Re: zone movable patches comments

[Andy Whitcroft <apw@shadowen.org>]

Yasunori Goto wrote:
>>> No I really don't see why kernelcore=toosmall is any better than
>>> movable_mem=toobig. And why do you think the admin knows how much
>>> memory is enough to run the kernel, or why should that be the same
>>> between different sized machines? If you have a huge machine, you
>>> need much more addressable kernel memory for the mem_map array
>>> before you even think about anything else.
>>>
>>> Actually, it is more likely that the admin knows exactly how much
>>> memory they need to reserve (eg. for their database's shared
>>> memory segment or to hot unplug or whatever), and in that case
>>> it is much better to be able to specify movable_mem= and just be
>>> given exactly what you asked for and the kernel can be given the
>>> rest.
> 
> If hot-unplug is invoked after bootup, then movable_mem will be
> useful to specify removable memory size. It is true.
> 
> However, if hot-add is invoked at first after bootup, 
> movable_mem is not so useful.
> I think admin expects hot-add memory will be removable zone in many
> case, because he wish the memory for his application rather than
> for kernel.
> But, movable mem can't specify size of hot-add memory in the future.
> I suppose "kernelcore" is desirable for its case.

I would have expected either would interact successfully with
hot-remove/hot-add.  It makes sense to the administrator to say "I will
be removing this much memory" movable_mem=N.  For the hot-add case I
would have expected a zero sized movable_mem would suffice, the new
memory being added to and expanding the zone as it goes.

I envisioned "kernelcore" and "movable_mem" (that name is nasty btw can
anyone think of a better one) being minimum's.  So the expansion of
ZONE_MOVABLE on hot-plug of memory fits that semantically.  I think what
I am saying is you really want movable_mem=, another sane use-case.

>>> If somebody is playing with this parameter, they definitely know
>>> what they are doing and they are not just blindly throwing it out
>>> over their cluster because it might be a good idea.
>> It feels very much that there are two usage models.  Those who know how
>> much "kernel" memory works for them and want whatever is left usable for
>> their small/huge page workloads, and those who know how much they need
>> for their DB and are happy for the system to have the rest.  Both seem
>> like valid use cases, both would have the same underlying implementation
>> a sized ZONE_MOVABLE.
>>
>> How about we have two kernel options "kernelcore=" and "movable=" which
>> would both size ZONE_MOVABLE.  Both would be the minimum sizes, so the
>> effective differences would be the rounding to whole pageblocks.
> 
> I would like to vote it due to above mentioned. :-)

-apw

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>