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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.
> 




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