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[Qemu-devel] Re: virtio: Add memory statistics reporting to the balloon

From: Anthony Liguori
Subject: [Qemu-devel] Re: virtio: Add memory statistics reporting to the balloon driver
Date: Tue, 10 Nov 2009 18:07:56 -0600
User-agent: Thunderbird (X11/20090825)

Rusty Russell wrote:
On Wed, 11 Nov 2009 08:22:42 am Anthony Liguori wrote:
Rusty Russell wrote:
On Tue, 10 Nov 2009 03:02:06 am Adam Litke wrote:
A simpler approach is to collect memory statistics in the virtio
balloon driver and communicate them to the host via the device config space.
There are two issues I see with this.  First, there's an atomicity problem
since you can't tell when the stats are consistent.  Second, polling is

A stats vq might solve this more cleanly?
This turns out to not work so nicely. You really need bidirectional communication. You need to request that stats be collected and then you need to tell the hypervisor about the stats that were collected. You don't need any real correlation between requests and stat reports either.

You register an outbuf at initialization time.  The host hands it back when
it wants you to refill it with stats.

That's strangely backwards. Guest send a stat buffer that's filled out, host acks it when it wants another. That doesn't seem bizarre to you?

This really models how target/actual work and I think it suggests that we want to reuse that mechanism for the stats too.

Sure, I want to.  You want to.  It's simple.

But the universe is remarkably indifferent to what we want.  Is it actually
sufficient or are we going to regret our laziness?

It's not laziness, it's consistency. How is actual different than free memory or any other stat?



Anthony Liguori

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