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Re: [Xen-devel] [PATCH] RFC: initial libxl support for xenpaging

  • To: xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: "Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>
  • Date: Fri, 17 Feb 2012 08:55:38 -0800
  • Cc: olaf@xxxxxxxxx, ian.campbell@xxxxxxxxxx
  • Delivery-date: Fri, 17 Feb 2012 16:55:59 +0000
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> Date: Fri, 17 Feb 2012 16:43:18 +0000
> From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> To: Olaf Hering <olaf@xxxxxxxxx>
> Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH] RFC: initial libxl support for
>       xenpaging
> Message-ID: <1329496998.3131.131.camel@xxxxxxxxxxxxxxxxxxxxxx>
> Content-Type: text/plain; charset="UTF-8"
> On Fri, 2012-02-17 at 16:03 +0000, Olaf Hering wrote:
>> On Fri, Feb 17, Ian Campbell wrote:
>> > That's exactly the sort of complexity we should not be exposing to the
>> > end user!
>> Of course not as is!
>> Right now one has to understand two values, maxmem and the size of the
>> memhog called "guest balloon driver". Paging adds a third one.
> Right, but that third one should not be "paging size" or anything like
> that, just like they should not really have to understand what "a memhog
> called "guest balloon driver"" is. Even today the thing we actually
> expose is just called "memory" in the configuration.
> The user cares about three things:
>      1. The maximum amount of memory a guest can use.
>      2. The amount of memory which the guest thinks it has.
>      3. The actual amount of memory the guest has
> The fact that these are implemented by some combination of paging and/or
> ballooning is not really of interest to the user. They only need to be
> aware that if #3 < #2 then there is a performance cost and that even if
> #2==#3 a guest which tries to use more memory than that will be suffer a
> performance penalty.
> My point is that the memory knobs should be exposed to the user of xl as
> semantically meaningful options without the need to refer to "the paging
> target" or "the balloon target" which is why there should not IMHO be
> "xl commands to adjust just ballooning and/or paging".

May I suggest you call these things 'memory' and 'footprint'? Users can
adjust the memory the guest thinks it has (#2 above), and the footprint
the domain will actually occupy (#3).

Then footprint can be adjusted by paging or something else. I don't want
to begin to think about how PoD would jive there. It only seems to affect
'footprint' during bootup.

> Ian.

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