github.com/gammazero/workerpool (go): Package workerpool queues work to a limited number of goroutines.  The purpose of the worker pool is to limit the concurrency of tasks executed by the workers. This is useful when performing tasks that require sufficient resources (CPU, memory, etc.), and running too many tasks at the same time would exhaust resources.  A task is a function submitted to the worker pool for execution. Submitting tasks to this worker pool will not block, regardless of the number of tasks. Incoming tasks are immediately dispatched to an available worker. If no worker is immediately available, or there are already tasks waiting for an available worker, then the task is put on a waiting queue to wait for an available worker.  The intent of the worker pool is to limit the concurrency of task execution, not limit the number of tasks queued to be executed. Therefore, this unbounded input of tasks is acceptable as the tasks cannot be discarded. If the number of inbound tasks is too many to even queue for pending processing, then the solution is outside the scope of workerpool. It should be solved by distributing load over multiple systems, and/or storing input for pending processing in intermediate storage such as a database, file system, distributed message queue, etc.  This worker pool uses a single dispatcher goroutine to read tasks from the input task queue and dispatch them to worker goroutines. This allows for a small input channel, and lets the dispatcher queue as many tasks as are submitted when there are no available workers. Additionally, the dispatcher can adjust the number of workers as appropriate for the work load, without having to utilize locked counters and checks incurred on task submission.  When no tasks have been submitted for a period of time, a worker is removed by the dispatcher. This is done until there are no more workers to remove. The minimum number of workers is always zero, because the time to start new workers is insignificant.  It is advisable to use different worker pools for tasks that are bound by different resources, or that have different resource use patterns. For example, tasks that use X Mb of memory may need different concurrency limits than tasks that use Y Mb of memory.  When there are no available workers to handle incoming tasks, the tasks are put on a waiting queue, in this implementation. In implementations mentioned in the credits below, these tasks were passed to goroutines. Using a queue is faster and has less memory overhead than creating a separate goroutine for each waiting task, allowing a much higher number of waiting tasks. Also, using a waiting queue ensures that tasks are given to workers in the order the tasks were received.  This implementation builds on ideas from the following:  http://marcio.io/2015/07/handling-1-million-requests-per-minute-with-golang http://nesv.github.io/golang/2014/02/25/worker-queues-in-go.html
Keywords: concurrency|worker-pool