問題描述
選擇 Linux I/O 調度程序 (Selecting a Linux I/O Scheduler)
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?
參考解法
方法 1:
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 > /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:
noopis 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 resourcesdeadlineis a lightweight scheduler which tries to put a hard limit on latencycfqtries 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.
方法 2:
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 $f; done
方法 3:
The aim of having the kernel support different ones is that you can try them out without a reboot; you can then run test workloads through the sytsem, measure performance, and then make that the standard one for your app.
On modern server-grade hardware, only the noop one appears to be at all useful. The others seem slower in my tests.
方法 4:
You can set this at boot by adding the "elevator" parameter to the kernel cmdline (such as in grub.cfg)
Example:
elevator=deadline
This will make "deadline" the default I/O scheduler for all block devices.
If you'd like to query or change the scheduler after the system has booted, or would like to use a different scheduler for a specific block device, I recommend installing and use the tool ioschedset to make this easy.
https://github.com/kata198/ioschedset
If you're on Archlinux it's available in aur:
https://aur.archlinux.org/packages/ioschedset
Some example usage:
# Get i/o scheduler for all block devices
[username@hostname ~]$ 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.
(by Robert S. Barnes、ephemient、Dani_l、MarkR、Tim Savannah)