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

[Pradeep Kiruvale <pradeepkiruvale@gmail.com>]

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On 2 May 2016 at 14:57, Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:

> On Thu, 28 Apr 2016 11:45:41 +0200
> Pradeep Kiruvale <pradeepkiruvale@gmail.com> wrote:
>
> > On 27 April 2016 at 19:12, Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> >
> > > On Wed, 27 Apr 2016 16:39:58 +0200
> > > Pradeep Kiruvale <pradeepkiruvale@gmail.com> wrote:
> > >
> > > > On 27 April 2016 at 10:38, Alberto Garcia <berto@igalia.com> 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
>
>

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