Selecting a Linux I O Scheduler

Question

I read that it s supposedly possible to change the I O scheduler for a particular device on a running kernel by writing to  sys block  disk  queue scheduler   For example I can see on my system   anon anon    cat  sys block sda queue scheduler  noop anticipatory deadline  cfq     that the default is the completely fair queuing scheduler   What I m wondering is if there is any use in including all four schedulers in my custom kernel   It would seem that there s not much point in having more than one scheduler compiled in unless the kernel is smart enough to select the correct scheduler for the correct hardware  specifically the  noop  scheduler for flash based drives and one of the others for a traditional hard drive   Is this the case

User · Accepted Answer

As documented in  usr src linux Documentation block switching-sched txt  the I O scheduler on any particular block device can be changed at runtime  There may be some latency as the previous scheduler s requests are all flushed before bringing the new scheduler into use  but it can be changed without problems even while the device is under heavy use     cat  sys block hda queue scheduler noop deadline  cfq    echo anticipatory  gt   sys block hda queue scheduler   cat  sys block hda queue scheduler noop  deadline  cfq   Ideally  there would be a single scheduler to satisfy all needs   It doesn t seem to exist yet   The kernel often doesn t have enough knowledge to choose the best scheduler for your workload    noop is often the best choice for memory-backed block devices  e g  ramdisks  and other non-rotational media  flash  where trying to reschedule I O is a waste of resources deadline is a lightweight scheduler which tries to put a hard limit on latency cfq tries to maintain system-wide fairness of I O bandwidth   The default was anticipatory for a long time  and it received a lot of tuning  but was removed in 2 6 33  early 2010    cfq became the default some while ago  as its performance is reasonable and fairness is a good goal for multi-user systems  and even single-user desktops    For some scenarios -- databases are often used as examples  as they tend to already have their own peculiar scheduling and access patterns  and are often the most important service  so who cares about fairness   -- anticipatory has a long history of being tunable for best performance on these workloads  and deadline very quickly passes all requests through to the underlying device

User · Answer

You can set this at boot by adding the  elevator  parameter to the kernel cmdline  such as in grub cfg   Example   elevator deadline   This will make  deadline  the default I O scheduler for all block devices   If you d like to query or change the scheduler after the system has booted  or would like to use a different scheduler for a specific block device  I recommend installing and use the tool ioschedset to make this easy   https   github com kata198 ioschedset  If you re on Archlinux it s available in aur   https   aur archlinux org packages ioschedset  Some example usage     Get i o scheduler for all block devices  username hostname     io-get-sched sda     bfq sr0     bfq    Query available I O schedulers  username hostname     io-set-sched --list mq-deadline kyber bfq none    Set sda to use  kyber   username hostname     io-set-sched kyber  dev sda Must be root to set IO Scheduler  Rerunning under sudo      sudo  password for username    Successfully set sda to  kyber      Get i o scheduler for all block devices to assert change  username hostname     io-get-sched sda     kyber sr0     bfq    Set all block devices to use  deadline  i o scheduler  username hostname     io-set-sched deadline Must be root to set IO Scheduler  Rerunning under sudo       Successfully set sda to  deadline     Successfully set sr0 to  deadline      Get the current block scheduler just for sda  username hostname     io-get-sched sda sda     mq-deadline   Usage should be self-explanatory  The tools are standalone and only require bash   Hope this helps   EDIT  Disclaimer  these are scripts I wrote

User · Answer

The aim of having the kernel support different ones is that you can try them out without a reboot  you can then run test workloads through the sytsem  measure performance  and then make that the standard one for your app   On modern server-grade hardware  only the noop one appears to be at all useful  The others seem slower in my tests

User · Answer

It s possible to use a udev rule to let the system decide on the scheduler based on some characteristics of the hw  An example udev rule for SSDs and other non-rotational drives might look like    set noop scheduler for non-rotating disks ACTION   add change   KERNEL   sd a-z    ATTR queue rotational    0   ATTR queue scheduler   noop    inside a new udev rules file  e g    etc udev rules d 60-ssd-scheduler rules   This answer is based on the debian wiki  To check whether ssd disks would use the rule  it s possible to check for the trigger attribute in advance     for f in  sys block sd  queue rotational  do printf   f    cat  f  done

User · Answer

The Linux Kernel does not automatically change the IO Scheduler at run-time  By this I mean  the Linux kernel  as of today  is not able to automatically choose an  optimal  scheduler depending on the type of secondary storage devise  During start-up  or during run-time  it is possible to change the IO scheduler manually   The default scheduler is chosen at start-up based on the contents in the file located at  linux-2 6  block Kconfig iosched  However  it is possible to change the IO scheduler during run-time by echoing a valid scheduler name into the file located at  sys block  DEV  queue scheduler  For example  echo deadline  gt   sys block hda queue scheduler

[linux] Selecting a Linux I/O Scheduler

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