chapters 1, 2, 3, 4, 5, 10, 14, 16 and part of 17).

Second part:
   * R. Milner. <I>Communicating and Mobile Systems.</I> Cambridge University Press, 1999. (chapters: 1, 2 (no 2.3), 3, 4 (no 4.1, 4.4, 4.5 and 4.6), 5 (no prop. 5.2, ex 5.3, lemma 5.4/5.5, theor. 5.6, def. 5.17, prop. 5.18, prop. 5.23), 6 (no 6.3) and 7 (only 7.2 and 7.3).
   * R. Cleaveland and S. Smolka. <I>Process Algebra.</I> In Encyclopedia of Electrical Engineering, John Wiley & Sons, 1999. Available at http://www.cs.umd.edu/~rance/publications/papers/ee99.ps.gz. (sect. 3.2, 4.1 and 4.2).
   * R. Cleaveland and O. Sokolsky.  <I>Equivalence and Preorder Checking for Finite-State Systems.</I>  In "Handbook of Process Algebra,"  pp. 391-424, Elsevier, 2001. Available at  http://www.cis.upenn.edu/~sokolsky/PAhandbook.pdf. (sect. 3.1).

Third part:
   * J. Parrow. <I>An Introduction to the pi-Calculus.</I> In  Handbook of Process Algebra, pages 479-543. Elsevier, 2001. Available at http://user.it.uu.se/~joachim/intro.ps
   * R. Milner. <I>The Polyadic pi-Calculus: A Tutorial.</I> In Logic and Algebra of Specification. Springer-Verlag, 1993. Available at http://www.lfcs.inf.ed.ac.uk/reports/91/ECS-LFCS-91-180/ECS-LFCS-91-180.ps
   * D. Sangiorgi, D. Walker. <I>The Pi-Calculus - A Theory of Mobile Processes.</I> Cambridge University Press, 2001. The reference book on the pi-calculus; maybe, quite difficult for a master student.
   * http://www.cs.unibo.it/~zavattar/Publication/icalp03.ps. Comparison of recursion and replication in process calculi.
   * http://www.cs.unibo.it/~sangio/DOC_public/pi.ps.gz. The classical notion of subtyping in the pi-calculus. 
   * http://www.cs.unibo.it/~zavattar/Publication/tcs00.ps. A comparison of three different kinds of output actions in a CCS-like language.
   * http://lucacardelli.name/Papers/Mobility%20and%20Security.A4.pdf. A nice survey on a calculus very trend a few years ago. There are two logically distinct parts: I+II+III and IV+V.
   * http://lucacardelli.name/Papers/Brane%20Calculi.pdf. A funny work on a formalism for bio-inspired computing. 
   * https://www.cs.bham.ac.uk/~mdr/research/papers/pdf/11-applied-pi.extended.pdf. A survey on one of the best known cryptographic versions of the pi-calculus.


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---++ Detailed Program

The course is split into 3 parts:
   * The first part studies foundational problems of concurrent systems:
      * sequential vs concurrent programs
      * Process synchronization (competition vs cooperation)
      * Safety and liveness properties; a hierarchy of liveness properties (deadlock freedom, starvation freedom, bounded bypass)
      * Mutual exclusion: atomic registers (algorithms by Peterson and Lamport; how to obtain a starvation free lock object from a deadlock free lock object), with specialized hardware primitives (test&set, compare&swap, fetch&add) and with non-atomic registers (Bakery algorithm).
      * Semaphores, monitors and their use for solving classical problems (producers-consumers, readers-writers, rendezvous, dining philosophers)
      * Transactional memory
      * Atomicity and its properties
      * mutex-free concurrency: liveness revisited (obstruction freedom, non-blocking and wait-freedom); mutex-free implementation of an unbounded stack; failure detectors Omega_X and Diamond_P, and the associated construction of non-blocking and wait-free implementations from an obstruction free implementation; hints on how to implement the Omega failure detector.
      * Universal object; consensus object and its universality; primitives hierarchy based on the consensus number.

   * The second part is focused on CCS (Calculus of Communicating Systems [Milner:1980]): 
      * non-determinism
      * Labeled transition systems
      * recursion
      * parallel composition (interleaving semantics)
      * synchronization
      * restriction
      * bisimulation: strong vs weak, axiomatization, logical characterization, algorithmic verification.

