computer science

Noop scheduler

Noop scheduler is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Noop scheduler rather than just read about it. In short: The NOOP scheduler is the simplest I/O scheduler for the Linux kernel. This scheduler was developed by Jens Axboe.

Key takeaways

Noop scheduler belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Noop scheduler to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Noop scheduler from memory before moving on to harder problems.

Reference excerpt

The NOOP scheduler is the simplest I/O scheduler for the Linux kernel. This scheduler was developed by Jens Axboe.

Overview The NOOP scheduler inserts all incoming I/O requests into a simple FIFO queue and implements request merging. This scheduler is useful when it has been determined that the host should not attempt to re-order requests based on the sector numbers contained therein. In other words, the scheduler assumes that the host is unaware of how to productively re-order requests. There are (generally) three basic situations where this situation is desirable:

If I/O scheduling will be handled at a lower layer of the I/O stack. Examples of lower layers that might handle the scheduling include block devices, intelligent RAID controllers, Network Attached Storage, or an externally attached controller such as a storage subsystem accessed through a switched Storage Area Network. Since I/O requests are potentially rescheduled at the lower level, resequencing IOPs at the host level uses host CPU time on operations that will just be undone at the lower level, increasing latency/decreasing throughput for no productive reason. Because accurate details of sector position are hidden from the host system. An example would be a RAID controller that performs no scheduling on its own. Even though the host has the ability to re-order requests and the RAID controller does not, the host system lacks the visibility to accurately re-order the requests to lower seek time. Since the host has no way of judging whether one sequence is better than another, it cannot restructure the active queue optimally and should, therefore, pass it on to the device that is (theoretically) more aware of such details. Because read/write head movement doesn't impact application performance enough to justify the reordering overhead. This is usually the case with non-rotational media such as flash drives or solid-state drives (SSDs). However, NOOP is not necessarily the preferred I/O scheduler for the above scenarios. Typical to performance tuning, all guidance shall be based on observed work load patterns (undermining one's ability to create simplistic rules of thumb). If there is contention for available I/O bandwidth from other applications, it is still possible that other schedulers will generate better performance by virtue of more intelligently carving up that bandwidth for the applications deemed most important. For example, running an LDAP directory server may benefit from deadline's read preference and latency guarantees. At the same time, a user with a desktop system running many different applications may want to have access to CFQ's tunables or its ability to prioritize bandwidth for particular applications over others (ionice). If there is no contention between applications, then there are little to no benefits from selecting a scheduler for the above-listed three scenarios. This is due to a resulting inability to deprioritize one workload's operations in a way that makes additional capacity available to another workload. In other words, if the I/O paths are not saturated and the requests for all the workloads fail to cause an unreasonable shifting around of drive heads (which the operating system is aware of), the benefit of prioritizing one workload may create a situation where CPU time spent scheduling I/O is wasted instead of providing desired benefits.

The Linux kernel also exposes the nomerges sysfs parameter as a scheduler-agnostic configuration, making it possible for the block layer's requests merging logic to be disabled either entirely, or only for more complex merging attempts. This reduces the need for the NOOP scheduler as the overhead of most I/O schedulers is associated with their attempts to locate adjacent sectors in the request queue in order to merge them. However, most I/O workloads benefit from a certain level of requests merging, even on fast low-latency storage such as SSDs.

Illustrations

Worked examples

Example 1 — a first encounter with Noop scheduler

Start with the simplest possible case. Write down what Noop scheduler claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Noop scheduler before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Noop scheduler ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Noop scheduler

In research: Noop scheduler appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Noop scheduler in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Noop scheduler is common in secondary-school and first-year university syllabi. It links to neighbouring topics Disk scheduling algorithms, so understanding it makes those chapters shorter.
In everyday life: Look for Noop scheduler outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Noop scheduler in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Noop scheduler means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Noop scheduler out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Noop scheduler in simple terms?

The NOOP scheduler is the simplest I/O scheduler for the Linux kernel. This scheduler was developed by Jens Axboe.

Why does Noop scheduler matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Noop scheduler?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Noop scheduler.