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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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".

Ian.


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