What is the difference between an on-line and off-line algorithm?

Question

These terms were used in my data structures textbook, but the explanation was very terse and unclear. I think it has something to do with how much knowledge the algorithm has a

Answer 1

An online algorithm processes the input only piece by piece and doesn't know about the actual input size at the beginning of the algorithm.

An often used example is scheduling: you have a set of machines, and an unknown workload. Each machine has a specific speed. You want to clear the workload as fast as possible. But since you don't know all inputs from the beginning (you can often see only the next in the queue) you can only estimate which machine is the best for the current input. This can result in non-optimal distribution of your workload since you cannot make any assumption on your input data.

An offline algorithm on the other hand works only with complete input data. All workload must be known before the algorithm starts processing the data.

Example:

Workload:
   1. Unit (Weight: 1)
   2. Unit (Weight: 1)
   3. Unit (Weight: 3)
Machines:
   1. Machine (1 weight/hour)
   2. Machine (2 weights/hour)

Possible result (Online):
   1. Unit -> 2. Machine  // 2. Machine has now a workload of 30 minutes
   2. Unit -> 2. Machine  // 2. Machine has now a workload of one hour
  either
   3. Unit -> 1. Machine  // 1. Machine has now a workload of three hours
  or 
   3. Unit -> 2. Machine  // 1. Machine has now a workload of 2.5 hours

 ==> the work is done after 2.5 hours

Possible result (Offline):
   1. Unit -> 1. Machine  // 1. Machine has now a workload of one hour
   2. Unit -> 1. Machine  // 1. Machine has now a workload of two hours
   3. Unit -> 2. Machine  // 2. Machine has now a workload of 1.5 hours

 ==> the work is done after 2 hours

Note that the better result in the offline algorithm is only possible since we don't use the better machine from the start. We know already that there will be a heavy unit (unit 3), so this unit should be processed by the fastest machine.

Answer 2

An on-line algorithm is one that receives a sequence of requests and performs an immediate action in response to each request.

In contrast,an off-line algorithm performs action after all the requests are taken.

This paper by Richard Karp gives more insight on on-line,off-line algorithms.

Answer 3

Cache Miss problem: In a computer system, cache is a memory unit used to avoid the speed mismatch between the faster processor and the slowest primary memory. The objective of using cache is to minimize the average access time by keeping some frequently accessed pages in the cache. The assumption is that these pages may be requested by the processor in near future. Generally, when a page request is made by the processor then the page is fetched from the primary or secondary memory and a copy of the page is stored in the cache memory. Suppose, the cache is full, then the algorithm implemented in the cache has to take immediate decision of replacing a cache block without knowledge of future page requests. The question arises: which cache block has to be replaced? (In worst case, it may happen that you replace a cache block and very next moment, the processor request for the replaced cache block). So, the algorithm must be designed in such a way that it take immediate decision upon the arrival of an incoming request with out advance knowledge of entire request sequence. This type of algorithms are known as ONLINE ALGORITHM

Answer 4

Wikipedia

The Wikipedia page is quite clear:

In computer science, an online algorithm is one that can process its input piece-by-piece in a serial fashion, i.e., in the order that the input is fed to the algorithm, without having the entire input available from the start. In contrast, an offline algorithm is given the whole problem data from the beginning and is required to output an answer which solves the problem at hand. (For example, selection sort requires that the entire list be given before it can sort it, while insertion sort doesn't.)

Let me expand on the above:

An offline algorithm requires all information BEFORE the algorithm starts. In the Wikipedia example, selection sort is offline because step 1 is Find the minimum value in the list. To do this, you need to have the entire list available - otherwise, how could you possibly know what the minimum value is? You cannot.

Insertion sort, by contrast, is online because it does not need to know anything about what values it will sort and the information is requested WHILE the algorithm is running. Simply put, it can grab new values at every iteration.

Still not clear?

Think of the following examples (for four year olds!). David is asking you to solve two problems.

In the first problem, he says:

"I'm, going to give you two balls of different masses and you need to drop them at the same time from a tower.. just to make sure Galileo was right. You can't use a watch, we'll just eyeball it."

If I gave you only one ball, you'd probably look at me and wonder what you're supposed to be doing. After all, the instructions were pretty clear. You need both balls at the beginning of the problem. This is an offline algorithm.

For the second problem, David says

"Okay, that went pretty well, but now I need you to go ahead and kick a couple of balls across a field."

I go ahead and give you the first ball. You kick it. Then I give you the second ball and you kick that. I could also give you a third and fourth ball (without you even knowing that I was going to give them to you). This is an example of an online algorithm. As a matter of fact, you could be kicking balls all day.

I hope this was clear :D

Answer 5

We can differentiate offline and online algorithms based on the availability of the inputs prior to the processing of the algorithm.

Offline Algorithm: All input information are available to the algorithm and processed simultaneously by the algorithm. With the complete set of input information the algorithm finds a way to efficiently process the inputs and obtain an optimal solution.

Online Algorithm: Inputs come on the fly i.e. all input information are not available to the algorithm simultaneously rather part by part as a sequence or over the time. Upon the availability of an input, the algorithm has to take immediate decision without any knowledge of future input information. In this process, the algorithm produces a sequence of decisions that will have an impact on the final quality of its overall performance.

Eg: Routing in Communication network:

Data Packets from different sources come to the nearest router. More than one communication links are connected to the router. When a new data packet arrive to the router, then the router has to decide immediately to which link the data packet is to be sent. (Assume all links are routed to the destination, all links bandwidth are same, all links are the part of the shortest path to the destination). Here, the objective is to assign each incoming data packet to one of the link without knowing the future data packets in such a way that the load of each link will be balanced. No links should be overloaded. This is a load balancing problem.

Here, the scheduler implemented in the router has no idea about the future data packets, but it has to take scheduling decision for each incoming data packets. In the contrast a offline scheduler has full knowledge about all incoming data packets, then it efficiently assign the data packets to different links and can optimally balance the load among different links.

Answer 6

An algorithm is said to be online if it does not know the data it will be executing on beforehand. An offline algorithm may see all of the data in advance.