linux-kernel - RE: [PATCH v6 kernel 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

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Date:   Wed, 18 Jan 2017 13:29:59 +0000
From:   "Li, Liang Z" <liang.z.li@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "amit.shah@...hat.com" <amit.shah@...hat.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "cornelia.huck@...ibm.com" <cornelia.huck@...ibm.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "dgilbert@...hat.com" <dgilbert@...hat.com>,
        "quintela@...hat.com" <quintela@...hat.com>
Subject: RE: [PATCH v6 kernel 0/5] Extend virtio-balloon for fast
 (de)inflating & fast live migration

> Am 21.12.2016 um 07:52 schrieb Liang Li:
> > This patch set contains two parts of changes to the virtio-balloon.
> >
> > One is the change for speeding up the inflating & deflating process,
> > the main idea of this optimization is to use {pfn|length} to present
> > the page information instead of the PFNs, to reduce the overhead of
> > virtio data transmission, address translation and madvise(). This can
> > help to improve the performance by about 85%.
> >
> > Another change is for speeding up live migration. By skipping process
> > guest's unused pages in the first round of data copy, to reduce
> > needless data processing, this can help to save quite a lot of CPU
> > cycles and network bandwidth. We put guest's unused page information
> > in a {pfn|length} array and send it to host with the virt queue of
> > virtio-balloon. For an idle guest with 8GB RAM, this can help to
> > shorten the total live migration time from 2Sec to about 500ms in
> > 10Gbps network environment. For an guest with quite a lot of page
> > cache and with little unused pages, it's possible to let the guest
> > drop it's page cache before live migration, this case can benefit from this
> new feature too.
> 
> I agree that both changes make sense (although the second change just
> smells very racy, as you also pointed out in the patch description), however I
> am not sure if virtio-balloon is really the right place for the latter change.
> 
> virtio-balloon is all about ballooning, nothing else. What you're doing is using
> it as a way to communicate balloon-unrelated data from/to the hypervisor.
> Yes, it is also about guest memory, but completely unrelated to the purpose
> of the balloon device.
> 
> Maybe using virtio-balloon for this purpose is okay - I have mixed feelings
> (especially as I can't tell where else this could go). I would like to get a second
> opinion on this.
> 

We have ever discussed the implementation for a long time, making use the current
virtio balloon seems better than the other solutions and is recommended by Michael.

Thanks!
Liang
> --
> 
> David