Re: [PATCH 2/2] console/serial: bump buffer from 16K to 32K

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.06.2022 17:23, Roger Pau Monné wrote:
> On Thu, Jun 23, 2022 at 03:32:30PM +0200, Jan Beulich wrote:
>> On 23.06.2022 11:08, Roger Pau Monne wrote:
>>> Testing on a Kaby Lake box with 8 CPUs leads to the serial buffer
>>> being filled halfway during dom0 boot, and thus a non-trivial chunk of
>>> Linux boot messages are dropped.
>>>
>>> Increasing the buffer to 32K does fix the issue and Linux boot
>>> messages are no longer dropped.  There's no justification either on
>>> why 16K was chosen, and hence bumping to 32K in order to cope with
>>> current systems generating output faster does seem appropriate to have
>>> a better user experience with the provided defaults.
>>
>> Just to record what was part of an earlier discussion: I'm not going
>> to nak such a change, but I think the justification is insufficient:
>> On this same basis someone else could come a few days later and bump
>> to 64k, then 128k, etc.
> 
> Indeed, and that would be fine IMO.  We should aim to provide defaults
> that work fine for most situations, and here I don't see what drawback
> it has to increase the default buffer size from 16kiB to 32kiB, and
> I would be fine with increasing to 128kiB if that's required for some
> use case, albeit I have a hard time seeing how we could fill that
> buffer.
> 
> If I can ask, what kind of justification you would see fit for
> granting an increase to the default buffer size?

Making plausible that for a majority of contemporary systems the buffer
is not large enough would be one aspect. But then there's imo always
going to be an issue: What if non-Linux Dom0 would be far more chatty?
What if Linux, down the road, was made less verbose (by default)? What
if people expect large enough a buffer to also suffice when Linux runs
in e.g. ignore_loglevel mode? We simply can't fit all use cases and at
the same time also not go overboard with the default size. That's why
there's a way to control this via command line option.

Jan