Re: [Xen-devel] [PATCH v3 3/3] xl: Improve xl documentation in regards to guest memory management

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Mon, Apr 08, 2013 at 05:19:00PM +0100, Ian Jackson wrote:
> Daniel Kiper writes ("[Xen-devel] [PATCH v3 3/3] xl: Improve xl documentation 
> in regards to guest memory management"):
> > Improve xl documentation in regards to guest memory management.
>
> Thanks.
>
> > +=item B<mem_set_enforce_limit=BOOLEAN>
> > +
> > +If enabled, then C<xl mem-set> by default will set a memory allocation
> > +target and enforce the maximum memory allocation for a given domain.
> > +If disabled, then C<xl mem-set> by default will set a memory allocation
> > +target only for a given domain.
>
> I think you mean "for each domain" rather than "for a given domain",
> and "will set only a memory allocation target for each domain" ?

Yes, thanks.

> > +The memory allocation target is a suggested amount of memory which
> > +should be allocated by a guest. It is nice when the guest follows this
> > +value but it may have its own idea about memory requirments. However,
> > +it could not allocate more memory than a ceiling known as the maximum
> > +memory allocation.
>
> I would write "However the guest is prevented by Xen from allocating
> more memory than a ceiling known as the "xen maximum" memory
> allocation" or some such.
>
> > --- a/docs/misc/libxl_memory.txt
> > +++ b/docs/misc/libxl_memory.txt
> > @@ -15,14 +15,19 @@
> >               |  |          |  |    |        | target  |          |
> >               |  | guest    |  |    | build  |         |          |
> >               |  |          |  |    | start  |         |          |
> > -      static |  |          |  |    |        |         |          |
> > -     maximum |  +----------+  |    +        +         +          +
> > -             |  |          |  |
> > -             |  |          |  |
> > +             |  |          |  |    |        |         |          |
> > +             |  +----------+  |    +        +         +          +
> > +      static |  |          |  |
> > +     maximum |  |          |  |
> >               |  | balloon  |  | build
> >               |  |          |  | maximum
> >               |  |          |  |
> > -             +  +----------+  +
> > +             |  +----------+  +
> > +             |  |          |
> > +             |  | memory   |
> > +             |  | hotplug  |
> > +             |  |          |
> > +             +  +----------+
>
> This change to the diagram implies that the target, and and the xen
> maximum, never include any hotplug memory.  I don't think that can be
> true.

Right, "xen maximum" should include hotplug memory. However, I think that
"actual target" should be split to "suggested target" and "actual target".
"suggested target" could be up to "static maximum". "actual target"
always include only video and guest memory (currently alloceted memory
by domain).

> Presumably hotplugged memory can be ballooned too.  So perhaps we need
> another column ?

I think that it is better to add comment that after
hotplugging memory could be ballooned down.

> >  #### ~/memory/target = MEMKB []
> >
> >  The current balloon target for the domain. The balloon driver within
> > -the guest is expected to make every effort to every effort use no more
> > -than this amount of RAM.
> > +the guest is expected to make every effort to use no more than this
> > +amount of RAM.
>
> Good fix.
>
> It might be worth adding:
>
>   "Guests whose actual allocation exceeds the target may experience
>    memory allocation failures."

... or "Guests which attempts to exceed the target may experience
memory allocation failures."

Daniel

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