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Re: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Mon, 9 Nov 2009 17:27:42 +0200
Cc: Robert Dunkley <Robert@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Mr. Teo En Ming \(Zhang Enming\)" <space.time.universe@xxxxxxxxx>, "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>, Nick Couchman <Nick.Couchman@xxxxxxxxx>
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On Mon, Nov 09, 2009 at 03:24:59PM +0000, Keir Fraser wrote:
> On 09/11/2009 15:17, "Pasi Kärkkäinen" <pasik@xxxxxx> wrote:
> >> Furthermore, while I'm not
> >> Linux developer and so not familiar with how Linux calculates buffering
> >> and caching, I do know that my Linux systems dynamically manage buffers
> >> and caches, and when memory is reduced or some application requires a
> >> larger amount of physical memory, Linux reduces the amount of data in
> >> buffers and caches.
> >> 
> > 
> > Yeah, it has to do with sizing the network buffers, caches etc..
> > 
> > It shouldn't _crash_, so Teo is seeing some bug I believe. But it has
> > always been "best practice" to limit dom0 memory - and prevent weird
> > things happening later (like "memory squeeze in netback driver").
> The issue is not really kernel data like network buffers and buffer cache.
> It is kernel memory metadata -- primarily the per-page info structure that
> the kernel maintains. The metadata doesn't get shrunk with memory size when
> ballooning out, hence it increases as a proportion of memory still assigned
> to the domain. That really is significant when aggressively ballooning down
> a large-memory domain.

Ok, thanks. That makes sense. 

-- Pasi

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