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

From: David Hildenbrand
Subject: Re: [Qemu-devel] [PATCH v6 kernel 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration
Date: Wed, 18 Jan 2017 11:09:30 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

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.



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