How can I determine the appropriate number of tasks with GCD or similar?

Testing, and profiling. What makes sense, and what works well is application specific.

Testing, and profiling. What makes sense, and what works well is application specific. Basically you need to decide on two things: The number of worker processes/threads to generate The size of the chunks they will work on Play with the two numbers, and calculate their throughput (tasks completed per second * number of workers).

Somewhere you'll find a good equilibrium between speed, number of workers, and number of tasks in a chunk. You can make finding the right balance even simpler by feeding your workers a bunch of test data, essentially a benchmark, and measuring their throughput automatically while adjusting these two variables. Record the throughput for each combination of worker size/task chunk size, and output it at the end.

The highest throughput is your best combination. Finally, if how long a particular task takes really depends on the task itself (e.g. Some tasks take X time, and while some take X*3 time, then you can can take a couple of approaches. Depending on the nature of your incoming work, you can try one of the following: Feed your benchmark historical data - a bunch of real-world data to be processed that represents the actual kind of work that will come into your worker grid, and measure throughput using that example data.

Generate random-sized tasks that cross the spectrum of what you think you'll see, and pick the combination that seems to work best on average, across multiple sizes of tasks If you can read the data in a task, and the data will give you an idea of whether or not that task will take X time, or X*3 (or something in between) you can use that information before processing the tasks themselves to dynamically adjust the worker/task size to achieve the best throughput depending on current workload. This approach is taken with Amazon EC2 where customers will spin-up extra VMs when needed to handle higher load, and spin them back down when load drops, for example. Whatever you choose, any unknown speed issue should almost always involve some kind of demo benchmarking, if the speed at which it runs is critical to the success of your application (sometimes the time to process is so small, that it's negligible).

Good luck!

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