RE: [Xen-devel] [PATCH] PoD: Handle operations properly when domain is dying

[Dave Scott <Dave.Scott@xxxxxxxxxxxxx>]

> A possibly related issue came up in a team discussion
> yesterday.  It is not uncommon for management tools to
> check available memory to decide if there is sufficient
> space to provision a new domain, for example when choosing
> a machine on which to start a new domain.  But there is
> no "lock" so if the answer is yes and the machine
> is chosen, but moments later some memory becomes used
> (by PoD or ballooning or ???), domain creation might
> fail due to insufficient memory.

In the xapi toolstack world we assume that all domains on a host will be able 
to balloon down to their "dynamic_min" and use this in our host-choosing logic. 
We bias the choice of host towards the ones with the most memory free... but 
ultimately if the guests don't respond then we'll still get an out-of-memory 
error.

We're also writing/debugging a host ballooning daemon which you can ask to 
reserve memory for stuff like a domain creation. It asks the domains to balloon 
down and then makes sure they stay down by setting maxmem. I'm in the middle of 
writing up how it's supposed to work, I'll send a link around for comments when 
I have a draft.

> Tmem has a "freeze" feature to avoid this problem, but
> tmem has the advantage that all domains will continue
> to function even if tmem is frozen... I'm not sure
> that's true for PoD.

This sounds interesting -- do you have a link to a document somewhere?

Cheers,
Dave

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