Shulou(Shulou.com)05/31 Report--

This article mainly introduces "how to view the I/O scheduler of Linux system", in daily operation, I believe many people have doubts on how to view the I/O scheduler of Linux system, Xiaobian consulted all kinds of information, sorted out simple and easy to use operation methods, and hoped to answer the doubts of "how to view the I/O scheduler of Linux system"! Next, please follow the small series to learn together!

Introduction to Linux I/O System

The Linux I/O Scheduler is a component of the Linux I/O architecture that sits between the generic block layer and block device drivers. As shown in Figure 1.

Figure 1. Linux I/O scheduler sits between the generic block layer and block device drivers

When a Linux kernel component wants to read or write some data, the kernel does not execute the request as soon as it is issued, but delays its execution. When a new block of data is transmitted, the kernel needs to check if it can pass. The Linux IO scheduler is between the generic block layer and the block device driver, so it receives requests from the generic block layer, tries to merge requests, and finds the most appropriate request to send to the block device driver. The block device driver then calls a function in response to the request.

Linux overall I/O architecture can be divided into seven layers, they are:

VFS Virtual File System: The kernel has to deal with a variety of file systems. The kernel abstracts this VFS, which is specially used to adapt to various file systems and provide a unified operating interface.

Disk cache: Disk cache is a software mechanism that keeps some data on disk in RAM, which allows faster response times for accessing that data. There are three types of disk cache in Linux: Dentry cache, Page cache, Buffer cache.

Mapping Layer: The kernel reads data from the block device, so the kernel must determine where the data is located on the physical device, which is done by the Mapping Layer.

Generic Block Layer: Since most I/O operations deal with block devices, Linux provides an abstraction layer for block device operations similar to the vfs layer. The lower layer interfaces with block devices with different attributes, and provides a unified Block IO request standard on the upper layer.

I/O scheduling layer: Most block devices are disk devices, so it is necessary to set up different schedulers according to the characteristics of such devices and application characteristics.

Block device driver: Block device driver provides advanced device operation interface.

Physical hard disk: This layer is the physical device.

5 Types of Linux I/O Scheduler

Linux has supported I/O schedulers since kernel 2.4, and so far there are five types: Linux 2.4 Linux Elevator, Linux 2.6 Deadline, Anticipatory, CFQ, Noop, where Anticipatory has been removed since Linux 2.6.33. Most Linux distributions currently use Deadline, CFQ, and Noop I/O schedulers. The following is a brief introduction in turn:

1 Linus Elevator

In kernel 2.4 it is the *** I/O scheduler. Its main role is to maintain a query request for each device, and when the kernel receives a new request, merge it if it can. If they cannot merge, they try sorting. If it cannot be merged and there is no suitable place to insert, it is placed in the *** of the request queue.

2 Anticipatory

Anticipatory means "expected, expected". As the name implies, when an I/O occurs, if another process requests an I/O operation, it will generate a default 6 millisecond guess time to guess what the next process requests I/O. This I/O scheduler optimizes service time for read operations, making short waits while providing one I/O so that the process can commit to another I/O. The Anticipatory algorithm was removed from Linux version 2.6.33 because CFQ can be configured to achieve the Anticipatory effect.

3 DeadLine

Deadline is an improvement on Linus Elevator, which prevents some requests from being processed for too long. It is also possible to distinguish between read operations and write operations. DEADLINE additionally provides FIFO queues for read I/O and write I/O, respectively. Deadline workflow is shown in Figure 2.

Figure 2 Deadline workflow

4 CFQ

CFQ is called Completely Fair Scheduler, Chinese name completely fair scheduler, it is now the default scheduler of many Linux distributions, CFQ is the kernel default I/O scheduler. It places synchronous requests submitted by processes into multiple process queues and then allocates time slices to each queue to access disk. CFQ distributes access to I/O bandwidth evenly for general purpose servers that are *** optional. CFQ creates a queue for each process and thread to manage the requests generated by the process, so as to ensure that each process can be well allocated to the I/O bandwidth. The I/O scheduler executes 4 requests for a process at a time. The algorithm is characterized by sorting I/O requests by address rather than responding in first-come, first-served order. Simply put, all synchronization processes are allocated time slices before queuing for disk access. The CFQ workflow is shown in Figure 3.

