RE: [Xen-devel] what happens when a PoD page is touched?

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 
> On Mon, May 16, 2011 at 10:22 AM, James Harper
> <james.harper@xxxxxxxxxxxxxxxx> wrote:
> > I've not seen any slowdown on boot with any version of Windows until
I
> > go to actually do the balloon down...
> 
> You mean, you don't see slowness on boot unless you boot with maxmem
> != memory (i.e,. in PoD mode)?

Not quite what I mean. If I boot with memory=512 and maxmem=4100, there
are no performance problems I have noticed until my balloon down driver
starts returning memory to xen very early in boot. In allocating this
memory, Windows zero's it before it gives it to me which means I am
touching 3.5GB of memory and presumably invoking the emergency sweep for
pretty much every page.

> 
> For one thing, after the balloon driver is finished with its initial
> allocation, there shouldn't be any sweeping; if there is, then it may
> be that you're not actually inflating the balloon as much as Xen
> thinks you should.
> 
> Before the balloon driver is done, the emergency sweep could be an
> issue.  We had a bunch of stuff in XenServer in our most recent
> release to try to mitigate this, but it was all pretty hacky and
> unsuitable for upstreaming.  I determined that it just needs to be
> re-written in a way that actually made things Better; I haven't gotten
> a chance to do that yet.

Yes it sounds like that might be the case.

> 
> It looks like ATM the sweep is limited to doing 2MiB at a time; so if
> you have a reasonably sized guest, every other superpage that gets hit
> will cause another sweep -- potentially a big problem.
> 
> I have a patch that attempts to greedily re-grab pages after a sweep;
> I'll check to see if it applies to tip, and if so, I'll send it to you
> to see if that helps any.
> 

Thanks

James

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