[Xen-devel] RE: xl features missing

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 19 Jul 2010, Dan Magenheimer wrote:
> What xl processes are racy for obtaining memory?  Is it possible
> that multiple xl processes are doing an "xl create" at the same time?

Multiple users on the same host must be able to call "xl create" at the
same time.

> 
> I'm not sure I understand completely, but one possible issue is
> that the function that asks "is there enough memory?" must not
> actually tell tmem to surrender the memory.  Also, tmem has
> a "freeze/thaw" mechanism to ensure additional memory isn't
> asynchronously absorbed (search for the instances of "tmem"
> in balloon.py to see this code) and they must always be paired,
> e.g. a freeze without a thaw is disastrous for tmem.
> Note that other dynamic memory mechanisms (even plain ballooning)
> don't have this freeze/thaw capability so there
> is always the potential for races, e.g. for
> 
> - Is there enough memory for a 1G domain?
> - Yes, it appears there is enough so I will proceed
> - Create the domain with 1G of memory
> 
> the last step must be allowed to fail gracefully (preferably
> with a decent error message).

Indeed.
We could also try to catch these errors and and go back to step one for
a few times.


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