75 lines
2.5 KiB
Markdown
75 lines
2.5 KiB
Markdown
---
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title: tdt4200ex3 -- 1d wave equation mpi
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date: 2025-09-22
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author: fredrik robertsen
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---
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## theory questions
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### 1)
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by parallelizing the code i get quite a speed-up when running four processes.
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this is as i expected, because my laptop has four cores available for parallel
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processing. i have done threading previously, which also gave a speed-up for
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four threads. thus it makes sense that we get a speed-up here. it means we must
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have more intensive calculations than the overhead of using mpi.
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here's an excerpt from `make check`:
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```
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./sequential
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Total elapsed time: 5.115875 seconds
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mpiexec -n 1 --oversubscribe ./parallel
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Total elapsed time: 5.347622 seconds
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mpiexec -n 4 --oversubscribe ./parallel
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Total elapsed time: 1.729224 seconds
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mpiexec -n 13 --oversubscribe ./parallel
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Total elapsed time: 2.717839 seconds
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```
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assuming i have implemented timing correctly, mpi is slightly slower with only
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a single process, as expected. similarly, for 13 processes we get a speed-up,
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but not by as much due to the overhead of having more processes that are not
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necessarily capable of running in parallel -- they are limited by my hardware.
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### 2)
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this is a ring-buffer (halo) topology 3-stencil. a process only communicates
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with its nearest neighbor. the calculations are a form of finite differences.
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### 3)
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_well-spoken [AI](https://kagi.com) answer:_
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**point-to-point communication:**
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- communication between **exactly two processes** (sender ↔ receiver)
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- examples: `mpi_send`, `mpi_recv`, `mpi_sendrecv`
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**collective communication:**
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- communication involving **all processes** in a communicator
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- examples: `mpi_bcast`, `mpi_reduce`, `mpi_allreduce`, `mpi_gather`, `mpi_scatter`
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**key differences:**
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| aspect | point-to-point | collective |
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| ------------------- | -------------- | ---------------- |
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| **participants** | 2 processes | all processes |
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| **synchronization** | optional | required |
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| **optimization** | manual | automatic |
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| **flexibility** | high | limited patterns |
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### 4)
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`int* a, b` is a classic c pitfall. my auto-formatter is configured to correctly
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format such a string to `int *a, b`, since pointers are a part of the variable
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name definition, not the type. as such, `b` is merely an `int`.
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this is slightly relevant for the entertaining c-ism `buffer[0] == 0[buffer]`
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(this relates to the fact that indexing brackets are syntactic sugar for pointer
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addition, which is commutative).
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