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Concurrent Systems (prof. Gorla) - AA 2016-2017

Teacher Daniele Gorla
Phone 06-4991 8434
Office Via Salaria 113, 3rd floor, room 310
Email gorla@diNOSPAM.uniroma1.it

The class is focused on the foundational aspects and on the formal/mathematical semantics of concurrent programming languages. 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.


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

Orario Aula
tue 8:30 (actually, 9) - 10:30 Aula G50 (Via Regina Elena)
thu 8:30 (actually, 9) - 10:30 Aula G50 (Via Regina Elena)

Text books

First part:

  • M. Raynal: Concurrent Programming: Algorithms, Principles and Foundations. Springer, 2013. (chapters 1, 2, 3, 4, 5, 10, 14, 16 and part of 17).

Second part:

  • R. Milner. Communicating and Mobile Systems. 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. Process Algebra. 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. Equivalence and Preorder Checking for Finite-State Systems. 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:

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


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).

Febr. 21st, 2017 ([R]: chapt.1, excluding sect.1.2.5): 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).

Febr. 23rd, 2017 ([R]: 2.1.1, 2.1.2, 2.1.3, 2.1.4): Atomic read/write registers; mutex for 2 processes (Peterson algorithm); generalizing Peterson's algorithm to n processes.

Febr 28th, 2017 ([R]: 2.1.5 - only the idea, no algo, no proofs; 2.1.6; 2.1.7 - no proofs; 2.1.8): Algorithms with better performances when no concurrency is present: with a tournament tree of 2-processes competitions (O(log n)); algorithms with constant time (Lamport and with timing assumptions).

March 2nd, 2017 ([R]: 2.2 and 5.2.3): 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.

March 7th, 2017 ([R]: 2.3.1, 2.3.2, 2.3.3): Safe registers: Lamport's Bakery algorithm and Aravind's bounded algorithm.

March 9th, 2017 ([R]: 3.1, 3.2.1, 3.2.2, 3.2.4): 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).

March 14th, 2017 ([R]: 3.3.1, 3.3.2, 3.3.4, 3.3.5): 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).

March 16th, 2017 ([R]: 10.1, 10.2, 10.3, 10.5): Software Transactional Memory. Opacity and a Logical clack-based STM system (TL2). Virtual World Consistency and a vector clock-based STM system.

March 21st, 2017 ([R]: chap. 4): Atomicity: formal definition and compositionality; possible variants and their non-composability.

March 23rd, 2016 ([R]: 5.1, 5.2.1, 5.2.2, 5.2.5, 5.2.6): Mutex-free concurrency: problems of mutex and notion of mutex-freedom, progress conditions. Examples: splitter, timestamp generator and stack (based on compare&swap and on swap plus fetch&add); progress conditions for these examples.

March 28th, 2017 ([R]: 5.3; 17.7, 17.7.1, 17.7.2 -- only the idea, not the formal protocol): From obstruction-freedom to stronger liveness properties through failure detectors (Omega_X and Diamond_P); hints on the implementation of failure detectors.

March 30th, 2017 ([R]: 14.1, 14.2, 14.3): Universal object and consensus object; universality of consensus (unbounded wait-free construction).

April 4th, 2017 ([R]: 14.5; 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): Binary vs multivalued consensus. Consensus number (for atomic R/W registers, test&set, swap, fetch&add, compare&swap) and consensus hierarchy.

April 6th, 2017 ([CCS]: 1; 2.1, 2.2): Automata for describing process behaviors (notion of LTS); inadequacy of trace equivalence for equating non-deterministic processes; simulation, double simulation and bisimulation.

April 11th, 2017 ([CCS]: 2.3, 2.4, 2.5): Properties of bisimulation. Syntax for non-deterministic processes: from LTS to the syntax and vice versa. Examples: counter and queue.

April 20th, 2017 ([CCS]: 3.1, 3.2): Process interaction, parallel composition and name restriction. A first proof technique for proving properties of LTSs: the case of image-finiteness.

April 27th, 2017 ([CCS]: 3.2, 3.4): A second proof technique for proving properties of LTSs: the case of closure under substitutions. Congruence of Bisimilarity.

May 2nd, 2017([CCS]: 3.3, 4.1, 4.2, 4.3.1): A simple example that uses bisimilarity for proving an implementation equivalent to its specification. Some basic properties of bisimilarity. Weak bisimilarity: basic properties, comparison with strong bisimilarity and fundamental laws.

May 4th, 2017 ([CCS]: 4.3.2, 4.3.3): Three sophisticated examples: the factory, the lottery and the scheduler.

May 9th, 2017 ([CCS]: 5.1): 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.

May 11th, 2017 ([CCS]: 5.2): 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.

May 16th, 2017 ([CCS]: 5.3): The Kennelakis and Smolka Algorithm for bisimulation on finite state LTSs; soundness and complexity.

May 18th, 2017 ([PI]: Chapt.1): The pi-calculus: from CCS, through value-passing CCS; syntax and reduction semantics; implementing recursive parametric definitions through replication.

May 23rd, 2017 ([PI]: 2.1, 2.2): The polyadic version of the calculus: type system and encoding in the monadic version. The asynchronous calculus and the encoding of synchrony into asynchrony.

May 25th, 2017 ([PI]: 2.3, 3.1 - only sketched - 3.2): The higher-order calculus; encoding lambda-calculus (sketch) and an object-oriented calculus.

Exam modalities

There are two possible ways to pass the exam:

  • First modality (better suited for students who regularly attend the classes)
    • a homework on the first two parts of the course;
    • a short presentation of a simple research paper about the topics of the third part of the course;
  • Second modality: oral exam on all the topics presented during the classes.


Starting Time Classes will start at 9am, both on Tuesdays and on Thursdays.

Beginning of the classes Classes will regularly start on February 21st, 2017.

Course Material and Googlegroup Students can have access to all text books and lecture notes by sending an email to the teacher and ask to be included in the googlegroup of the curse. The first posts of this group contain in attachment all what you need for studying. Please, register with the email address that you most frequently access, because important and urgent news about the course will be primarily posted on this group.

-- DanieleGorla - 20 Jan 2016

-- DanieleGorla - 03 Mar 2005

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Topic revision: r108 - 2017-05-29 - DanieleGorla

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