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Questions tagged [false-sharing]

False sharing is the condition, where in parallel programs, memory cache lines are shared by two or more threads and writes on one cache line would force other cores working on the same line to re-validate their cache. This is a concurrency anti-pattern.

Questions with this tag should be about a suspected or actual false sharing problem.

False sharing is the condition in which in parallel programs, in which memory cache lines which are shared by two or more threads. Writes on one cache line would force other cores working in the same line to re-validate their cache. This is a concurrency anti-pattern.

enter image description here

Note that in the diagram above, Thread 1 writes to A and never B, yet Thread 2 must re-validate its cache to continue computation.

Common ways to alleviate false sharing include storing a thread local result to update to a shared spaced once the computation is completed, and/or spacing contiguous memory blocks that are shared, so they are not on the same cache line.

More information:

Wikipedia

C++ Today Blog Article

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does false sharing occur when data is read in openmp?

If I have a C++ program with OpenMP parallelization, where different threads constantly use some small shared array only for reading data from it, does false sharing occur in this case? in other words, is false sharing related only to memory write…
John Smith
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Increased speed despite false sharing

I've been doing some tests on OpenMP and made this program that should not scale because of false sharing of the array "sum". The problem I have is that it does scale. Even "worse": with 1 thread: 4 seconds (icpc), 4 seconds (g++) with 2 threads: 2…
InsideLoop
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Does false sharing also occur when threads only write to the same cache block?

If we have two cores which read and write to different memory position in the same cache block, both cores are forced to reload that cache block again and again, although it is logically not necessary. This is what we call false sharing. However,…
user1494080
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What is "False Sharing" in Parallel programming .net 4.0

Can any one please share me the knowledge of "False Sharing" in Parallel programming .net 4.0 ? Would be great if you can explain with an example. Thanks in advance . i want the maximum performance for my code .
NO Name
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False sharing and volatile

Good day, I recently found an annotation introduced in Java 8 called Contended. From this mailing list I read what is false sharing and how annotation allows objects or fields to allocate an entire cache line. After some research I found that if two…
Almas Abdrazak
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Loading an entire cache line at once to avoid contention for multiple elements of it

Assuming that there are three pieces of data that I need from a heavily contended cache line, is there a way to load all three things "atomically" so as to avoid more than one roundtrip to any other core? I don't actually need a correctness…
Curious
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False sharing of guarded member variables?

Consider: class Vector { double x, y, z; // … }; class Object { Vector Vec1, Vec2; std::mutex Mtx1, Mtx2; void ModifyVec1() { std::lock_guard Lock(Mtx1); /* … */ } void ModifyVec2() { std::lock_guard Lock(Mtx2); /* … */ } }; If either…
metalfox
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Dose Segment in ConcurrentHashMap has false sharing problems?

java.util.concurrent.ConcurrentHashMap uses a Segment array as Mutexand Segment Object is small than cache line. Does this lead to false sharing?
user6102088
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Can't reproduce false cache line sharing problem in Rust

I'm trying to reproduce example 6 of the Gallery of Processor Cache Effects. The article gives this function (in C#) as an example how to test false sharing: private static int[] s_counter = new int[1024]; private void UpdateCounter(int position) { …
mvlabat
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Prevent False Sharing without using padding

I'm currently learning about pthreads in C and came across the issue of False Sharing. I think I understand the concept of it and I've tried experimenting a bit. Below is a short program that I've been playing around with. Eventually I'm going to…
Ardembly
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False sharing in Cuda GPUs: does it exist / similar to CPUs?

I understand that in symmetric multiprocessor (SMP) systems, false sharing may occur due to the individual caches in each cores, for the following code: http://software.intel.com/en-us/articles/avoiding-and-identifying-false-sharing-among-threads 01…
Qiangzini
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C++ Using `.reserve()` to pad `std::vector`s as a way of protecting against multithreading cache invalidation and false sharing

I have a program with the general structure shown below. Basically, I have a vector of objects. Each object has member vectors, and one of those is a vector of structs that contain more vectors. By multithreading, the objects are operated on in…
Matt Munson
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When shoud we use `CacheLinePad` to avoid false sharing?

It's well-known that using pad to make a struct exclusive one or more cache line is good for performance. But for what scene, we should add a pad like the following to improve performance? Are there some rules of thumb here? import…
wymli
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When examining False Sharing, why are there more L1d cache misses when running with sibling-threads than when running with independent threads

( I know that there have been a few somewhat related questions asked in the past, but I wasn't able to find a question regarding L1d cache misses and HyperThreading/SMT. ) After reading for a couple of days about some super interesting stuff like…
Zeosleus
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volatile increments with false sharing run slower in release than in debug when 2 threads are sharing the same physical core

I'm trying to test the performance impact of false sharing. The test code is as below: constexpr uint64_t loop = 1000000000; struct no_padding_struct { no_padding_struct() :x(0), y(0) {} uint64_t x; uint64_t y; }; struct padding_struct…
Yuki N
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