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I'm kinda new too linux still, But suprised to see nothing about this subject. (Since it's relevant to Actual Desktop performance.) I read ALOT about this subject and, I'd like ...
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- 01-10-2009 #1
CPU Direct I/O Scheduler (suse default install is disabled.)
I read ALOT about this subject and, I'd like to explain to n00bies on how to set the kernel shcheduler.
-- Anticipatory IO Scheduler (AS)
The anticipatory scheduler is the default scheduler in older 2.6 kernels – if you've not specified one, this is the one that will be loaded. It implements request merging, a one-way elevator, read and write request batching, and attempts some anticipatory reads by holding off a bit after a read batch if it thinks a user is going to ask for more data. It tries to optimise for physical disks by avoiding head movements if possible – one downside to this is that it probably give highly erratic performance on database or storage systems.
-- Deadline Scheduler
"The deadline scheduler implements request merging, a one-way elevator, and imposes a deadline on all operations to prevent resource starvation. Because writes return instantly within Linux, with the actual data being held in cache, the deadline scheduler will also prefer readers – as long as the deadline for a write request hasn't passed. The kernel docs suggest this is the preferred scheduler for database systems, especially if you have TCQ aware disks, or any system with high disk performance."
-- Complete Fair Queueing Scheduler (CFQ)
"The complete fair queueing scheduler implements both request merging and the elevator, and attempts to give all users of a particular device the same number of IO requests over a particular time interval. This should make it more efficient for multiuser systems. It seems that Novel SLES sets cfq as the scheduler by default, as does the latest Ubuntu release. As of the 2.6.18 kernel, this is the default schedular in kernel.org releases."
-- NOOP (No-Op)
"The NOOP scheduler inserts all incoming I/O requests into a simple, unordered FIFO queue and implements request merging.
The scheduler assumes I/O performance optimization will be handled at some other layer of the I/O hierarchy; e.g., at the block device; by an intelligent HBA such as a Serial Attached SCSI (SAS) RAID controller or by an externally attached controller such as a storage subsystem accessed through a switched Storage Area Network)
NOOP scheduler is best used with solid state devices such as flash memory or in general with devices that do not depend on mechanical movement to access data (meaning typical "hard disk" drive technology consisting of seek time primarily, plus rotational latency). Such non-mechanical devices do not require re-ordering of multiple I/O requests, a technique that groups together I/O requests that are physically close together on the disk, thereby reducing average seek time and the variability of I/O service time."