21 lines
1.8 KiB
XML
21 lines
1.8 KiB
XML
= Homework 2
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My algorithm works like this:
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- First I generate $n/p$ integers on each process.
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- Then I jump directly into the recursive step:
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- I choose the pivot using the algorithm where each process picks a random element, and the median of those is picked.
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- The way I moved $S$ and $L$ arrays around is:
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1. First `MPI_Alltoallv` the plan for _which_ processors are going to be sent to, including exact calculations of which local index is being copied from and to.
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2. Then, each processor loops through all the processors and if they have something to send, they send it.
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3. This way, I can coordinate all of the senders/receivers and the ones with nothing to send don't do anything.
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- For the recursion, I opted to make the recursive step have different lengths. (*NOTE:* The reason I have a different "capacity" than "length" is because for the `displs` array I opted to have them all be the same length, so there's extra padding on the shorter ones)
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- If the boundary between $S$ and $L$ falls between a $n/p$ segment, I'd extend the one before and shorten the one after.
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- Then, I recursively process all the $S$'s and all the $L$'s separately using `MPI_Comm_split`.
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- Once it's done processing, I reverse the exact operation that extends / shortens the arrays. This ensures everything is always back to $n/p$ at the end.
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- Everything is collected back at the end via a `Send`/`Recv` to save on allocations.
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Allocations are all on the order of $O(p + n/p)$.
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Unfortunately I didn't finish debugging segfaults in time, and have this report prepared for the parts of the assignment that I _did_ do. It works on small integers (capped at 100) but for some reason segfaults at address `(nil)` at the end... I spent several hours debugging but have not discovered how this occurs.
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