Deadline scheduler
Encyclopedia
Deadline scheduler is an I/O scheduler
for the Linux kernel
which was written in 2002 by Jens Axboe
.
Before serving the next request, the Deadline scheduler decides which queue to use. Read queues are given a higher priority, because processes
usually block on read operations. Next, the Deadline scheduler checks if the first request in the deadline queue has expired. Otherwise, the scheduler serves a batch of requests from the sorted queue. In both cases, the scheduler also serves a batch of requests following the chosen request in the sorted queue.
By default, read requests have an expiration time of 500 ms, write requests expire in 5 seconds.
An early version of the scheduler was published by Axboe in September 2002.
The kernel documentation suggest this is the preferred scheduler for database
systems, especially with Tagged Command Queuing
(TCQ) aware disks, or any system with large numbers of disks.
I/O scheduling
Input/output scheduling is a term used to describe the method computer operating systems decide the order that block I/O operations will be submitted to storage volumes...
for the Linux kernel
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
which was written in 2002 by Jens Axboe
Jens Axboe
Jens Axboe is a Linux kernel hacker. He is the current Linux kernel maintainer of the block layer and other block devices, along with contributing the CFQ I/O scheduler, Noop scheduler, Deadline scheduler and splice IO architecture. Jens is also the author of the blktrace utility and kernel parts,...
.
Overview
The goal of the Deadline scheduler is to guarantee a start service time for a request. It does that by imposing a deadline on all I/O operations to prevent starvation of requests. It also maintains two deadline queues, in addition to the sorted queues (both read and write). Deadline queues are basically sorted by their deadline (the expiration time), while the sorted queues are sorted by the sector number.Before serving the next request, the Deadline scheduler decides which queue to use. Read queues are given a higher priority, because processes
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
usually block on read operations. Next, the Deadline scheduler checks if the first request in the deadline queue has expired. Otherwise, the scheduler serves a batch of requests from the sorted queue. In both cases, the scheduler also serves a batch of requests following the chosen request in the sorted queue.
By default, read requests have an expiration time of 500 ms, write requests expire in 5 seconds.
An early version of the scheduler was published by Axboe in September 2002.
The kernel documentation suggest this is the preferred scheduler for database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
systems, especially with Tagged Command Queuing
Tagged Command Queuing
Tagged Command Queuing is a technology built into certain ATA and SCSI hard drives. It allows the operating system to send multiple read and write requests to a hard drive. ATA TCQ is not identical in function to the more efficient native command queuing used by SATA drives...
(TCQ) aware disks, or any system with large numbers of disks.
Other I/O schedulers
- CFQCFQCompletely Fair Queuing is an I/O scheduler for the Linux kernel which was written in 2003 by Jens Axboe.-Description:CFQ places synchronous requests submitted by processes into a number of per-process queues and then allocates timeslices for each of the queues to access the disk...
- Anticipatory schedulerAnticipatory schedulingAnticipatory scheduling is an algorithm for scheduling hard disk input/output. It seeks to increase the efficiency of disk utilization by "anticipating" synchronous read operations....
- Noop schedulerNoop schedulerThe 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....