Linux

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Linux

Kernel Boot-Parameter

Different distributions ship different boot-parameters. Look them up via:

   1 man 7 bootparam

Here are some important kernel command line parameters that should not be forgotten.

   1 GRUB_CMDLINE_LINUX_DEFAULT="quiet zwap.enabled=1 cgroup.enable=memory swapaccount=1 scsi_mod.use_blk_mq=1 nomodeset"

Virtual Memory

https://www.kernel.org/doc/Documentation/sysctl/vm.txt

Swappiness

Default: 60
Configuration:
1. at runtime via procfs
2. at boottime via sysctl

/proc/sys/vm/swappiness

   1 # cat /proc/sys/vm/swappiness
   2 60
   3 # echo 5 > /proc/sys/vm/swappiness
   4 # cat /proc/sys/vm/swappiness
   5 5

/etc/sysctl.d/vm.conf

   1 vm.swappiness = 5

Apply configuration via sysctl.

   1 # sysctl --system
   2 * Applying /etc/sysctl.d/30-baloo-inotify-limit.conf ...
   3 fs.inotify.max_user_watches = 524288
   4 * Applying /etc/sysctl.d/30-postgresql-shm.conf ...
   5 * Applying /etc/sysctl.d/30-tracker.conf ...
   6 fs.inotify.max_user_watches = 65536
   7 * Applying /usr/lib/sysctl.d/50-coredump.conf ...
   8 kernel.core_pattern = |/lib/systemd/systemd-coredump %P %u %g %s %t 9223372036854775808 %e
   9 * Applying /etc/sysctl.d/99-sysctl.conf ...
  10 * Applying /etc/sysctl.d/vm.conf ...
  11 vm.swappiness = 5
  12 vm.dirty_background_ratio = 8
  13 vm.dirty_expire_centisecs = 3000
  14 vm.dirty_ratio = 32
  15 vm.dirty_writeback_centisecs = 500
  16 * Applying /etc/sysctl.conf ...

IO-Scheduler

On a hypervisor the scheduler bfq seems to be reasonable.
On a VM with no disk or controller pass-through none should be used. This avoids optimizing the queues twice, which is inefficient and contra-productive. The hypervisor will optimize the io-request anyway.

Make alternative schedulers available

BLK-MQ is nowadays broadly available and enabled in distributions. Using multiple queues on multicore systems with fast storage promises some performance gains.

But when I took a look on available schedulers only "mq-deadline" and "none" were available.

   1 cat /sys/class/block/sda/queue/scheduler
   2 [mq-deadline] none

This is because these scheduler are shipped as a kernel module and need to be loaded first into the kernel via modprobe.

   1 ls /lib/modules/$(uname -r)/kernel/block
   2 bfq.ko  kyber-iosched.ko

Modules may be loaded manually:

   1 modprobe bfq
   2 modprobe kyber-iosched

Modules may also be loaded automatically at boot-time via /etc/modules.

   1 for SCHEDULER in bfq kyber-iosched; do
   2         if \! grep -q "$SCHEDULER" /etc/modules && \
   3            \! grep -qr "$SCHEDULER" /etc/modules-load.d; then
   4                 echo "$SCHEDULER" >> /etc/modules
   5         fi
   6 done

Set IO-Scheduler permanently

kernel-cmdline

Method seems not to be working any longer.
Service affecting.

/etc/default/grub

   1 GRUB_CMDLINE_LINUX_DEFAULT="quiet elevator=$SCHEDULER"

Refresh grub config and reboot.

   1 update-grub2
   2 systemctl reboot

udev-rule

Works at run- and at boot-time!
More selective because disks may be filtered with a regex.

/etc/udev/rules.d/60-persistent-storage-scheduler.rules

   1 # GUEST SHOULD NOT REORDER STORAGE REQUESTS -> NONE
   2 ACTION=="add|change", KERNEL=="[sv]d[a-z]", ATTR{queue/scheduler}="none"
   3 # HYPERVISOR SHOULD NOT REORDER STORAGE REQUESTS IN PARALLEL QUEUES
   4 #ACTION=="add|change", KERNEL=="sd[ab]", ATTR{queue/scheduler}="bfq"
   5

Reload udev-rules

Reload will probably happen automatically but the "trigger" is necessary.

   1 udevadm control --reload-rules && udevadm trigger

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