Figure 3 CFQ workflow

5 NOOP

NOOP full name No Operation, Chinese name elevator scheduler, this algorithm realizes the simplest FIFO queue, all I/O requests roughly according to the order of first come, first served operation. NOOP implements a simple FIFO queue that organizes I/O requests like an elevator's workhorse. It is the simplest I/O scheduler in the Linux kernel based on the concept of first-in, first-out (FIFO) queues. This scheduler is best suited for solid state drives. The workflow of NOOP is shown in Figure 4.

Figure 4 NOOP workflow

I/O scheduler selection

At present, mainstream Linux distributions use three I/O schedulers: DeadLine, CFQ and NOOP. Generally speaking, Deadline is suitable for most environments, especially file servers with a large number of writes. In principle, DeadLine is a scheduling algorithm that takes improving the throughput of mechanical hard disks as the starting point. It tries to ensure that I/O requests are scheduled when they reach the deadline. It is very suitable for businesses with single services and heavy I/O pressure, such as Web servers and database applications. CFQ allocates equal bandwidth to all processes, suitable for multi-user systems with a large number of processes. CFQ is a relatively general scheduling algorithm. It is a scheduling algorithm that takes processes as the starting point to ensure fairness as much as possible. Allocate equal bandwidth to all processes, suitable for desktop multitasking and multimedia applications. NOOP is the best choice for flash devices and embedded systems. For SSDs, NOOP is ***, DeadLine is second, and CFQ efficiency is ***.

View I/O Scheduler for Linux Systems

Viewing the I/O scheduler of a Linux system is generally divided into two parts, one is to view the I/O scheduler used by the Linux system as a whole, and the other is to view the I/O scheduler used by a disk.

To view the currently supported I/O schedulers, use the following command:

Listing 1. View currently supported I/O schedulers

# dmesg | grep -i scheduler [ 1.508820] io scheduler noop registered [ 1.508827] io scheduler deadline registered [ 1.508850] io scheduler cfq registered (default)

The code in Listing 1 shows that cfq is the current I/O scheduler.

To view the IO scheduling algorithm I/O scheduler of a hard disk, use the following command:

Listing 2. View the I/O scheduler used by a hard disk

# cat /sys/block/sda/queue/scheduler noop deadline [cfq]

Listing 2 shows that the scheduler currently in use is cfq, the one enclosed in parentheses.

Modify I/O scheduler of Linux system

There are three ways to modify the Linux I/O scheduler: using shell commands, using grubby commands, or modifying the grub configuration file. The following are introduced in turn:

Using shell commands

Changing the I/O scheduler under Linux is simple. No kernel update required, shell commands can be used to modify:

listing 3. Use the shell command.

#echo noop > /sys/block/sdb/queue/scheduler

The command in Listing 3 sets noop to a disk I/O scheduler that you can change at any time without restarting your computer.

*** Modify default I/O scheduler

Use shell command to modify I/O scheduler, only temporary modification, after system restart, modified scheduler will be invalid, to modify the default scheduler, there are two ways to use grubby command or directly edit the grub configuration file.

Using the grubby command

For example, to adjust the I/O scheduler from cfq to DeadLine, the command is as follows:

Listing 4. Using the grubby command

# grubby --grub --update-kernel=ALL --args="elevator=deadline"

The command in Listing 4 sets kernel load parameters so that when the machine restarts, the system automatically changes the I/O scheduler for all devices to DeadLine.

Modify a profile using the editor

You can also edit the grub configuration file directly. By modifying the grub configuration file, the system automatically changes the I/O scheduler of all devices to cfq. The operation process is as follows:

Listing 5 Using vi editor to modify grub configuration files

#vi cat /etc/default/grub #Modify the fifth line, add # elevator= cfq at the end of the line and save the file, recompile the configuration file,#grub2-mkconfig -o/boot/grub2/grub.cfg

Restart your computer system.

At this point, the study of "how to view the I/O scheduler of Linux system" is over, hoping to solve everyone's doubts. Theory and practice can better match to help everyone learn, go and try it! If you want to continue learning more relevant knowledge, please continue to pay attention to the website, Xiaobian will continue to strive to bring more practical articles for everyone!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.