   * The third part will present possible enhancements of the previous material (every lecture will be on a different topic). By following the students' interests, some of the following topics can be presented: 
      * communication: name creation and passage
      * polyadic communication (type system for correct communications and encodability in the base calculus)
      * asynchronous communication and encodability of synchrony
      * higher-order communication and its encodability in the base calculus
      * encoding of the lambda-calcolus
      * a model for object-oriented languages
      * concurrent calculi with cryptography or distribution
      * truly concurrent semantics


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---++ Diary

Here, [R] denotes Raynal's book; [CCS] and [PI] denote teacher's lecture notes on CCS and pi-calculus (that are a unified and coherent presentation of the texts for the second and the third part of the course listed above).


<b><font color="#008000">Feb 23rd, 2022 </font>([R]: chapt.1, excluding sect.1.2.5; 2.1.1, 2.1.2, 2.1.3. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_1.pdf][Lecture Notes]]): </b>
Sequential vs Multiprocess Program; process synchronization (competition and cooperation); the mutual exclusion problem; safety and liveness properties; a hierarchy of liveness properties (bounded bypass; starvation freedom; deadlock freedom).
Atomic read/write registers; mutex for 2 processes (Peterson algorithm).

<b><font color="#008000">Feb 25th, 2022 </font>([R]: 2.1.4; 2.1.5 - only the idea, no algo, no proofs; 2.1.6; 2.1.7. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_2.pdf][Lecture Notes]]):</b>
Generalizing Peterson's algorithm to n processes.
Algorithms with better performances when no concurrency is present: with a tournament tree of 2-processes competitions (O(log n)); 
Lamport's fast mutex algorithm (with constant time when there is no contention).

<b><font color="#008000">March 2nd, 2022 </font>([R]: 2.2 and 5.2.3; 2.3.1, 2.3.2. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_3.pdf][Lecture Notes]] - [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_4.pdf][Lecture Notes]]): </b>
From Deadlock freedom to Starvation freedom using atomic r/w-registers; mutex with specialized HW primitives (test&set; swap; compare&set; fetch&add); the ABA problem with the compare&set.
Safe registers: Lamport's Bakery algorithm

<b><font color="#008000">March 4th, 2022 </font>([R]: 2.3.3. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_4.pdf][Lecture Notes]]): </b>
Aravind's bounded algorithm.

<b><font color="#008000">March 9th, 2022 </font>([R]: 3.1, 3.2.1, 3.2.2, 3.2.4. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_5.pdf][Lecture Notes]]): </b>
Concurrent objects. Semaphores and their implementation. Use of semaphores in the producer/consumer problem (both for single producer/consumer and for multiple ones) and in the readers/writers problem (both with weak/strong priority to the readers and with weak priority to the writers).

<b><font color="#008000">March 11th, 2022 </font>([R]: 3.3.1, 3.3.2, 3.3.4, 3.3.5. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_5.pdf][Lecture Notes1]], 
[[https://twiki.di.uniroma1.it/pub/TC/WebHome/dining-philosophers.pdf][Lecture Notes2]]): </b>
Monitors: concept and implementation through semaphores; implementing rendez-vous through monitors.
Readers/writers through monitors.
The problem of the "Dining Philosophers": solutions that break symmetry (through semaphores) and a symmetric solution (through monitors).

<b><font color="#008000">March 16th, 2022 </font>: </b>
First classwork.

<b><font color="#008000">March 18th, 2022 </font>([R]: chap. 4. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_6.pdf][Lecture Notes]]): </b>
Atomicity: formal definition and compositionality; possible variants and their non-composability.

<b><font color="#008000">March 23rd, 2022 </font>([R]: 10.1, 10.2, 10.3, 10.5. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_7.pdf][Lecture Notes]]): </b>
Software Transactional Memory. Opacity and a Logical clock-based STM system (TL2). Virtual World Consistency and a vector clock-based STM system
(REMARK: to better understand the difference between virtual world consistency and opacity, look at this paper: https://www.sciencedirect.com/science/article/pii/S0304397512004021/pdf?md5=78dd63551d097b6a3212285669d20ff8&pid=1-s2.0-S0304397512004021-main.pdf, sections 2.6 and 3.2).

<b><font color="#008000">March 25th, 2021 </font>([R]: 5.1, 5.2.1, 5.2.2, 5.2.5, 5.2.6. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_8.pdf][Lecture Notes]]): </b>
Mutex-free concurrency: problems of mutex and notion of mutex-freedom, progress conditions. Examples: splitter, timestamp generator and stack (based on swap plus fetch&add; based on compare&set); progress conditions for these examples.

