Re: [Xen-devel] xenstored memory leak

[Juergen Gross <jgross@xxxxxxxx>]

On 13/07/16 14:40, Andrew Cooper wrote:
> On 13/07/16 13:21, Juergen Gross wrote:
>> On 06/07/16 09:31, Juergen Gross wrote:
>>> While testing some patches for support of ballooning in Mini-OS by using
>>> the xenstore domain I realized that each xl create/destroy pair would
>>> increase memory consumption in Mini-OS by about 5kB. Wondering whether
>>> this is a xenstore domain only effect I did the same test with xenstored
>>> and oxenstored daemons.
>>>
>>> xenstored showed the same behavior, the "referenced" size showed by the
>>> pmap command grew by about 5kB for each create/destroy pair.
>>>
>>> oxenstored seemed to be even worse in the beginning (about 6kB for each
>>> pair), but after about 100 create/destroys the value seemed to be
>>> rather stable.
>>>
>>> Did anyone notice this memory leak before?
>> I think I've found the problem:
>>
>> qemu as the device model is setting up a xenstore watch for each backend
>> type it is supporting. Unfortunately those watches are never removed
>> again. This sums up to the observed memory leak.
>>
>> I'm not sure how oxenstored is avoiding the problem, may be by testing
>> socket connections to be still alive and so detecting qemu has gone.
>> OTOH this won't help for oxenstored running in another domain than the
>> device model (either due to oxenstore-stubdom, or a driver domain with
>> a qemu based device model).
> 
> I do seem to remember something along those lines.
> 
>>
>> I'll post a qemu patch to remove those watches on exit soon.
> 
> That is good, but there needs to be further consideration as to how to
> clean up after a crashed qemu/etc.

Hmm, I see two possibilities:

a) Add a possibility to do xs_unwatch() with a path and a token with
   wildcards (e.g.: xs_unwatch(xs, "/local/domain/0/backend/qdisk",
   "be:*:97:*") for removing all qdisk watches for domid 97) and use
   this when doing the xenstore cleanup from libxl on domain
   destruction.

b) Instead of watching /local/domain/<dm-domid>/backend/<type> let qemu
   create /local/domain/<dm-domid>/backend/<type>/<domid> and watch
   this. When this directory is being removed by libxl all the watches
   will be gone automatically.


Juergen

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