Anticipatory scheduling
Encyclopedia
Anticipatory scheduling is an algorithm
for scheduling hard disk
input/output
. It seeks to increase the efficiency of disk utilization by "anticipating" synchronous read operations.
"Deceptive idleness" is a situation where a process
appears to be finished reading from the disk when it is actually processing data in preparation of the next read operation. This will cause a normal work-conserving I/O scheduler to switch to servicing I/O from an unrelated process. This situation is detrimental to the throughput of synchronous reads, as it degenerates into a seeking workload.
Anticipatory scheduling overcomes deceptive idleness by pausing for a short time (a few milliseconds) after a read operation in anticipation of another close-by read requests.
Anticipatory scheduling yields significant improvements in disk utilization for some workloads. In some situations the Apache web server may achieve up to 71% more throughput from using anticipatory scheduling.
The Linux
anticipatory scheduler may reduce performance on disks using TCQ
, high performance disks, and hardware RAID
arrays. An anticipatory scheduler (AS) was the default Linux kernel
scheduler between 2.6.0 and 2.6.18, by which time it was replaced by the CFQ
scheduler.
As of kernel version 2.6.33, the Anticipatory scheduler (AS) has been removed from the Linux kernel
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
for scheduling hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
input/output
Input/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
. It seeks to increase the efficiency of disk utilization by "anticipating" synchronous read operations.
"Deceptive idleness" is a situation where a process
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...
appears to be finished reading from the disk when it is actually processing data in preparation of the next read operation. This will cause a normal work-conserving I/O scheduler to switch to servicing I/O from an unrelated process. This situation is detrimental to the throughput of synchronous reads, as it degenerates into a seeking workload.
Anticipatory scheduling overcomes deceptive idleness by pausing for a short time (a few milliseconds) after a read operation in anticipation of another close-by read requests.
Anticipatory scheduling yields significant improvements in disk utilization for some workloads. In some situations the Apache web server may achieve up to 71% more throughput from using anticipatory scheduling.
The Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
anticipatory scheduler may reduce performance on disks using TCQ
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...
, high performance disks, and hardware RAID
RAID
RAID is a storage technology that combines multiple disk drive components into a logical unit...
arrays. An anticipatory scheduler (AS) was the default 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....
scheduler between 2.6.0 and 2.6.18, by which time it was replaced by the CFQ
CFQ
Completely 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...
scheduler.
As of kernel version 2.6.33, the Anticipatory scheduler (AS) has been removed from the Linux kernel