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

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Message-ID: <20160304075538.GC9100@rkaganb.sw.ru>
Date:	Fri, 4 Mar 2016 10:55:39 +0300
From:	Roman Kagan <rkagan@...tuozzo.com>
To:	"Dr. David Alan Gilbert" <dgilbert@...hat.com>
CC:	Liang Li <liang.z.li@...el.com>, <ehabkost@...hat.com>,
	<kvm@...r.kernel.org>, <mst@...hat.com>, <quintela@...hat.com>,
	<linux-kernel@...r.kernel.org>, <qemu-devel@...gnu.org>,
	<linux-mm@...ck.org>, <amit.shah@...hat.com>,
	<pbonzini@...hat.com>, <akpm@...ux-foundation.org>,
	<virtualization@...ts.linux-foundation.org>, <rth@...ddle.net>
Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration
 optimization

On Thu, Mar 03, 2016 at 05:46:15PM +0000, Dr. David Alan Gilbert wrote:
> * Liang Li (liang.z.li@...el.com) wrote:
> > The current QEMU live migration implementation mark the all the
> > guest's RAM pages as dirtied in the ram bulk stage, all these pages
> > will be processed and that takes quit a lot of CPU cycles.
> > 
> > From guest's point of view, it doesn't care about the content in free
> > pages. We can make use of this fact and skip processing the free
> > pages in the ram bulk stage, it can save a lot CPU cycles and reduce
> > the network traffic significantly while speed up the live migration
> > process obviously.
> > 
> > This patch set is the QEMU side implementation.
> > 
> > The virtio-balloon is extended so that QEMU can get the free pages
> > information from the guest through virtio.
> > 
> > After getting the free pages information (a bitmap), QEMU can use it
> > to filter out the guest's free pages in the ram bulk stage. This make
> > the live migration process much more efficient.
> 
> Hi,
>   An interesting solution; I know a few different people have been looking
> at how to speed up ballooned VM migration.
> 
>   I wonder if it would be possible to avoid the kernel changes by
> parsing /proc/self/pagemap - if that can be used to detect unmapped/zero
> mapped pages in the guest ram, would it achieve the same result?

Yes I was about to suggest the same thing: it's simple and makes use of
the existing infrastructure.  And you wouldn't need to care if the pages
were unmapped by ballooning or anything else (alternative balloon
implementations, not yet touched by the guest, etc.).  Besides, you
wouldn't need to synchronize with the guest.

Roman.