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Tuesday, April 3, 2007

Planning a multi-threaded application

I have used analogies to teach programming to people for over two decades. A while ago I realized that analogies aren't only good for teaching but sometimes you can apply the concepts from something completely different to programming (and you can apply programming concepts to other aspects of life).

For example, I used to talk about Relational Database Management Systems (RDBMS) using file cabinets, drawers, folders, index cards, etc. and I realized that systems libraries used when I was a kid (before computers were affordable) actually worked and the concepts helped to form RDBMS we see today. In other words, before computers existed, secretaries, clerks, librarians, etc. had to keep track of data (books, client information, etc.). If I wanted to search for a book by author they had a set of index cards sorted by author. If I wanted to search for a book by title they had a set of index cards sorted by title. The actual books were sorted, on the shelves, according to the Dewey Decimal System. If a book was added to the library, they had to insert it on a shelf (leave room for new books so you don't have to shift the entire collection) and an index card for each index was created. Same sort of thing happens for a database. You insert the record for the book in the database and then you update all the indices.

So, how does this relate to multi-threaded applications? Well, we just need to look at a multi-threaded application differently. A professor at U of T used things like Human, Male, Female, Baby, etc. or Animal, Mammal, etc. to teach object oriented design (OOD). I learned to used project management software; people were 'resources'. You planned the activities of resources. You created dependencies between two resources, e.g. Karen might be working on something but she need work from Bob opn day 4. So at day 4 there is a dependency between Bob and Karen. If the work Bob is doing will take 9 days, I want Bob to start 5 days before Karen or I want someone to help Bob so it will get done early or I want work for Karen to do while she is waiting for Bob to finish. Basic techniques to ensure my 'resources' are used efficiently.

What if we look at the threads in an application as 'resources' and use project planning software to make sure all the different threads are working together well. Or what about systems with multiple processors. Each processor could be a resource and I want to make sure they are utilitized efficiently. If the math co-processor will take 9 seconds to complete a computation and the main cpu will need the result in 4 seconds, I have the same options as with Karen and Bob. I can start the math computation 5 seconds before the cpu needs it, I can have 5 seconds of other work for the cpu while it waits for the math computation, I can put more math co-processors. As I type this I also realized, I could put a faster math co-processor. This means for the human scenario, I can put someone who is faster than Bob on the task so it will get completed in 4 days rather than 9 days.

So for everyone who thinks learning how to use project management software is only for managing project, think again.

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