Friday, November 13, 2009

OPL: Shared Queue, Speculation, & Digital Circuits

Shared Queue:

What experience have you had with shared queues, and what factors did you use to decide whether to use single vs. multiple queues?

+I do not have any experience with these. I have not had to do concurrent programming yet.

An enqueue or dequeue seems like a very small operation. Why is there so much concern over contention to the queue? What factors cause this?

+Although the act of enqueuing or dequeuing may seem quick or simple, there is obviously a drastically negative impact of having contention in these situations that could easily cause the queue to behave unexpectedly or enter an inconsistent state if locking is not done properly. You could also easily end up with a single queue being a severe bottleneck if the tasks are small, there are many workers, or both.

Seeing as how the authors lay out quite a bit of code to support the variations of these patterns, they seem to mitigate their own concern as to the value of "simple protocols.” Why would someone opt for the less efficient implementation, given that the concerns are already outlined, and sample code available?

+I cannot think of a good reason to use the simple protocols other than the fact that it may be easier for n00bs to debug.



Speculation:

Speculation seems like a straightforward pattern, though potentially a quite powerful one. What factors would you consider to decide if speculation is an appropriate/beneficial pattern to apply?

+The pattern boils down to optimistic parallelization. This reminds me of my computer arch class, 433, and our examination of branch prediction.

+You break sequential dependencies assuming that the majority of the time that breakage will not affect the correctness of the problem. In the cases where this does cause a problem, which should be a small percentage if you are applying this pattern correctly, the cost of going back and fixing those problems should be outweighed by the benefits gained from the optimistic parallelization.

+Regarding what factors need to be considered, I would like the ability either to know the statistical distributions of the inputs or to prototype a system in a reasonable amount of time to check the validity of apply this pattern to the given data or domain.

+I think the HTML lexer was extremely helpful edifying this pattern for me. While the abstract discussion all makes sense and seems well written, I just could not figure out what the tasks and dependencies could look like. I wish there were a few more examples that are concrete.

Seeing as how you may not have data about the statistical distribution of inputs to expect before you build the program, and therefore perhaps not know the average cost of rollback, etc., is it prudent to implement the speculation pattern at the outset, or wait to evolve the application to include speculative optimizations down the road? What if the performance increase that speculation has the potential to generate is necessary to meet the minimum requirements of the specification? Is speculation still a viable pattern to apply, or would effort be better spent on other parallel optimizations with more certain outcome?

+I do not think that it is prudent to implement this pattern if you have no concept of whether it will be successful or not. The same applies to almost any architectural or design decisions that you would make.



Circuits:

Was this pattern what you expected from the name?

+Nope. I am not sure how the name fits the pattern that well. Sure, we are operating on the bits as circuits in a CPU would, but I feel that I had to think about how the name & pattern related rather than it being something more obvious. Then again, it does attempt to create an analogy with something that we all know.

It seems like a great deal of this pattern is described through examples of clever transformations for specific problems. Do these examples give you enough information to go about applying this pattern to a problem dissimilar to the given examples? Can you think of a way of describing these transformations at a more abstract yet actionable level than the examples do?

+I could theoretically think of creative ways to map other problems onto this pattern, but this pattern seems like such an extreme way to implement a pattern, that I imagine it would be a last resort that would only be considered if all other parallelization attempts were inadequate or unsuccessful. Though I agree that using 32 or 64 bits to represent what only needs 1, I feel that you would really have to be trying to wring every idle cycle out of your systems for this pattern to be something that you would turn to.

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