Question

I’ve implemented something similar to consumer-producer problem using a unbounded linked blocking queue. I have the producer putting objects to the queue and consumers taking it. When I tested the program, doubling the amound of thread each trial while still processing the same amount of objects, the time for all trials seems to be constant. Is it suppose to be constant? Or more thread means faster processing? Not sure if it is my code that is causing the slowness or the synchonization for the shared resource. Any ideas?

Answer 1

It entirely depends on what the bottleneck is:

• If the consumers are processing the elements as fast as the producer is producing them, then adding more consumers won’t help
• If the consumers are the bottleneck, then adding more producers will just mean you build up a backlog of work
• If the consumers are all sharing a single resource which is already maxed out (e.g. a saturated network connection or disk) then adding more threads won’t help
• If the consumers synchronize on a shared resource which forces them to work in a serial way for a large portion of the time, then adding more threads won’t help
• If the consumers are CPU-bound and you’ve got enough threads to already max out your CPU usage, then adding more threads won’t help

You need to look at what’s going on while your program is running:

• What does the length of your work queue look like? Does it just keep growing? Is it always close to 0?
• What does your CPU usage look like? What about network / disk usage?

Then analyze what your code is doing, and work out what how parallelizable you expect the problem to be