<b><font color="#008000">March 30th, 2022 </font>([R]: 5.3.1, 5.3.2, 5.3.3, 5.3.4; 17.7, 17.7.1, 17.7.2 -- only the idea, not the formal protocol. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_9.pdf][Lecture Notes]]): </b>
From obstruction-freedom to non-blocking through failure detector Omega_X; hints on the implementation of Omega_X.
From obstruction-freedom to wait-freedom through failure detector Diamond_P; hints on the implementation of Diamond_P.


<b><font color="#008000">April 1st, 2022 </font>([R]: 14.1, 14.2, 14.3, 14.5. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_10.pdf][Lecture Notes]]): </b>
Universal object and consensus object; universality of consensus (unbounded wait-free construction).

<b><font color="#008000">April 6th, 2021 </font>([R]: 16.1, 16.2, 16.3, 16.4.1, 16.4.3, 16.4.4, 16.4.5 -- adapted to test&set -- 16.5.1, 16.6. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_11.pdf][Lecture Notes]]): </b>
Binary vs multivalued consensus. 
Consensus number (for atomic R/W registers, test&set, swap, fetch&add, compare&swap) and consensus hierarchy.

<b><font color="#008000">April 8th, 2022 </font>([CCS]: chapters 1 and 2. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_12.pdf][Lecture Notes]]): </b>
Automata for describing process behaviors (notion of LTS); inadequacy of trace equivalence for equating non-deterministic processes; simulation, double simulation and bisimulation. Properties of bisimilarity. 

<b><font color="#008000">April 13th, 2022 </font>: </b>
Second classwork.

<b><font color="#008000">April 20th, 2022 </font>([CCS]: chapters 1 and 2.[[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_12.pdf][Lecture Notes]] and [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_13.pdf][Lecture Notes]]): </b>
Syntax for non-deterministic processes: from LTS to the syntax and vice versa. Examples: counter and queue.
Process interaction, parallel composition and name restriction. 

<b><font color="#008000">April 22nd, 2022 </font>([CCS]: chapter 3.1, 3.2, 3.3. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_13.pdf][Lecture Notes]]): </b>
A first proof technique for proving properties of LTSs: the case of image-finiteness.
A second proof technique for proving properties of LTSs: the case of closure under substitutions.
A simple example that uses bisimilarity for proving an implementation equivalent to its specification.

<b><font color="#008000">April 27th, 2022</font>([CCS]: 3.4, 4.1, 4.2. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_14.pdf][Lecture Notes]]): </b>
Congruence of Bisimilarity. 
Weak bisimilarity: basic properties, comparison with strong bisimilarity and fundamental laws.

<b><font color="#008000">April 29th, 2022</font>([CCS]: 4.3. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_14.pdf][Lecture Notes]]): </b>
Examples using weak bisimilarity: the factory, the lottery and the scheduler.

<b><font color="#008000">May 4th, 2022</font>: </b>
Third classwork.

<b><font color="#008000">May 6th, 2022 </font>([CCS]: 5.1. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_15.pdf][Lecture Notes]]): </b>
An inference system for strong bisimilarity: soundness and completeness for finite processes.
The tau laws for weak bisimilarity. Verifying the equivalence of a specification and an implementation through the inference system.

<b><font color="#008000">May 11th, 2022 </font>([CCS]: 5.3. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_16.pdf][Lecture Notes]]): </b>
The Kennelakis and Smolka Algorithm for bisimulation on finite state LTSs; soundness and complexity.

<b><font color="#008000">May 13th, 2022 </font>([CCS]: 5.2. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_17.pdf][Lecture Notes]]): </b>
A logical characterization of bisimilarity and its use to show process inequivalences; sub-logics for double simulation and for trace equivalence; a logic for weak bisimilarity.

<b><font color="#008000">May 18th, 2022 </font>([PI]: Chapt.1. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_18.pdf][Lecture Notes]]): </b>
The pi-calculus: from CCS, through value-passing CCS; syntax and reduction semantics; implementing recursive parametric definitions through replication.

<b><font color="#008000">May 20th, 2022 </font>([PI]: Chapter 2.1, 2.2, 2.3. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_19.pdf][Lecture Notes]]): </b>
The polyadic version of the calculus and a type system for correct communications.
Encoding the polyadic version of the calculus in the monadic version.
The asynchronous calculus and the encoding of synchrony into asynchrony.
The higher-order calculus, encoding replication and encoding HOpi in the first order paradigm.

