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Questions tagged [concurrency]

Questiont about issues of concurrency such as synchronization and deadlocks.

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32 views

Deadlocks: Why if 2 threads are waiting for a lock from each other results in a deadlock?

I am confused as to why 2 threads waiting for a lock from each other will result in a deadlock. Let's say I have a Thread 1 that needs to execute contents in Lock 1 and then Lock 2 and a Thread 2 that ...
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1answer
102 views

Invariant vs Assertion vs lemma

I am reading Distributed Algorithms by Nancy Lynch. I have come across lemmas, assertions and invariants, but I do not understand the difference between them. I think lemma means an intermediate ...
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1answer
55 views

What will you do if multiple users access your application at the same time?

I'm an experienced Software Engineer but very weak in concurrency because of no prior experience in that. I've been interviewing with several companies in which I was asked similar kinds of questions ...
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1answer
58 views

PetersonNP, mechanical mutual exclusion proof

Good day everyone, I'm currently trying to carry out the PetersonNP (a.k.a. FilterLock) correctness proof (mutual exclusion). I've found several proof sketches on concurrency books but I'm interested ...
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3answers
103 views

Cache coherence is worthless and does nothing?

When you perform a write on a multi-cpu system, relevant cache flushes are done to ensure that the other cpus see the change immediately. BUT That write was issued from an independent thread on a ...
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3answers
88 views

concurrency vs parallelism

I am so confused between concurrency vs parallelism in multi-core processors. If a concurrent process has many threads and if the processor is multi-core then can each thread run on multiple cores? If ...
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1answer
57 views

Prove correctness of a solution to the critical section problem in general?

I was wondering if there is any formal, general way to prove the correctness of a candidate solution to the critical section problem in synchronization. For example, in the image enclosed, I have ...
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2answers
109 views

concurrency in multiple core

I know the basic definition between parallelism and concurrency. But there is not much information about concurrency in multiple core. what is the difference between concurrency in single-core vs ...
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17 views

Companion volume to Concurrency verification of Willem-Paul de Roever

At the moment I am working hard with the book "Concurrency Verification Introduction to Compositional and Non-compositional Methods" written by Willem-Paul de Roever et al. In the ...
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42 views

How to Concurrently Add a Row to a Relational Database

Say I have a relational database and I want to support allowing multiple writers to add create a new row in the same table. How is this implemented? I've read a lot about locks for dealing with ...
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1answer
31 views

Why does taking advantage of locality matter in multithreaded systems?

As we all know, when a given thread/process reaches a memory address it does not have cached, the execution will (for the most part) freeze up until said data is fetched from memory. What I don't ...
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1answer
60 views

How to use Parallel Semaphores for Dining Philosophers Problem

I am in an intro to OS class and we are learning about mutual exclusion and semaphores. One classical problem we learned about was the dining philosophers problem. My professor touched on the ...
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1answer
81 views

Why sequential consistency model is widely used if it is unsound

From this talk and these lecture notes here and here I learned that sequential consistency does not actually model what really happens in practice. However, it is also pointed out that most of the ...
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1answer
31 views

Fast software pipelining for stream processing

I have a pipelined stream processing situation, where a number of streams is linked in the computational process S0 -> S1 -> S2 -> .... You can understand &...
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0answers
34 views

How many different values can a shared variable take in concurrent computing?

Question: Is there a way to find out number of different values that a shared variable can take in concurrent computing, in general, without listing all the possibilities and then counting the ones ...
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30 views

How the task is divided between multiple cores to perform in multiprocessing systems?

I am trying to find how single task is divided between two cores? what are the methods makes them work at the same time without interfering with each other?
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1answer
24 views

Uncertainty of coroutines writing to single socket

A program runs in a low-spec hardware utilizes coroutines writes to a single socket but how does the socket know when the data should be sent as there could more coroutines writing given N time. I ...
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21 views

Algorithm for locking based on a shared resource (dictionary)

I am working with a shared resource with the following specifications acts as a dictionary (values can be obtained with key name: dictionary['key']) does not have ...
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0answers
32 views

Dekker's Algorithm spin on intent instead of turn

For Dekker's algorithm given below (from Wikipedia), why is it that we wait for the turn to change, instead of waiting for p1 to set it's flag to false? It seems to me that it would be safer (or more ...
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36 views

How does 2PL protocol apply to schedule of multiple transactions?

If a schedule follows the two-phase locking protocol, does every transaction have to follow 2PL, or is it the case that all locks must appear all unlocks in the whole schedule? I am a little confused ...
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1answer
118 views

Does upgrading after all locks and before all unlocks violate 2PL?

2PL's rule says: Upgrading of lock (from S(a) to X (a)) is allowed in growing phase. But what if upgrading lies after the last lock and before the first unlock? Does it still count "in the ...
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27 views

Calculus for Communicating Systems vs. Calculus of Communicating Systems

Robin Milner uses both terms: Calculus for Communicating Systems and Calculus of Communicating Systems and, AFAIK, writes CCS for both. Is there any difference between the two? Is there a difference ...
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1answer
100 views

Two-phase locks: why is it better?

I'm reading Arpaci's Operating Systems: Three Easy Pieces, the chapter on Locks. At the end of the chapter, they present Two-phase locks (section 28.16). They say A two-phase lock realizes that ...
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29 views

Why is shared register's consensus number 1?

