Challenging Multithreading Problems

Is there some resource for challenging multi-threading problems? Would like to pose these to interviewees if possible. Tired of asking the same wai开发者_运维知识库t-notify questions that everyone gets right these days, but can't visualise a real scenario where multi-threading was employed.

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The problem is that concurrent programming is a difficult topic. If you (the interviewer) are not fully on top of it, it will be difficult for you to tell if the interviewee knows their stuff. It is very easy to come up with solutions to concurrency problems that have subtle flaws. Conversely, it is unfair on candidates¹ if you reject them because you think their answers are wrong when they are actually correct.

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^{1 - and bad for your organisation. If the candidate actually knows more about multi-threading than you, then you arguably need to employ him. Other factors being equal, of course.}

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Java Concurrency In Practice. I like to know if candidate understand data race, CAS, Michael Scott Queue and other concurrent data structures and how concurrent thread safety is important with growing number of cores.

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As multithreading is hard (as others have pointed out) I would suggest having this in an actual programming session where the potential employee is given a programming problem preferrably based on something that has actually happened along with one of your experienced programmers so you can actually SEE how the problem was attempted solved, and the experienced programmer can evaluate what happened.

Must not be too complex, but complex enough that your expereinced programmer get enough information.

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Well, if you want to have fun with the poor sap, ask him about Dekker's Algorithm (and Peterson's variation thereof). If you're feeling nasty, ask him if he has ever used either one on real multiprocessor hardware.

If you feel extra-nasty, ask him to show you a technique suitable for lock-free true concurrent single-reader single-writer unidirectional communications, between two processors with shared memory, in which the only atomic operations are single-word reads and writes. There is no read-modify-write instruction, on either side, and the processor architectures need not be the same. (Yes, such a technique exists.)

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I wouldn't ask too specific/detailed questions. But the above mentioned book 'Concurrency in practice' is a good helper. Just go there chapter-wise and read out the pin-points, e.g.:

• Explain difference between mutable/immutable
• What does it mean to share data in concurrency setup
• What problems do you solve with concurrency
• etc.

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First you try to get the real scenario of it, and then ask job seekers. For this you should pose a questions like what is real scenario for multithreading? Hope it will help you.

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I got one in a interview recently. Get the candidate to write a Servlet that implements an accurate in memory hit counter indexed by URL (to serve a javascript style hit counter on a number of web page). Try it for yourself, it's not as easy as it sounds. The solution is a cut down implementation of the Memoizer pattern from Concurrency in Practice.