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Re: [Xen-devel] [PATCH for-xen-4.5] console: increase initial conring size

On Tue, Dec 02, 2014 at 03:58:53PM +0000, Jan Beulich wrote:
> >>> Daniel Kiper <daniel.kiper@xxxxxxxxxx> 12/02/14 3:58 PM >>>
> >In general initial conring size is sufficient. However, if log
> >level is increased on platforms which have e.g. huge number
> >of memory regions (I have an IBM System x3550 M2 with 8 GiB RAM
> >which has more than 200 entries in EFI memory map) then some
> >of earlier messages in console ring are overwritten. It means
> >that in case of issues deeper analysis can be hindered. Sadly
> >conring_size argument does not help because new console buffer
> >is allocated late on heap. It means that it is not possible to
> >allocate larger ring earlier. So, in this situation initial
> >conring size should be increased. My experiments showed that
> >even on not so big machines more than 26 KiB of free space are
> >needed for initial messages. In theory we could increase conring
> >size buffer to 32 KiB. However, I think that this value could be
> >too small for huge machines with large number of ACPI tables and
> >EFI memory regions. Hence, this patch increases initial conring
> >size to 64 KiB.
> >
> >Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> I think it was made clear before that just saying "for-xen-4.5" without any
> further rationale is insufficient. Please explain what makes this so important
> a change that it needs to go in now.

What is not clear in commit message? It describes a bug, how to fix it
and why in that way. Do you need anything else?

> Apart from that, as Olaf validly replied, setting up a dynamically allocated
> buffer earlier would seem like a much better course of action.

I though about that before posting this patch (I did not know beforehand about
Olaf's work made in 2011). However, I stated that it is too late to make so
intrusive changes. I think we should (sadly) apply this "band-aid" right
now because, as you can see, this bug hits more and more people. On the
other hand I agree that we should finally fix this issue in better way.
Even I am able to add this thing to my TODO list but it is quite long
so I do not know when it will happen.


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