I'm trying to understand the consensus number concept. It is trivially clear that a shared register doesn't create any problem when there is only one thread using it. By why does its consensus number ...
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0answers
40 views

Concurrent Garbage-Collectiong/Compacting Memory Allocator

I'm developing an algorithm for concurrent heap garbage collection/compaction. It will be used in low latency systems that need to scale well to a lot of clients, e.g. web servers. I thought about an ...
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1answer
222 views

What is the difference between a lock convoy and lock/thread contention?

From wikipedia on lock convoy: A lock convoy occurs when multiple threads of equal priority contend repeatedly for the same lock. Unlike deadlock and livelock situations, the threads in a lock convoy ...
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76 views

Count number of ways in which atomic operation(s) of n different processes can be interleaved

PROBLEM: Count the number of ways in which atomic operation(s) of n different processes can be interleaved. A process may crash mid way before completion. Suppose there are a total of n different ...
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1answer
321 views

Estimating P in Amdahl's Law theoretically and in practice

In parallel computing, Amdahl's law is mainly used to predict the theoretical maximum speedup for program processing using multiple processors. If we denote the speed up by S then Amdahl’s law is ...
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12 views

parallelized data structure

I'm searching a data structure that supports O(1) concurrent insertions (thread number is known in advance) and iteration over its elements (insertion order doesn't need to be preserved). While one ...
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1answer
170 views

Difference between consensus number n and consensus number infinite

In book Concurrent Programming: Algorithms, Principles, and Foundations of Michel Raynal, in Section 16.5.1, Theorem 75 says Compare&swap objects have infinite consensus number. and in ...
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49 views

Unconditionally fair, weakly fair, and strongly fair scheduling

I am trying to understand the difference between weakly fair and strongly fair scheduling. For example, what scheduling policy would ensure that a process delayed at its first await statement will ...
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0answers
43 views

Bakery algorithm: How does it work with single-writer safe registers?

I'm trying to understand the bakery algorithm and read that the Bakery algorithm uses single-writer safe registers. But I am not able to understand the correctness of bakery algorithm with safe ...
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0answers
70 views

Why is the Black-White Bakery Algorithm considered bounded?

As stated in Lamport's papers for the bakery algorithm he states that the ticket numbers are unbounded specifically The range of values of number is unbounded. and Fortunately, practical ...
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1answer
70 views

Does Dekkers solutions to critical section problem ensure progress?

I was reading concurrency control section from Operating Systems book by William Stallings. In this book, he gives three attempts by Dekker to give solution to critical section problem: Attempt 1 <...
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2answers
254 views

Why the most dominant programming languages didn't follow CSP thread model?

I was trying to ask this question in StackOverflow, but later realized that this question is more relevant to general computer science, not specific engineering problems. If you think it's not, please ...
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2answers
206 views

Is this a deadlock?

Consider we have two processes P1 and P2. There are also two semaphores M and N which are initialized to zero. Here's how they execute: ...
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43 views

n thread consensus queues

Its generally agreed that wait free queues have a consensus number of 2. But what about this following code for a consensus protocol for a queue. How is it that this does not go against the idea above?...
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94 views

How to prove that an algorithm has no deadlock

I need to mathematically prove that this algorithm is deadlock-free: ...
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1answer
767 views

What is google docs solution to handling multiple users editing and reading at the same time?

I have always been curious as to how google solved the solution of real time collaboration in their google docs product where they allow multiple users to edit and read the same document in real time. ...
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1answer
33 views

Distributed algorithm can be executed in a single process

Looking at a "Distributed" algorithm from a textbook on distributed algorithms (e.g. Building a spanning tree, Broadcast/Convergecast.) I found that one can implement it using Golang's concurrency ...
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0answers
79 views

Concurrent Element-wise Reduction Algorithm (multi-threaded) (C++)

I'd like to implement a high-performance implementation of a multi-threaded reduction, element-wise, on x86 CPUs. Without loss of generality, assume the reduction operation is a sum of integers (so, ...
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0answers
25 views

Comparing wait-signal alternatives for synchronizing two piece of programs

I came across following problem: A certain computation generates two arrays a and b such that ...
2
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1answer
128 views

Designing solution to critical section problem with two semaphores vs single semaphore

I came across following problem: Let two processes P1 and P2 are there: ...
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0answers
26 views

Does this state certify to be called as a deadlock?

I came across following problem: Barrier is a synchronization construct where a set of processes synchronizes globally i.e., each process in the set arrives at the barrier and waits for all others ...
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4answers
3k views

Whats exact definition of 'atomicity' in programming?

the definition of 'atomicity' says that a transaction should be able to be terminated without being touched or manipulated by possibly concurrent running actions during its process. But does that also ...
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1answer
85 views

Are two CCS processes equivalent with respect to weak bisimilarity if and only if they satisfy exactly the same set of HML formulas?

I was skimming this recent paper and I was struck by the following statement: two processes are equivalent with respect to weak bisimilarity if and only if they satisfy exactly the same set of HML ...
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58 views
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1answer
245 views

Can a schedule be conflict serializable but not serializable?

A schedule is called conflict serializable if it can be transformed into a serial schedule by swapping non-conflicting operations. Then, My question is: Can a schedule be conflict serializable but ...
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1answer
586 views

What is a counterexample for Lamport's distributed mutual exclusion algorithm with non-FIFO message queues?

Lamport's distributed mutual exclusion algorithm (also described here) solves mutual exclusion problem for $N$ processes with $3(N-1)$ messages per request ("take and release lock" cycle). It ...

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