Xen devel - Aug 2005 - RE: Re: [Xen-changelog] fail domU creation if memory need couldn''t be succeed after ballooning out dom0

> In general, we should try to avoid using timeouts at all 
> cost.  If we do have to use a timeout, then we should make 
> sure it''s long enough that the timeout is never going to 
> happen.  It''s surprisingly easy to have enough load on your 
> system such you miss a 2 second timeout.
Yep, in general I''m all for making timeouts long enough to be visible.

However, this timeout is different: its something we expect may fail (as
opposed to an internal failure of the tools). We''re asking domain 0 to
try and shrink itself. It may be unable to, in which case trying for a
few seconds and then failing the domain create seems like a good
approach. If domain 0 can''t page enough stuff out after a second or
two,
its unlikely to succeed without operator intervention (e.g. killing
something).

2s is perhaps overly short, but I doubt going larger than 5s would
result in any greater chance of success, and would just anoy users.

Ian

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Ian Pratt wrote:
>>In general, we should try to avoid using timeouts at all 
>>cost.  If we do have to use a timeout, then we should make 
>>sure it''s long enough that the timeout is never going to 
>>happen.  It''s surprisingly easy to have enough load on your 
>>system such you miss a 2 second timeout.
>>    
>>
>
>Yep, in general I''m all for making timeouts long enough to be
visible.
>
>However, this timeout is different: its something we expect may fail (as
>opposed to an internal failure of the tools).
>
Is it really possible to fail?  I just did:

echo 42 > /proc/xen/balloon

On domain-0 and the system crashed.  It would be nice if it actually 
failed but currently the only thing this timeout will do is prevent us 
from working when we could have :-)

How about when we fix the balloon driver, we have it write a node in the 
store that indicates whether the balloon has succeeded or failed and 
then the tools can simply set a watch and wait for something to happen?

Regards,

Anthony Liguori
> We''re asking domain 0 to
>try and shrink itself. It may be unable to, in which case trying for a
>few seconds and then failing the domain create seems like a good
>approach. If domain 0 can''t page enough stuff out after a second or
two,
>its unlikely to succeed without operator intervention (e.g. killing
>something).
>
>2s is perhaps overly short, but I doubt going larger than 5s would
>result in any greater chance of success, and would just anoy users.
>
>Ian
>
>  
>

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