Concurrent Systems (prof. Gorla)  AA 20202021
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).
Since no student is currently willing to physically attend the class, all lectures will take place remotely. If somebody prefers to come in the lecture room, please send me an email and reserve your sit by using Infoprof: I'll come in presence then!
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, 2021 ([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, 2021 ([R]: 2.1.4; 2.1.5  only the idea, no algo, no proofs; 2.1.6; 2.1.7): 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 fast mutex algorithm (with constant time when there is no contention). March 3rd, 2021 ([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 5th, 2021 ([R]: 2.3.1, 2.3.2, 2.3.3): Safe registers: Lamport's Bakery algorithm and Aravind's bounded algorithm.
March 10th, 2021 ([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 12th, 2021 ([R]: 3.3.1, 3.3.2, 3.3.4, 3.3.5): 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 17th, 2021 ([R]: chap. 4): Atomicity: formal definition and compositionality; possible variants and their noncomposability. March 19th, 2021 ([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 24th, 2021 ([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. March 26th, 2021 ([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 7th, 2021 ([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 9th, 2021 ([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 14th, 2021 ([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. April 16th, 2021 ([CCS]: chapters 1 and 2): Syntax for nondeterministic processes: from LTS to the syntax and vice versa. Examples: counter and queue. Process interaction, parallel composition and name restriction. April 21st, 2021 ([CCS]: chapter 3.1, 3.2, 3.3): 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 23rd, 2021([CCS]: 3.4, 4.1, 4.2): Congruence of Bisimilarity. Weak bisimilarity: basic properties, comparison with strong bisimilarity and fundamental laws. April 28th, 2021([CCS]: 4.3): Examples using weak bisimilarity: the factory, the lottery and the scheduler. April 30th, 2021 ([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 5th, 2021 ([CCS]: 5.3): The Kennelakis and Smolka Algorithm for bisimulation on finite state LTSs; soundness and complexity. May 7th, 2021 ([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 12th, 2021 ([PI]: Chapt.1): The picalculus: from CCS, through valuepassing CCS; syntax and reduction semantics; implementing recursive parametric definitions through replication. May 14th, 2021 ([PI]: Chapter 2.1, 2.2): 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. May 19th, 2021 ([PI]: Chapt. 3): A labelled transition system for the picalculus; bisimilarity and bisimulation congruence; weak bisimilarity. May 21st, 2021 ([PI]: Chapt. 2.3, 4): The higherorder calculus, encoding replication and encoding HOpi in the first order paradigm. Encoding the lazy lambdacalculus and an objectoriented calculus. May 26th, 2021 : Solutions of last homework and final discussion on the topics presented.
Exam modalitiesThere are two possible ways to pass the exam:

NewsClasses will regularly start on Feb 24th 2021. Just for the first class, the lecture will start at 8.30. 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. Furthermore, you'll receive all information for remotely attending the classes on such googlegroup (zoom meeting number and password). Of course, students can also come in presence, by following Sapienza's rules.  DanieleGorla  20 Jan 2016 
 DanieleGorla  03 Mar 2005
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