Xen devel - Dec 2009 - [PATCH 3/7] libxenlight resend: 8mb physmap slack for pv domains

Contemplate 8MB of slack for PV domains, since they
do ballooning (or flipping network rx) and need
some extra room in their pfn space.

Note that this does not allocate any extra memory
to the domain, it simply extends the physmap with
some extra room for "bounce bufffering back" pfn''s
that are yielded to dom0.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.com>



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On Wed, 2 Dec 2009, Andres Lagar-Cavilla wrote:
> Contemplate 8MB of slack for PV domains, since they
> do ballooning (or flipping network rx) and need
> some extra room in their pfn space.
> 
> Note that this does not allocate any extra memory
> to the domain, it simply extends the physmap with
> some extra room for "bounce bufffering back" pfn''s
> that are yielded to dom0.
> 
Vincent is probably traveling and for this reason he didn''t reply to
your last email about this patch.

I don''t think he would be happy until you move the 8mb slack into a
configurable parameter in libxl_domain_build_info, defaulted to 8mb in
init_build_info.


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I can cut the patch that way, however ... a fixed 8mb slack for pv domains
has been in place at least since Dec 08 2006, changeset 12803. No one
seems to have needed an optionally larger or smaller slack ;)
Andres
> On Wed, 2 Dec 2009, Andres Lagar-Cavilla wrote:
>> Contemplate 8MB of slack for PV domains, since they
>> do ballooning (or flipping network rx) and need
>> some extra room in their pfn space.
>>
>> Note that this does not allocate any extra memory
>> to the domain, it simply extends the physmap with
>> some extra room for "bounce bufffering back" pfn''s
>> that are yielded to dom0.
>>
>
> Vincent is probably traveling and for this reason he didn''t reply
to
> your last email about this patch.
>
> I don''t think he would be happy until you move the 8mb slack into
a
> configurable parameter in libxl_domain_build_info, defaulted to 8mb in
> init_build_info.
>
>


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No extra memory is being allocated at all in any way. The only impact is 
a wee bit larger p2m table.
Andres
Vincent Hanquez wrote:
> On Wed, Dec 02, 2009 at 06:00:31PM +0000, Andres Lagar-Cavilla wrote:
>   
>> Contemplate 8MB of slack for PV domains, since they
>> do ballooning (or flipping network rx) and need
>> some extra room in their pfn space.
>>
>> Note that this does not allocate any extra memory
>> to the domain, it simply extends the physmap with
>> some extra room for "bounce bufffering back" pfn''s
>> that are yielded to dom0.
>>     
>
> but the memory is still going off from somewhere isn''t it ?
> all those random memory constants spread around in xend took us quite some
time
> to get things right in XCP and XenServer, and also for keeping the library
as
> dumb as possible, I''ld rather have the toolstack set the field in
the structure
> (even if it end up beeing 8M all the time) than starting adding those
things.
>
>   

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On Wed, Dec 02, 2009 at 06:00:31PM +0000, Andres Lagar-Cavilla
wrote:
> Contemplate 8MB of slack for PV domains, since they
> do ballooning (or flipping network rx) and need
> some extra room in their pfn space.
> 
> Note that this does not allocate any extra memory
> to the domain, it simply extends the physmap with
> some extra room for "bounce bufffering back" pfn''s
> that are yielded to dom0.
but the memory is still going off from somewhere isn''t it ?
all those random memory constants spread around in xend took us quite some time
to get things right in XCP and XenServer, and also for keeping the library as
dumb as possible, I''ld rather have the toolstack set the field in the
structure
(even if it end up beeing 8M all the time) than starting adding those things.

-- 
Vincent

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The p2m is *inside* the pv guest. So the only "impact" is 
infinitesimally increasing the chances of an oom_killer going postal. 
This is not for hvm''s with external p2m''s
Andres
Vincent Hanquez wrote:
> On Thu, Dec 03, 2009 at 04:03:58PM +0000, Andres Lagar-Cavilla wrote:
>   
>> No extra memory is being allocated at all in any way. The only impact
is
>> a wee bit larger p2m table.
>>     
>
> yes, and that consumes memory. not sure that''s one we care about
in XCP,
> but as a rule to not make any exceptions, i want the amount to be
explicetly
> tracked.
>
> note that also, whilst xend didn''t have much done regarding memory
stuff,
> xapi (the ocaml daemon running instead of xend in XCP) have load of changes
> regarding how we allocate and track things.
>
>   

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On Thu, Dec 03, 2009 at 04:03:58PM +0000, Andres Lagar-Cavilla
wrote:
> No extra memory is being allocated at all in any way. The only impact is 
> a wee bit larger p2m table.
yes, and that consumes memory. not sure that''s one we care about in
XCP,
but as a rule to not make any exceptions, i want the amount to be explicetly
tracked.

note that also, whilst xend didn''t have much done regarding memory
stuff,
xapi (the ocaml daemon running instead of xend in XCP) have load of changes
regarding how we allocate and track things.

-- 
Vincent

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Contemplate a memory space slack for PV domains,
since they do ballooning (or flipping network rx)
and need some extra room in their pfn space.

Note that this does not allocate any extra memory
to the domain, it simply extends the physmap with
some extra room for "bounce bufffering back" pfn''s
that are yielded to dom0.

The default slack is set at 8MB.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.com>



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I don''t think this one got Acked. I''ve applied all your other
patches
however.

 -- Keir

On 03/12/2009 17:05, "Andres Lagar-Cavilla"
<andres@lagarcavilla.com> wrote:
> Contemplate a memory space slack for PV domains,
> since they do ballooning (or flipping network rx)
> and need some extra room in their pfn space.
> 
> Note that this does not allocate any extra memory
> to the domain, it simply extends the physmap with
> some extra room for "bounce bufffering back" pfn''s
> that are yielded to dom0.
> 
> The default slack is set at 8MB.
> 
> Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.com>
> 


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