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Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support
From: |
Igor Mammedov |
Subject: |
Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support |
Date: |
Tue, 26 Apr 2016 09:52:36 +0200 |
On Tue, 26 Apr 2016 10:39:23 +0530
Bharata B Rao <address@hidden> wrote:
> On Mon, Apr 25, 2016 at 11:20:50AM +0200, Igor Mammedov wrote:
> > On Wed, 16 Mar 2016 10:11:54 +0530
> > Bharata B Rao <address@hidden> wrote:
> >
> > > On Wed, Mar 16, 2016 at 12:36:05PM +1100, David Gibson wrote:
> > > > On Tue, Mar 15, 2016 at 10:08:56AM +0530, Bharata B Rao wrote:
> > > > > Add support to hot remove pc-dimm memory devices.
> > > > >
> > > > > Signed-off-by: Bharata B Rao <address@hidden>
> > > >
> > > > Reviewed-by: David Gibson <address@hidden>
> > > >
> > > > Looks correct, but again, needs to wait on the PAPR change.
> > [...]
> > >
> > > While we are here, I would also like to get some opinion on the real
> > > need for memory unplug. Is there anything that memory unplug gives us
> > > which memory ballooning (shrinking mem via ballooning) can't give ?
> > Sure ballooning can complement memory hotplug but turning it on would
> > effectively reduce hotplug to balloning as it would enable overcommit
> > capability instead of hard partitioning pc-dimms provides. So one
> > could just use ballooning only and not bother with hotplug at all.
> >
> > On the other hand memory hotplug/unplug (at least on x86) tries
> > to model real hardware, thus removing need in paravirt ballooning
> > solution in favor of native guest support.
>
> Thanks for your views.
>
> >
> > PS:
> > Guest wise, currently hot-unplug is not well supported in linux,
> > i.e. it's not guarantied that guest will honor unplug request
> > as it may pin dimm by using it as a non migratable memory. So
> > there is something to work on guest side to make unplug more
> > reliable/guarantied.
>
> In the above scenario where the guest doesn't allow removal of certain
> parts of DIMM memory, what is the expected behaviour as far as QEMU
> DIMM device is concerned ? I seem to be running into this situation
> very often with PowerPC mem unplug where I am left with a DIMM device
> that has only some memory blocks released. In this situation, I would like
> to block further unplug requests on the same device, but QEMU seems
> to allow more such unplug requests to come in via the monitor. So
> qdev won't help me here ? Should I detect such condition from the
> machine unplug() handler and take required action ?
I think offlining is a guests task along with recovering from
inability to offline (i.e. offline all + eject or restore original state).
QUEM does it's job by notifying guest what dimm it wants to remove
and removes it when guest asks it (at least in x86 world).
>
> On x86, if some pages are offlined and subsequently other pages couldn't
> be offlined, then I see the full DIMM memory size remaining
> with the guest. So I infer that on x86, QEMU memory unplug either
> removes full DIMM or nothing. Is that understanding correct ?
I wouldn't bet that it's guarantied behavior but it should be this way.
>
> Regards,
> Bharata.
>
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Igor Mammedov, 2016/04/25
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Bharata B Rao, 2016/04/26
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support,
Igor Mammedov <=
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Michael Roth, 2016/04/26
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Thomas Huth, 2016/04/27
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Igor Mammedov, 2016/04/27
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Thomas Huth, 2016/04/27
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Igor Mammedov, 2016/04/27
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Michael Roth, 2016/04/27
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Igor Mammedov, 2016/04/28
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Bharata B Rao, 2016/04/27
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, David Gibson, 2016/04/28
- Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support, Igor Mammedov, 2016/04/29