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Re: [Qemu-devel] [PATCH v6 kernel 0/5] Extend virtio-balloon for fast (d

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH v6 kernel 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration
Date: Wed, 18 Jan 2017 17:38:53 +0200

On Wed, Jan 18, 2017 at 11:09:30AM +0100, David Hildenbrand wrote:
> 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.

As long as the interface is similar, it seems to make
sense for me - why invent a completely new device that
looks very much like the old one?

So this boils down to whether the speedup patches are merged.

> -- 
> David

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