Mailing List Archive

> No one really expects a dom0_mem of 16Mb to work reliably.
>
> So perhaps it's not a good default value? I'm not currently seeing the
> value of a low default, given that people who need a small value can
> easily set it. May I suggest a default value of 64Mb?
>
> Benefits include reduced FAQs on this list, better out-of-the-box
> robustness, and optimization for the common case.
>
> If there are benefits to sticking with 16Mb, a quick summary would be
> appreciated.

Sensible defaults are probably either a fixed proportion of total
system RAM, or just to print an error and hang. Either of these is
easy to implement.

-- Keir


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Keir Fraser wrote:
> Sensible defaults are probably either a fixed proportion of total
> system RAM, or just to print an error and hang. Either of these is
> easy to implement.

I wonder, is there a meaningful difference in behaviour between giving
dom0 just the memory it should have, or giving dom0 all the memory
there is and then just ballooning free space for the virtual machines?

If ballooning is a working option, the default could be even to give
all memory to dom0. (Ofcourse excluding what is needed for Xen to
properly function.)

-- Naked




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Jared> [P]erhaps [...] a dom0_mem of 16Mb [is] not a good default
Jared> value?

Keir> Sensible defaults are probably either a fixed proportion of
Keir> total system RAM, or just to print an error and hang. Either
Keir> of these is easy to implement.

Given only those two choices, I definitely think that "require
dom0_mem to be set otherwise error out" is the preferred choice.

Using a percentage of total RAM seems error-prone and will cause even
more community problems because of different behaviors on different
machines. (In other words, you'll now need to ask not only "what did
you set dom0_mem to?", but also "how much total is available?"). If
the default is say 25%, then a 64Mb machine is right back to the
problematic 16Mb, while a 2Gb machine would reserve an unneeded
half-a-gig. Correctness and consistency should come first, so I think
the "percentage of available" approach should be rejected (though
supporting a "dom0_mem=25%" would be a reasonable enhancement).

I personally still think that having an actual value for a default
would be somewhat preferred over "dom0_mem is now a required
parameter", as that's a behavior change instead of a simple tuning.
Since all the documentation already says "dom0_mem=131072", simply
setting that to be the default would likely generate the "least
suprise" for users and work in almost all cases.

Ultimately, it's strictly a matter of opinion, and the community is
unlikely to provide a clear consensus as to the "best" approach. If a
change seems warranted based on the recurring FAQs, I'm sure the
development team will make an appropriate choice.

-- jared@wordzoo.com

"Truth is a great flirt." -- Franz Liszt


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