<b><font color="#008000">May 25th, 2022 </font>([PI]: Chapt. 4. [[https://twiki.di.uniroma1.it/pub/TC/WebHome/lez_21.pdf][Lecture Notes]]): </b>
Encoding the lazy lambda-calculus and an object-oriented calculus.

<b><font color="#008000">May 27th, 2022</font>: </b>
Fourth classwork.


<!--



<b><font color="#008000">March 16th, 2020 </font>([R]: chapter 2): </b>
First classwork.

<b><font color="#008000">March 30th, 2020 </font>([R]: chapters 3, 4 and 10): </b>
Second classwork.

<b><font color="#008000">April 20th, 2020 </font>([R]: chapts 5, 14 and 16): </b>
Third classwork.

<b><font color="#008000">May 13th, 2020 </font>([CCS]: chapters 1-5): </b>
Fourth classwork.

<b><font color="#008000">May 27th, 2020 </font>([PI]: chapters 1, 2, 3): </b>
Fifth classwork.

<b><font color="#008000">May 27th, 2022 </font>([PI]: Chapt. 3): </b>
A labelled transition system for the pi-calculus; bisimilarity and bisimulation congruence; weak bisimilarity.


-->

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---++ Exam modalities

There are two possible ways to pass the exam:

<!--

   * First modality (better suited for students who regularly attend the classes): classworks, approx one every two weeks. Students should pass 3 over 5 classworks. There will be 3 classworks for the first part and 2 for the second+third part; students must pass at least 2 from the first part and 1 from the second part of the course. The final mark will be the average of the 3 better marks in such classworks (FEASIBILITY OF THIS MODALITY NEEDS TO BE CHECKED, ACCORDING TO THE CURRENT COVID REGULATIONS)

   * First modality (better suited for students who regularly attend the classes): homeworks, one every week, to be presented during the Wednesday class. On Friday afternoon, I'll send in the googlegroup the text of 3/4 exercises on the topics presented during the classes of the week. You have time to solve the exercises on your own and send them to me your solutions by email. Every week, 3 or 4 students will present their solutions and publicly discuss them with me and the class. The policy for selecting the students is two-fold: FIFO (who arrives first in sending me the email gets the opportunity to present) and number of presentations already done (if you have done few presentations, you've the priority). Every discussion will have a mark (REMARK: not only on the solution itself that you propose, but mostly on the way in which you present it and on how you reply to my questions -- that can also be on topics that I explained during the course). To pass the exam, you must present your solutions at least 4 times during the term (2 times on the first part of the course, and 2 times on the second one). The final mark will be obtained as the average of the 4 best marks obtained at every presentation.

-->

   * First modality (better suited for students who regularly attend the classes): classworks, approx one every two weeks. Students should pass 3 over 4 classworks. There will be 2 classworks for the first part and 2 for the second+third part. The final mark will be the average of the 3 better marks in such classworks
   * Second modality: written and oral exam on all the topics presented during the classes.


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-- Users.DanieleGorla - 03 Mar 2005
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Topic attachments
I	Attachment	History	Action	Size	Date	Who
pdf	dining-philosophers.pdf	r1	manage	112.3 K	2022-03-08 - 07:36	DanieleGorla
pdf	lez_1.pdf	r1	manage	1473.6 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_10.pdf	r1	manage	5373.8 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_11.pdf	r1	manage	4924.2 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_12.pdf	r1	manage	367.2 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_13.pdf	r1	manage	1103.1 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_14.pdf	r1	manage	1526.9 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_15.pdf	r1	manage	892.4 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_16.pdf	r1	manage	270.9 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_17.pdf	r1	manage	586.8 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_18.pdf	r1	manage	586.0 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_19.pdf	r1	manage	724.6 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_2.pdf	r1	manage	1049.8 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_20.pdf	r1	manage	1420.8 K	2022-02-22 - 08:39	DanieleGorla
pdf	lez_21.pdf	r1	manage	637.5 K	2022-02-22 - 08:40	DanieleGorla
pdf	lez_3.pdf	r1	manage	4236.6 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_4.pdf	r1	manage	4872.0 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_5.pdf	r1	manage	884.5 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_6.pdf	r1	manage	3557.7 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_7.pdf	r1	manage	4991.0 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_8.pdf	r1	manage	5092.2 K	2022-02-22 - 08:38	DanieleGorla
pdf	lez_9.pdf	r1	manage	5861.8 K	2022-02-22 - 08:38	DanieleGorla
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