linux-kernel - Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization

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Date:	Thu, 10 Mar 2016 13:21:30 +0300
From:	Roman Kagan <rkagan@...tuozzo.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"Li, Liang Z" <liang.z.li@...el.com>,
	"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
	"ehabkost@...hat.com" <ehabkost@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"quintela@...hat.com" <quintela@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"amit.shah@...hat.com" <amit.shah@...hat.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"rth@...ddle.net" <rth@...ddle.net>, <riel@...hat.com>
Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration
 optimization

On Wed, Mar 09, 2016 at 07:39:18PM +0200, Michael S. Tsirkin wrote:
> On Wed, Mar 09, 2016 at 08:04:39PM +0300, Roman Kagan wrote:
> > On Wed, Mar 09, 2016 at 05:41:39PM +0200, Michael S. Tsirkin wrote:
> > > On Wed, Mar 09, 2016 at 05:28:54PM +0300, Roman Kagan wrote:
> > > > For (1) I've been trying to make a point that skipping clean pages is
> > > > much more likely to result in noticable benefit than free pages only.
> > > 
> > > I guess when you say clean you mean zero?
> > 
> > No I meant clean, i.e. those that could be evicted from RAM without
> > causing I/O.
> 
> They must be migrated unless guest actually evicts them.

If the balloon is inflated the guest will.

> It's not at all clear to me that it's always preferable
> to drop all clean pages from pagecache. It is clearly is
> going to slow the guest down significantly.

That's a matter for optimization.  The current value for
/proc/meminfo:MemAvailable (which is being proposed as a member of
balloon stats, too) is a conservative estimate which will probably cover
a good deal of cases.

> > I must be missing something obvious, but how is that different from
> > inflating and then immediately deflating the balloon?
> 
> It's exactly the same except
> - we do not initiate this from host - it's guest doing
>   things for its own reasons
> - a bit less guest/host interaction this way

I don't quite understand why you need to deflate the balloon until the
VM is on the destination host.  deflate_on_oom will do it if the guest
is really tight on memory; otherwise there appears to be no reason for
it.  But then inflation followed immediately by deflation doubles the
guest/host interactions rather than reduces them, no?

> > it's just the granularity that makes things slow and
> > stands in the way.
> 
> So we could request a specific page size/alignment from guest.
> Send guest request to give us memory in aligned units of 2Mbytes,
> and then host can treat each of these as a single huge page.

I'd guess just coalescing contiguous pages would already speed things
up.  I'll try to find some time to experiment with it.

Roman.