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Add section about false sharing problems to documentation #1819

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merged 5 commits into from
Feb 5, 2024
Merged

Add section about false sharing problems to documentation #1819

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193fa9f
Add section to docs about false sharing
bennibolm Jan 30, 2024
c3d24c9
Fix typos
bennibolm Jan 30, 2024
6fd8f77
Fix typo
bennibolm Jan 30, 2024
aa1949f
Merge branch 'main' into false-sharing-in-docs
bennibolm Feb 5, 2024
54114e4
Implement suggestions
bennibolm Feb 5, 2024
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11 changes: 11 additions & 0 deletions docs/src/performance.md
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Expand Up @@ -267,3 +267,14 @@ requires. It can thus be seen as a proxy for "energy used" and, as an extension,
timing result, you need to set the analysis interval such that the
`AnalysisCallback` is invoked at least once during the course of the simulation and
discard the first PID value.

## Performance issues with multi-threaded reductions
[False sharing](https://en.wikipedia.org/wiki/False_sharing) is a known performance issue
for systems with distributed caches. It also occurred for the implementation of a thread
parallel bounds checking routine for the subcell IDP limiting
in [PR #1736](https://github.com/trixi-framework/Trixi.jl/pull/1736).
After some [testing and discussion](https://github.com/trixi-framework/Trixi.jl/pull/1736#discussion_r1423881895),
it turned out that initializing a vector of length `n * Threads.nthreads()` and only using every
n-th entry instead of a vector of length `Threads.nthreads()` fixes the problem.
Since there are no processors with caches over 128B, we use `n = 128B / size(uEltype)`.
Now, the bounds checking routine of the IDP limiting scales as hoped.
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