Re: [PATCH v2] tools/libxl: make default of max event channels dependant on vcpus [and 1 more messages]

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.04.2020 13:54, Jürgen Groß wrote:
> On 06.04.20 13:11, Jan Beulich wrote:
>> On 06.04.2020 13:00, Ian Jackson wrote:
>>> Julien Grall writes ("Re: [PATCH v2] tools/libxl: make default of max event 
>>> channels dependant on vcpus"):
>>>> There are no correlation between event channels and vCPUs. The number of
>>>> event channels only depends on the number of frontend you have in your
>>>> guest. So...
>>>>
>>>> Hi Ian,
>>>>
>>>> On 06/04/2020 11:47, Ian Jackson wrote:
>>>>> If ARM folks want to have a different formula for the default then
>>>>> that is of course fine but I wonder whether this might do ARMk more
>>>>> harm than good in this case.
>>>>
>>>> ... 1023 event channels is going to be plenty enough for most of the use
>>>> cases.
>>>
>>> OK, thanks for the quick reply.
>>>
>>> So, Jürgen, I think everyone will be happy with this:
>>
>> I don't think I will be - my prior comment still holds on there not
>> being any grounds to use a specific OS kernel's (and to be precise
>> a specific OS kernel version's) requirements for determining
>> defaults. If there was to be such a dependency, then OS kernel
>> [variant] should be part of the inputs to such a (set of) formula(s).
> 
> IMO this kind of trying to be perfect will completely block a sane
> heuristic for being able to boot large guests at all.

This isn't about being perfect - I'm suggesting to leave the
default alone, not to improve the calculation, not the least
because I've been implying ...

> The patch isn't about to find an as stringent as possible upper
> boundary for huge guests, but a sane value being able to boot most of
> those.
> 
> And how should Xen know the OS kernel needs exactly after all?

... the answer of "It can#t" to this question.

> And it is not that we talking about megabytes of additional memory. A
> guest with 256 vcpus will just be able to use additional 36 memory
> pages. The maximum non-PV domain (the probably only relevant case
> of another OS than Linux being used) with 128 vcpus would "waste"
> 32 kB. In case the guest misbehaves.

Any extra page counts, or else - where do you draw the line? Any
single page may decide between Xen (not) being out of memory,
and hence also not being able to fulfill certain other requests.

> The alternative would be to do nothing and having to let the user
> experience a somewhat cryptic guest crash. He could google for a
> possible solution which would probably end in a rather high static
> limit resulting in wasting even more memory.

I realize this. Otoh more people running into this will improve
the chances of later ones finding useful suggestions. Of course
there's also nothing wrong with trying to make the error less
cryptic.

Jan