kernel-packages team mailing list archive

[Bpkroth <1346687@xxxxxxxxxxxxxxxxxx>]

I hate to reopen an old bug, but I haven't seen this discussed anywhere
else, and I think there is some validity in this.

Without the use of the cfq scheduler on block devices (virtual or not),
the blkio cgroup controller doesn't perform any IO accounting (see
below).

Without that, tools like systemd-cgtop don't show any I/O stats.
Determining which (group of) processes is using the most IO can be very
valuable even in a virtual environment.

It also prevents the use of ionice (which depends on cfq) on certain
tasks within the VM, which again still have merit inside a VM, and the
hypervisor can't affect once it reaches that level in the I/O path.

Now, for accounting at least, whether that's a fix that should be done
in the choice of block device queue scheduler or the blkio cgroup
controller, is perhaps up for debate.  But as it stands, users/operators
are prevented from making the choice of elevator for themselves which
seems wrong to me.  If I want to pay the CPU cost for performing IO
accounting if not scheduling inside a VM, even if the underlying storage
may turn around and reorder things with its own more global optimization
view, I should still be allowed to.

</cent></cent>

Thanks,
Brian


This was done on a VMware VM since obviously I can't demonstrate it in a KVM VM, but the idea applies to physical machines as well.  Tested running both 3.16 and 4.5 kernels.

(get the current process' blkio cgroup)
# grep blkio /proc/self/cgroup 
7:blkio:/system.slice/ssh.service

(yeah, I know, I was missing pam_systemd in the stack, but whatever)

(check the current scheduler being used)
# cat /sys/block/sda/queue/scheduler 
[noop] deadline cfq 

(check the current stats)
# cat /sys/fs/cgroup/blkio/system.slice/ssh.service/blkio.io_serviced
Total 0

(induce some IO)
# time find /var/log -type f -print0 2>/dev/null | xargs -0 cat > /dev/null 2>/dev/null

real    0m4.539s
user    0m0.000s
sys     0m0.048s

(check the new blk io stats)
# cat /sys/fs/cgroup/blkio/system.slice/ssh.service/blkio.io_serviced
Total 0

(no change)

(switch to the cfq scheduler)
# echo cfq | sudo tee /sys/block/sda/queue/scheduler
cfq

(verify that the change happened)
# cat /sys/block/sda/queue/scheduler
noop deadline [cfq]

(check the current blkio stats)
# cat /sys/fs/cgroup/blkio/system.slice/ssh.service/blkio.io_serviced
8:0 Read 0
8:0 Write 0
8:0 Sync 0
8:0 Async 0
8:0 Total 0
Total 0

(look new fields)

(clear the page cache to make sure our next test actually causes io)
# echo 1 | sudo tee /proc/sys/vm/drop_caches
1

(induce io again)
# time find /var/log -type f -print0 2>/dev/null | xargs -0 cat > /dev/null 2>/dev/null
                                 
real    0m2.386s                 
user    0m0.000s                 
sys     0m0.040s   


(check the blkio stats again)
# cat /sys/fs/cgroup/blkio/system.slice/ssh.service/blkio.io_serviced
8:0 Read 1827                    
8:0 Write 0                      
8:0 Sync 0                       
8:0 Async 1827                   
8:0 Total 1827                   
Total 1827

(look, data!)

-- 
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https://bugs.launchpad.net/bugs/1346687

Title:
  Ubuntu 14 KVM Guest I/O Elevator Non-configurable

Status in linux package in Ubuntu:
  Invalid

Bug description:
  Ubuntu 14.04 guest runs pretty much standard standard configuration:
  KVM hypervisor, ACPI and APIC enabled, virtio disk bus, raw storage
  format...

  While running this project, I am also involved in testing SSD drives
  across different Linux distributions and their support for discard
  option (TRIM).

  To cut the story short, Ubuntu-based KVM guest did not set any
  scheduler on the boot disk vda):

  # lsblk -io KNAME,TYPE,SCHED,ROTA,DISC-GRAN,DISC-MAX
  KNAME TYPE SCHED ROTA DISC-GRAN DISC-MAX
  sr0 rom deadline 1 0B 0B
  vda disk 1 0B 0B
  vda1 part 1 0B 0B
  vda2 part 1 0B 0B
  vda5 part 1 0B 0B
  dm-0 lvm 1 0B 0B
  dm-1 lvm 1 0B 0B

  That is in stark contrast with other distributions running almost
  identical KVM guest configuration (with the exception of the root file
  system using BTRFS instead of EXT4 on SUSE).

  Oracle Linux 6.5:

  # lsblk -io KNAME,TYPE,SCHED,ROTA,DISC-GRAN,DISC-MAX
  KNAME TYPE SCHED ROTA DISC-GRAN DISC-MAX
  sr0 rom deadline 1 0B 0B
  vda disk deadline 1 0B 0B
  vda1 part deadline 1 0B 0B
  vda2 part deadline 1 0B 0B
  dm-0 lvm 1 0B 0B
  dm-1 lvm 1 0B 0B

  OpenSUSE 13.1:

  # lsblk -io KNAME,TYPE,SCHED,ROTA,DISC-GRAN,DISC-MAX
  KNAME TYPE SCHED ROTA DISC-GRAN DISC-MAX
  sr0 rom cfq 1 0B 0B
  vda disk cfq 1 0B 0B
  vda1 part cfq 1 0B 0B
  vda2 part cfq 1 0B 0B
  dm-0 lvm 1 0B 0B
  dm-1 lvm 1 0B 0B

  Indeed, checking elevator capabilities for boot disk on Ubuntu KVM
  guest showed:

  # cat /sys/block/vda/queue/scheduler
  none

  Other Linux distributions show more options:

  # cat /sys/block/vda/queue/scheduler
  noop deadline [cfq]

  As well, attempts to change the elevator on Ubuntu guest fail. For
  example:

  # echo noop > /sys/block/vda/queue/scheduler

  # echo $?
  0

  # cat /sys/block/vda/queue/scheduler
  none

  Setting it globally in /etc/default/grub and updating it via update-
  grub2 fails too.

  In the meantime, to automate check of elevators (schedulers) and
  discard support for SSD drives and thin-provisioned volumes on Linux,
  I wrote simple Perl script:

  http://www.circlingcycle.com.au/Unix-sources/Linux-check-IO-scheduler-
  and-discard-support.pl.txt

  Part of the results on RHEL 6.5 server would look like:

  INFO: I/O elevator (scheduler) and discard support summary
  INFO: Hard Disk sdb configured with I/O scheduler "cfq"
  INFO: SSD sda configured with I/O scheduler "deadline" supports discard operation
  INFO: Hard Disk sdc configured with I/O scheduler "cfq"
  INFO: Hard Disk sdi configured with I/O scheduler "cfq"
  INFO: Hard Disk sdh configured with I/O scheduler "cfq"

  The Ubuntu KVM guest runs latest paches:

  Linux ubuntu14-vm2.circlingcycle.com.au 3.13.0-24-generic
  #47-Ubuntu SMP Fri May 2 23:30:00 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

  Regards,

  Dusan Baljevic VK2COT

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