Concurrent Systems (prof. Gorla)  AA 20192020
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 waitfree 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, nondeterminism, 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.
Timetable2nd semester (end of February > end of May).
Text booksFirst part:
Second part:
Third part:
Detailed ProgramThe course is split into 3 parts:
DiaryHere, [R] denotes Raynal's book; [CCS] and [PI] denote teacher's lecture notes on CCS and picalculus (that are a unified and coherent presentation of the texts for the second and the third part of the course listed above).
Feb 24th, 2020 ([R]: chapt.1, excluding sect.1.2.5; 2.1.1, 2.1.2, 2.1.3): 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). Feb 26th, 2020 ([R]: 2.1.4; 2.1.5  only the idea, no algo, no proofs; 2.1.6; 2.1.7  no proofs; 2.1.8): Generalizing Peterson's algorithm to n processes. Algorithms with better performances when no concurrency is present: with a tournament tree of 2processes competitions (O(log n)); Lamport's algorithm (with constant time); another algorithm with constant time (but with timing assumptions). March 9th, 2020 ([R]: 2.2 and 5.2.3): From Deadlock freedom to Starvation freedom using atomic r/wregisters; mutex with specialized HW primitives (test&set; swap; compare&set; fetch&add); the ABA problem with the compare&set. March 11th, 2020 ([R]: 2.3.1, 2.3.2, 2.3.3): Safe registers: Lamport's Bakery algorithm and Aravind's bounded algorithm. March 16th, 2020 ([R]: chapter 2): First classwork. March 18th, 2020 ([R]: 3.1, 3.2.1, 3.2.2, 3.2.4, 3.3.1, 3.3.2, 3.3.4, 3.3.5): 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). Monitors: concept and implementation through semaphores; implementing rendezvous 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 23rd, 2020 ([R]: chap. 4): Atomicity: formal definition and compositionality; possible variants and their noncomposability.
March 25th, 2020 ([R]: 10.1, 10.2, 10.3, 10.5): Software Transactional Memory. Opacity and a Logical clockbased STM system (TL2). Virtual World Consistency and a vector clockbased 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=1s2.0S0304397512004021main.pdf, sections 2.6 and 3.2). March 30th, 2020 ([R]: chapters 3, 4 and 10): Second classwork. April 1st, 2020 ([R]: 5.1, 5.2.1, 5.2.2, 5.2.5, 5.2.6): Mutexfree concurrency: problems of mutex and notion of mutexfreedom, progress conditions. Examples: splitter, timestamp generator and stack (based on swap plus fetch&add; based on compare&set); progress conditions for these examples. April 6th, 2020 ([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): From obstructionfreedom to nonblocking through failure detector Omega_X; hints on the implementation of Omega_X. From obstructionfreedom to waitfreedom through failure detector Diamond_P; hints on the implementation of Diamond_P. April 8th, 2020 ([R]: 14.1, 14.2, 14.3, 14.5): Universal object and consensus object; universality of consensus (unbounded waitfree construction). Binary vs multivalued consensus. April 15th, 2020 ([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): Consensus number (for atomic R/W registers, test&set, swap, fetch&add, compare&swap) and consensus hierarchy. April 20th, 2020 ([R]: chapts 5, 14 and 16): Third classwork. April 22nd, 2020 ([CCS]: chapters 1 and 2): Automata for describing process behaviors (notion of LTS); inadequacy of trace equivalence for equating nondeterministic processes; simulation, double simulation and bisimulation. Properties of bisimilarity. Syntax for nondeterministic processes: from LTS to the syntax and vice versa. Examples: counter and queue. April 27th, 2020 ([CCS]: chapter 3.1, 3.2, 3.3): Process interaction, parallel composition and name restriction. A first proof technique for proving properties of LTSs: the case of imagefiniteness. 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. April 29th, 2020([CCS]: 3.4, 4.1, 4.2, 4.3.2, 4.3.3): Congruence of Bisimilarity. Weak bisimilarity: basic properties, comparison with strong bisimilarity and fundamental laws. Examples using weak bisimilarity: the lottery and the scheduler. May 4th, 2020 ([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 6th, 2020 ([CCS]: 5.3): The Kennelakis and Smolka Algorithm for bisimulation on finite state LTSs; soundness and complexity. May 11th, 2020 ([CCS]: 5.2): A logical characterization of bisimilarity and its use to show process inequivalences; sublogics for double simulation and for trace equivalence; a logic for weak bisimilarity. May 13th, 2020 ([CCS]: chapters 15): Fourth classwork.
May 18th, 2020 ([PI]: Chapt.1): The picalculus: from CCS, through valuepassing CCS; syntax and reduction semantics; implementing recursive parametric definitions through replication. May 20th, 2020 ([PI]: Chapter 2): 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 higherorder calculus, encoding replication and encoding HOpi in the first order paradigm. May 25th, 2020 ([PI]: Chapter 3): Encoding the lazy lambdacalculus and an objectoriented calculus. May 27th, 2020 ([PI]: chapters 1, 2, 3): Fifth classwork.
Exam modalitiesThere are two possible ways to pass the exam:

NewsPhysical Classes are suspended from March 5th on . Lessons will take place remotely via Cisco Webex platform (according to the modalities specified in the google group) and will follow the usual schedule. Classes will regularly start on Feb 24th 2020. There will be no classes on March 2nd and 4th. 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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