Re: [Qemu-discuss] [Qemu-devel] iolimits for virtio-9p

[Pradeep Kiruvale]

On 2 May 2016 at 14:57, Greg Kurz <address@hidden> wrote:

On Thu, 28 Apr 2016 11:45:41 +0200
Pradeep Kiruvale <address@hidden> wrote:

> On 27 April 2016 at 19:12, Greg Kurz <address@hidden> wrote:
>
> > On Wed, 27 Apr 2016 16:39:58 +0200
> > Pradeep Kiruvale <address@hidden> wrote:
> >
> > > On 27 April 2016 at 10:38, Alberto Garcia <address@hidden> wrote:
> > >
> > > > On Wed, Apr 27, 2016 at 09:29:02AM +0200, Pradeep Kiruvale wrote:
> > > >
> > > > > Thanks for the reply. I am still in the early phase, I will let you
> > > > > know if any changes are needed for the APIs.
> > > > >
> > > > > We might also have to implement throttle-group.c for 9p devices, if
> > > > > we want to apply throttle for group of devices.
> > > >
> > > > Fair enough, but again please note that:
> > > >
> > > > - throttle-group.c is not meant to be generic, but it's tied to
> > > >   BlockDriverState / BlockBackend.
> > > > - it is currently being rewritten:
> > > >   https://lists.gnu.org/archive/html/qemu-block/2016-04/msg00645.html
> > > >
> > > > If you can explain your use case with a bit more detail we can try to
> > > > see what can be done about it.
> > > >
> > > >
> > > We want to use  virtio-9p for block io instead of virtio-blk-pci. But in
> > > case of
> >
> > 9p is mostly aimed at sharing files... why would you want to use it for
> > block io instead of a true block device ? And how would you do that ?
> >
>
> *Yes, we want to share the files itself. So we are using the virtio-9p.*

You want to pass a disk image to the guest as a plain file on a 9p mount ?
And then, what do you do in the guest ? Attach it to a loop device ?

Yes, would like to mount as  a 9p drive and create file inside that and read/write.

This was the experiment we are doing, actual use case no idea. My work is to do

a feasibility test does it work or not.

 

> *We want to have QoS on these files access for every VM.*
>

You won't be able to have QoS on selected files, but it may be possible to
introduce limits at the fsdev level: control all write accesses to all files
and all read accesses to all files for a 9p device.

 

That is right, I do not want to have QoS for individual files but to whole fsdev device.

>
> >
> > > virtio-9p we can just use fsdev devices, so we want to apply throttling
> > > (QoS)
> > > on these devices and as of now the io throttling only possible with the
> > > -drive option.
> > >
> >
> > Indeed.
> >
> > > As a work around we are doing the throttling using cgroup. It has its own
> > > costs.
> >
> > Can you elaborate ?
> >
>
> *We saw that we need to create cgroups and set it and also we observed lot
> of iowaits *
> *compared to implementing the throttling inside the qemu.*
> *This we did observe by using the virtio-blk-pci devices. (Using cgroups Vs
> qemu throttling)*
>

 

Just to be sure I get it right.

You tried both:
1) run QEMU with -device virtio-blk-pci and -drive throttling.*
2) run QEMU with -device virtio-blk-pci in its own cgroup

And 1) has better performance and is easier to use than 2) ?

And what do you expect with 9p compared to 1) ?

That was just to understand the cost of cpu 

 io throttling inside the qemu vs using cgroup.

The bench-marking we did to reproduce the numbers and understand the cost mentioned in 

http://www.linux-kvm.org/images/7/72/2011-forum-keep-a-limit-on-it-io-throttling-in-qemu.pdf

Thanks,

Pradeep 

 

>
> Thanks,
> Pradeep