Re: [Qemu-devel] Virtio-9p and cgroup io-throttling

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Fri, 8 Apr 2016 14:55:29 +0200
Pradeep Kiruvale <pradeepkiruvale@gmail.com> wrote:

> Hi Greg,
> 
> FInd my replies inline
> 
> >
> > > Below is the way how I add to blkio
> > >
> > > echo "8:16 8388608" >
> > > /sys/fs/cgroup/blkio/test/blkio.throttle.write_bps_device
> > >
> >
> > Ok, this just puts a limit of 8MB/s when writing to /dev/sdb for all
> > tasks in the test cgroup... but what about the tasks themselves ?
> >
> > > The problem I guess is adding these task ids to the "tasks" file in
> > cgroup
> > >
> >
> > Exactly. :)
> >
> > > These threads are started randomly and even then I add the PIDs to the
> > > tasks file the cgroup still does not do IO control.
> > >
> >
> > How did you get the PIDs ? Are you sure these threads you have added to the
> > cgroup are the ones that write to /dev/sdb ?
> >
> 
> *Yes, I get PIDs from /proc/Qemu_PID/task*
> 

And then you echoed the PIDs to /sys/fs/cgroup/blkio/test/tasks ?

This is racy... another IO thread may be started to do some work on /dev/sdb
just after you've read PIDs from /proc/Qemu_PID/task, and it won't be part
of the cgroup.

> 
> 
> >
> > > Is it possible to reduce these number of threads? I see different number
> > of
> > > threads doing IO at different runs.
> > >
> >
> > AFAIK, no.
> >
> > Why don't you simply start QEMU in the cgroup ? Unless I miss something,
> > all
> > children threads, including the 9p ones, will be in the cgroup and honor
> > the
> > throttle setttings.
> >
> 
> 
> *I started the qemu with cgroup as below*
> 
> *cgexec -g blkio:/test qemu.......*
> *Is there any other way of starting the qemu in cgroup?*
> 

Maybe you can pass --sticky to cgexec to prevent cgred from moving
children tasks to other cgroups...

There's also the old fashion method:

# echo $$ > /sys/fs/cgroup/blkio/test/tasks
# qemu.....

This being said, QEMU is a regular userspace program that is completely cgroup
agnostic. It won't behave differently than 'dd if=/dev/sdb of=/dev/null'.

This really doesn't look like a QEMU related issue to me.

> Regards,
> Pradeep
> 

Cheers.

--
Greg

> 
> >
> > > Regards,
> > > Pradeep
> > >
> >
> > Cheers.
> >
> > --
> > Greg
> >
> > >
> > > On 8 April 2016 at 10:10, Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> > >
> > > > On Thu, 7 Apr 2016 11:48:27 +0200
> > > > Pradeep Kiruvale <pradeepkiruvale@gmail.com> wrote:
> > > >
> > > > > Hi All,
> > > > >
> > > > > I am using virtio-9p for sharing the file between host and guest. To
> > test
> > > > > the shared file I do read/write options in the guest.To have
> > controlled
> > > > io,
> > > > > I am using cgroup blkio.
> > > > >
> > > > > While using cgroup I am facing two issues,Please find the issues
> > below.
> > > > >
> > > > > 1. When I do IO throttling using the cgroup the read throttling works
> > > > fine
> > > > > but the write throttling does not wok. It still bypasses these
> > throttling
> > > > > control and does the default, am I missing something here?
> > > > >
> > > >
> > > > Hi,
> > > >
> > > > Can you provide details on your blkio setup ?
> > > >
> > > > > I use the following commands to create VM, share the files and to
> > > > > read/write from guest.
> > > > >
> > > > > *Create vm*
> > > > > qemu-system-x86_64 -balloon none .......-name vm0 -cpu host -m 128
> > -smp 1
> > > > > -enable-kvm -parallel .... -fsdev
> > > > > local,id=sdb1,path=/mnt/sdb1,security_model=none,writeout=immediate
> > > > -device
> > > > > virtio-9p-pci,fsdev=sdb1,mount_tag=sdb1
> > > > >
> > > > > *Mount file*
> > > > > mount -t 9p -o trans=virtio,version=9p2000.L sdb1 /sdb1_ext4
> > 2>>dd.log &&
> > > > > sync
> > > > >
> > > > > touch /sdb1_ext4/dddrive
> > > > >
> > > > > *Write test*
> > > > > dd if=/dev/zero of=/sdb1_ext4/dddrive bs=4k count=800000
> > oflag=direct >>
> > > > > dd.log 2>&1 && sync
> > > > >
> > > > > *Read test*
> > > > > dd if=/sdb1_ext4/dddrive of=/dev/null >> dd.log 2>&1 && sync
> > > > >
> > > > > 2. The other issue is when I run "dd" command inside guest  it
> > creates
> > > > > multiple threads to write/read. I can see those on host using iotop
> > is
> > > > this
> > > > > expected behavior?
> > > > >
> > > >
> > > > Yes. QEMU uses a thread pool to handle 9p requests.
> > > >
> > > > > Regards,
> > > > > Pradeep
> > > >
> > > > Cheers.
> > > >
> > > > --
> > > > Greg
> > > >
> > > >
> >
> >