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

To: Pasi Kärkkäinen <pasik@xxxxxx>, Nick Couchman <Nick.Couchman@xxxxxxxxx>
Subject: Re: [Xen-devel] Re: [Xen-users] Max. PV and HVM Guests
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 09 Nov 2009 15:24:59 +0000
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Mr. Teo En Ming \(Zhang Enming\)" <space.time.universe@xxxxxxxxx>, "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>, Robert Dunkley <Robert@xxxxxxxxx>
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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.

 -- Keir



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