Re: [Xen-devel] [PATCH v12 00/26] COarse-grain LOck-stepping Virtual Machines for Non-stop Service

[Lars Kurth <lars.kurth@xxxxxxxxxx>]

On 24/03/2016 16:21, "Ian Jackson" <Ian.Jackson@xxxxxxxxxxxxx> wrote:

>Changlong Xie writes ("[PATCH v12 00/26] COarse-grain LOck-stepping
>Virtual Machines for Non-stop Service"):
>> This patchset implemented the COLO feature for Xen.
>> For detail/install/use of COLO feature, refer to:
>> http://wiki.xen.org/wiki/COLO_-_Coarse_Grain_Lock_Stepping
>
>Thanks for this resend.  I have now worked my way through all the
>patches.  There are mostly only trivial problems, which will be easily
>fixed.
>
[snip]
>
>I recommend that you focus on fixing this patch, urgently, and post a
>new version of perhaps just that patch ("v12.1 14/26" perhaps) ASAP.
>
>I am prepared to do some fixup myself but (i) I'm not sure I fully
>understand the new colo code well enough and (ii) I am going to be
>away, now, until Wednesday.  So relying on extensive help from me
>would be unwise.

I think it would be preferable, and buy extra time, given that Friday
and Monday are public holidays in Europe and not in China.

>There is one other overall area of concern I have with COLO.  It's
>evident that to make use of this code, there are a number of moving
>parts which are not in xen.git and which I haven't seen.  As a result
>the API between libxl/xl and those other parts can't really be
>considered stable.

Given that for a feature to not be experimental, APIs need to be
declared stable, this is a clear point in favour for declaring
COLO experimental, assuming a) v12.1 14/26 can be provided in time
And b) that Wei is happy with the proposal.

>This is IMO fine at this stage of the project's lifecycle in xen.git.
>(I hope Wei, as co-maintainer of libxl, will agree.)
>
>However, we really ought to be testing this code in osstest.  To do
>that we need a complete recipe for setting it up.  Ideally we would
>like code contributions for osstest.  That would also be a useful
>exercise to make sure that implementations of all the important
>components are available.

I agree that for a large feature such as COLO, having some test code
would be extremely desirable. Although this is something which we
can live without for 4.7, but not in the long-run. What I don't know
how hard this would be to do.

>In the Xen 4.8 cycle I think we will need to look at this, with a view
>to moving COLO out of the `experimental' maturity level.

Agreed. As I said, it is unreasonable to expect to declare a large
feature like COLO stable without being able to declare APIs stable
first.

>Lars, do you have any input on this ?

In addition, for a new large component such as COLO, we do also have
a chicken-and-egg problem. Once it is in xen.git, we do need a
Maintainer. Without it, we would not be able to provide security
support, which is another point in favour of declaring the feature
experimental first.

Regards
Lars

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