[Xen-devel] RE: tmem - really default to on?

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 05.02.10 19:44 >>>
>> >>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 04.02.10 22:01 >>>
>> >A) There is some order>0 memory allocation in Xen domain creation
>> >that doesn't fall back to order=0, that I've not seen in my testing
>> >but shows up in your systems (or has very recently been added).
>> >If true, this is a problem not only for tmem but also for all other
>> >memory optimization work and we need to identify and (if possible)
>> >fix it.
>> 
>> This is the case - x86's shadow code does all its allocations as order-
>> 2
>> with no (possible the way things are designed) fallback.
>
>Hmmm.... so this would affect HVM creation but not PVM, but also
>could cause PVM live migration to fail, correct?

Yes.

>> Also, the domain structure itself is of order 4 (38k), obviously
>> without
>> fallback (and even with address range restriction, though that one
>> is affecting only really big machines, and it could be lifted to
>> effectively not be a restriction anymore, i.e. just serve documentation
>> purposes).
>
>This has likely been avoided by luck when lots of memory is
>flushed from tmem and returned to the Xen heap and consolidated.
>
>Are you suggesting that the domain structure could/should have
>two sizes, dynamically chosen by machine size?  Or something
>else?

No, it just should be split into parts each of which fits in a page
independent of architecture. But that's nothing I would consider
realistic for 4.0.

>In any case, I'd still suggest turning tmem off in your dom0
>is the best short-term solution.

I'm still not following you here: For one, I can't recall a way to turn
of tmem on a per-domain basis. Then I can't see why it should be
only our Dom0 to be affected. And finally I can't see how the same
couldn't happen when only DomU-s use tmem.

Jan


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