linux-kernel - Re: virtio: Add memory statistics reporting to the balloon driver

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Date:	Wed, 11 Nov 2009 13:13:23 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Anthony Liguori <aliguori@...ibm.com>
Cc:	agl@...ux.vnet.ibm.com, qemu-devel@...gnu.org,
	Avi Kivity <avi@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: virtio: Add memory statistics reporting to the balloon driver

On Wed, 11 Nov 2009 10:37:56 am Anthony Liguori wrote:
> Rusty Russell wrote:
> > 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?

Yep!  But that's a limitation of our brains, not the infrastructure ;)

Think of the stats as an infinite stream of data.  Read from it at your
leisure.  This is how, for example, console output works.

> > 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?

Because it's a COLLECTION of stats.  For example, swap in should be < swap
out.  Now, the current Linux implementation of all_vm_events() is non-atomic
anyway, so maybe we can just document this as best-effort.  I'm saying that
if it *is* a problem, I think we need a vq.

But it raises the question: what stats are generally useful cross-OS?  Should
we be supplying numbers like "unused" (free) "instantly discardable" (ie.
clean), "discardable to disk" (ie. file-backed), "discardable to swap"
(ie. swap-backed) and "unswappable" instead?

(I just made those up, of course, but it seems like that would give a fair
indication of real memory pressure in any OS).

Thanks,
Rusty.
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