Questions tagged [concurrency]
Questiont about issues of concurrency such as synchronization and deadlocks.
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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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8 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
71 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
26 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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31 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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25 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
23 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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20 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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27 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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31 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
47 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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26 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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84 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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25 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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38 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
115 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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24 views
Strategies for event-driven concurrency
Is there any good formal investigation or at least definition of different strategies of event-driven concurrent programming?
I've read that there exist readiness based (pull based) and completion ...
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1answer
51 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
207 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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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
132 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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46 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
40 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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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
60 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
187 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
138 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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38 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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84 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
526 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
28 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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60 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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24 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
97 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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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
2k 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
68 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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214 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
467 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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48 views
Prove that set of operations form a commutative Monoid
this is my first post on this exchange.
I am looking for some help with defining a proof that a set of operations I have designed forms a commutative monoid.
(Disclaimer: I am not sure that I have ...
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2answers
61 views
System restarted before transaction is committed
Consider the following log sequence of two transactions on a bank account, with initial balance 12000, that transfer 2000 to a mortgage payment and then apply a 5% interest.
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1answer
124 views
Can 2-philosophers problem be analogous to dining philosophers problem?
Two philosophers A and B, two forks numbered "1", "2"; A needs both "1" and "2" for eating, so does B.
Is this theoretical a dining philosopher problem? I'm questioning about the forks philosopher A ...
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97 views
How can I implement locking a range of values in a sorted sequence?
I am trying to write a general purpose locking algorithm which allows me to lock a range of contiguous values in a sorted array. To start with, these could be exclusive locks - so two concurrent locks ...
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1answer
2k views
Thoughts on definition for disadvantage of concurrent processing?
I'm a Computer Science student currently learning about concurrent processing. I found the benefits and disadvantages in my book vague, did some online research and put together a more detailed way of ...
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2answers
68 views
What is the formal statement of the dining philosopher's problem?
I've read about it in a few places and I'm not sure I get it.
Are the philosopher's allowed to act simultaneously?
Do they each take 1 action simultaneously, then go on to their next action?
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69 views
Do we really need a mutex in this case?
Wikipedia's solution for multi Producer-Consumer problem uses both mutexes and semaphores.
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1answer
697 views
Which Peterson’s Solution is right?
I have seen two different Peterson’s Solution to mutual exclusion, one is from Modern Operating Systems:
...
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189 views
how to find the depth of a concurrency bug
How to find the depth of a concurrency bug
The below program runs on two separate threads and shares the variable x. Variables t1 and t2 are local to their threads. The program has a concurrency bug ...
