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

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Date:	Wed, 9 Mar 2016 16:22:11 +0300
From:	Roman Kagan <rkagan@...tuozzo.com>
To:	"Dr. David Alan Gilbert" <dgilbert@...hat.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	"Li, Liang Z" <liang.z.li@...el.com>,
	"ehabkost@...hat.com" <ehabkost@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mst@...hat.com" <mst@...hat.com>,
	"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>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"rth@...ddle.net" <rth@...ddle.net>
Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration
 optimization

On Fri, Mar 04, 2016 at 06:51:21PM +0000, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (pbonzini@...hat.com) wrote:
> > 
> > 
> > On 04/03/2016 15:26, Li, Liang Z wrote:
> > >> > 
> > >> > The memory usage will keep increasing due to ever growing caches, etc, so
> > >> > you'll be left with very little free memory fairly soon.
> > >> > 
> > > I don't think so.
> > > 
> > 
> > Roman is right.  For example, here I am looking at a 64 GB (physical)
> > machine which was booted about 30 minutes ago, and which is running
> > disk-heavy workloads (installing VMs).
> > 
> > Since I have started writing this email (2 minutes?), the amount of free
> > memory has already gone down from 37 GB to 33 GB.  I expect that by the
> > time I have finished running the workload, in two hours, it will not
> > have any free memory.
> 
> But what about a VM sitting idle, or that just has more RAM assigned to it
> than is currently using.
>  I've got a host here that's been up for 46 days and has been doing some
> heavy VM debugging a few days ago, but today:
> 
> # free -m
>               total        used        free      shared  buff/cache   available
> Mem:          96536        1146       44834         184       50555       94735
> 
> I very rarely use all it's RAM, so it's got a big chunk of free RAM, and yes
> it's got a big chunk of cache as well.

One of the promises of virtualization is better resource utilization.
People tend to avoid purchasing VMs so much oversized that they never
touch a significant amount of their RAM.  (Well, at least this is how
things stand in hosting market; I guess enterprize market is similar in
this regard).

That said, I'm not at all opposed to optimizing the migration of free
memory; what I'm trying to say is that creating brand new infrastructure
specifically for that case doesn't look justified when the existing one
can cover it in addition to much more common scenarios.

Roman.