Re: [Qemu-devel] balloon driver

From: Mark Williamson <mark.williamson@cl.cam.ac.uk>
To: qemu-devel@nongnu.org
Cc: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] balloon driver
Date: Sun, 9 Jul 2006 02:32:54 +0100	[thread overview]
Message-ID: <200607090232.55272.mark.williamson@cl.cam.ac.uk> (raw)
In-Reply-To: <946a3fda0607081806r56d18980vd00f570de44a1466@mail.gmail.com>

> It seems the point of the balloon driver is to avoid forcing the host
> to swap.  For example, suppose I start a new guest OS.  I check the
> memory usage on the host and everything looks pretty good, maybe 30MB
> used.  Then suppose I run a recursive grep command in a Linux source
> tree on the guest.  The host memory usage will climb to the maximum
> allotted memory as the guest OS fills its page cache with pages of
> kernel source.  Now, I go back to the host and decide I want to run
> something a little memory intensive.  The host has to swap and
> dutifully copies those pages of kernel source to swap.  Much better
> would be if I could just chuck those pages and give them back to the
> host, no swapping at all.
>
> Even if the guest has to swap, the reasoning is that the guest is in a
> much better position to figure out what to swap than if the host were
> forced to.

Worse, the guest might decide to swap out a page that's already swapped in by 
the host, forcing it to be read in again only to be immediately written out 
to disk by the guest :-(

> It is a rather crude approach and I'm not sure how much practical
> benefit there is, but I'll probably go ahead and code it up (at least
> for a Linux host) if only for myself as it looks pretty simple.
> (Linux 2.6.16 added the ability to punch holes in tmpfs files so all
> the hard work should be done).  I just wondered if there was anything
> I was missing or if anyone had considered it before.

Xen has a balloon driver - you might like to take a look at that for starters, 
and maybe borrow some code from it!  There's been occasional talk to the 
effect that a number of projects might as well share functionality like 
ballooning since a number of systems require it.

Cheers,
Mark

>
> - E
>
>
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-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!