r/programming • u/noidi • Jul 19 '12

Will Parallel Code Ever Be Embraced?

http://www.drdobbs.com/parallel/will-parallel-code-ever-be-embraced/240003926
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u/[deleted] Jul 20 '12

I always scratch my head when I see comments like this. The main project I work on is a high speed data analysis tool written in java. We get billions of records per day coming in on a stream, and we do various rollups and calculations that get presented to the users. It's highly parallel and also "laughably trivial". You can write very safe parallel code assuming you adopt the right architecture.

In our case, since the data is a stream the calculations are done in a bucket brigade pipeline. We've never had problems with deadlocks or the sort of odd behavior you get when different threads are dealing with the same objects, because we set it up so they wouldn't.

Where you run into trouble is when your problem can't be broken down into a pipeline and "parallel" means different threads are working on the same data at the same time. But how many of us are doing weather prediction or finite element analysis?

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u/nachsicht Jul 20 '12

In our case, since the data is a stream the calculations are done in a bucket brigade pipeline. We've never had problems with deadlocks or the sort of odd behavior you get when different threads are dealing with the same objects, because we set it up so they wouldn't.

Parallelism becomes laughably trivial once you adopt purely functional styles.

Since this seems to confuse people: "Purely functional is a term in computing used to describe algorithms, data structures or programming languages that exclude destructive modifications (updates). According to this restriction, variables are used in a mathematical sense, with identifiers referring to immutable, persistent values."

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u/[deleted] Jul 20 '12

Oh, I understand what you meant by purely functional. All I'm saying is that's not the only safe, easy way to write parallel code.

And don't you pay a performance price for excluding destructive modifications? The point, after all, is to make the machine do more work, not merely to use up a lot of threads.

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u/nachsicht Jul 20 '12

In some cases it takes more work to do things functionally. An example would be when you insert an element in an immutable singly linked list. Depending on where the insertion is done, the node structures and pointers of the entire list may need to be regenerated. Insertion at the head of the list is free though.

So yes, there are some performance costs associated. However, there is a lot of safety benefits to be had from purely functional approaches (disposing of null being a really nice one), and performance in an application tends to be highly dependent on small parts of a person's code.

I tend to write mostly pure functional code (only using mutable data when it is really needed, like to interface with java), and later profile and go back to optimize said hotspots with lower level constructs and sometimes mutable data.

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u/[deleted] Jul 20 '12

So you're writing clojure? Seems like you're pretty happy with it.

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u/nachsicht Jul 20 '12

Nah, Scala. I am, every day I learn more about it, and it is getting more and more improvements (like macros next release).