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

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

On Fri, 8 Apr 2016 11:51:05 +0200
Pradeep Kiruvale <pradeepkiruvale@gmail.com> wrote:

> Hi Greg,
> 
> Thanks for your reply.
> 
> 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 ?

> 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.

> 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
> >
> >