When should I use AsParallel() in linq/plinq

Question

I'm looking make use of the advantages of parallel programming in linq by using plinq, im not sure I understand the use entirely apart from the fact its going to make use of all cpu cores more efficiently so for a large query it might be quicker. Can I just simply call AsParallel() on linq calls to make use of th eplinq functionality and it will always be quicker? Or should I only use it when there is a lot of data to query or process?

Answer 1

You can't just assume that execution in parallel is always faster. It depends. In some situations you will gain a lot on multi-core processors by doing things in parallel. In other cases, you will just slow the things down, since parallel loops have a small overheat over simple loops.

For example, see my other answer which explains why embedded parallel loops can be a disaster.

Now, the best way to know if it is a good idea to use parallel loop in a precise context is to test both parallel and not parallel implementations and to measure the time they take.

Answer 2

To further add to the answer, it also depends on your data. Going a little 'old school' for a moment you could go down the road of loop unrolling, using for instead of foreach and so on and so forth.

However, you really need to ensure you aren't micro-optimising. Depending on your data fetches and the size of data (certainly with paged data) then you can probably get away with not using it.

That's not to say that making your linq mult-core aware isn't cool. But be aware of the setup costs of doing something like that and so be able to weigh up the benefits against the complexities of maintaining and debugging that code.

If your algorithm is already top notch then looking at the plinq extensions, a map reduce mechanism or similar may be the way to go. But first check your algorithm and your overall benefits. Operating on the right kind of collection (etc) in the right kind of way will always bring its own benefits (and problems!).

What are you trying to solve?