Disk Scheduling in Linux

Disk I/O Schedulers (Theory and in context of Linux)

How A Disk Works  A disk is a !unch of data !locks"  A disk has a disk head"  Data can only !e accessed when the head is on that !lock"  Disk head continues to #o$e for inco#in% disk access (read/write) re&uests"  Data transfer is fast (once the disk head %ets there)"

Our 'ro!le#  Suose we ha$e #ultile re&uests for disk !locks " Which should we access first* +es, order does #atter  a lot"

'ro!le# State#ent  Inut –   –   – 

A set of re&uests" The current disk head location" Al%orith# state (direction**)

 Outut – 

.ext disk !lock to %et  order of re&uest co#letion"

 – 

Total seek distance in ter#s of cylinders/sectors"

The 1oals  2axi#i3e throu%hut  Oerations er second"

 2axi#i3e fairness  All disk re&uests treated e&ually"

 A$oid Star$ation  And $ery $ery lon% waits"

 4eal Ti#e 5oncerns (if any)

'riorities are 6$erywhere  6$ery Oeratin% syste# assi%ns riorities to all sorts of thin%s  4e&uests for 4A2  4e&uests for the 5'7  4e&uests for network access  Disk Request priority.

'ossi!le Al%orith#s  'essi#ist 8 5hoose the disk re&uest furthest fro# the current disk head osition"  Oti#al 8 5hoose the disk re&uest closest to the current disk head osition"  959S 8 Ser$e the re&uests in their arri$al order"  6le$ator 8 2o$e !ack and forth, sol$in% re&uests as you %o"  5yclic 6le$ator 8 2o$e toward the !otto#, sol$in% re&uests as you %o"  – 

When you:$e sol$ed the lowest re&uest, seek all the way to the hi%hest re&uest"

 – 

'erfor#ance enalty occurs here"

 Deadline Scheduler 8 6ach re&uest has a deadline"  –  Ser$ice re&uests usin% cyclic ele$ator"  – 

When a deadline is threatened, ski directly to that re&uest"

Anticiatory Schedulin%  5oncet  – 

Assu#e that an I/O re&uest will !e closely followed !y another near!y one"

 Al%orith#  –   –   –   – 

After ser$icin% a re&uest  WAIT" +es, this #eans do nothin% e$en thou%h there is  work to !e done" If a near!y re&uest occurs soon, ser$ice it" If not  cyclic ele$ator (58S5A./58LOO;)"

5o#letely 9air
4T <=

Disatcher

9ilter

Disk
e"%", =?#s @al$e

<>

<>?

Disk

Why this is i#ortant to know*

 0ecause Linux has three/four $ery different schedulers, and you as a user ha$e a choice to do a selection"

Linux Disk Schedulin% 

Linux shis with different I/O schedulers" 

Deadline



.oo



Anticiatory, and



59<

Check which I/O schedulers your Linux distribution is shipped with? Clearly mention it as a part o your assi!nment report.

Linux Disk Schedulin% 

There are #any differences (discussed earlier) !etween these schedulin% al%orith#s (su##ary !elow)







59<- This is the default al%orith# in #ost Linux distri!utions" It atte#ts to distri!ute all I/O !andwidth e$enly a#on% all rocesses re&uestin% I/O" It is ideal for #ost uroses" .OO'- The noo al%orith# atte#ts to use as little cu as ossi!le" It acts as a !asic 9I9O &ueue exectin% the hardware controller to handle the erfor#ance oerations of the re&uests" Anticiatory- This al%orith# atte#ts to reorder all disk I/O oerations to oti#i3e disk seeks" It is desi%ned to increase erfor#ance on syste#s that ha$e slow disks" Deadline- This schedulin% al%orith# laces I/O re&uests in a riority &ueue so each is %uaranteed to !e ran within a certain ti#e" It is often used in real8ti#e oeratin% syste#s"

What you ha$e to do today* 



 Since the >""=? kernel user has !een %i$en the otion to easily adBust the disk scheduler on the fly without a re!oot"  +ou can see which scheduler you are currently runnin% !y usin% the followin% co##and-

C cat /sys/!lock/sda/&ueue/scheduler noo anticiatory deadline cf&E

// all or so#e of the# on your linux distri!ution*



 In this case sda is the !lock de$ice in &uestion"



 We can see it is currently runnin% the cf& scheduler"



 5han%in% the Disk I/O scheduler

C echo noo F /sys/!lock/sda/&ueue/scheduler C cat /sys/!lock/sda/&ueue/scheduler nooE anticiatory deadline cf& 



 5han%in% schedulers on the fly allows you to test and !ench#ark the al%orith#s for your secific alication"  Once the chan%e is issued, any current I/O oerations will !e executed !efore the new scheduler %oes into effect, so the chan%e will not !e instantaneous"

Al%orith# Analysis  Oti#al  – 

It is known to ha$e hi%hest erfor#ance in oerations er second"

 – 

It:s unfair to re&uests toward the ed%es of the disk"

 – 

It allows for star$ation"

 959S  – 

It:s fair and a$oids star$ation"

 – 

It:s #ediu# lousy erfor#ance"

 – 

So#e si#le OSs use this"

 6le$ator (.on85yclic !y D69A7LT)  – 

'erfor#ance %ood

 – 

9air, !ut files near the #iddle of the disk %et #ore attention"

Al%orith# Analysis """contGd 

5yclic 6le$ator (58S5A., 58LOO;)

 – 







It:s fair and star$ation8roof"

 – 

 It:s $ery %ood erfor#ance 8 Al#ost as %ood as ele$ator"

 – 

 It:s used in real life (and e$ery text!ook)"

Deadline Scheduler  – 

1i$es 'riority to 4eal Ti#e 'rocesses, 9air otherwise"

 – 

.o star$ation, unless a real ti#e rocess #is!eha$es"

Anticiatory Schedulin%  – 

9air, .o star$ation" .o suort for real ti#e"

 – 

2akes assu#tions a!out how rocesses work in real life"

59<  –  Has 4eal Ti#e suort" 9air"  –  .o star$ation is ossi!le" Allows for riorities"

0ench#arks  In 4eal Oeratin% Syste#s   'erfor#ance is ne$er o!$ious"  .o one uses the text!ook al%orith#"  0ench#arkin% is e$erythin%"  Theory is useful, if it hels you !ench#ark !etter"

Try the followin% exeri#ents 5oy =10 file On disk fro# one directory location to another" On 7S0 fro# one directory location to another" 9ro# us! to disk" 9ro# disk to us!" 5o#are erfor#ance of different schedulin% olicies and reort the result for your #achine for the linux distri!ution you ha$e installed on your #achine" 0ench#ark =- 5oy = 10 file on disk 'olicy 59< Deadline .oo

4eal ti#e

Sys ti#e

7ser ti#e

7se the ta!le Te#late on left for Other !ench#arks Listed a!o$e" +our reort should ha$e  ta!les each for the results o!tained on @2 and host #achine"