Re: [RFC PATCH] page_alloc: use first half of higher order chunks when halving

[Matthew Rushton <mvrushton@gmail.com>]

On 04/01/14 05:22, Tim Deegan wrote:
> At 06:48 -0400 on 01 Apr (1396331306), Konrad Rzeszutek Wilk wrote:
>> On Mon, Mar 31, 2014 at 08:25:43PM -0700, Matthew Rushton wrote:
>>> Are there any concerns about the proposed Xen change as a reasonable
>>> work around for the current implementation? Thank you!
>> So I finally understood what the concern was about it - the balloon
>> mechanics get the pages in worst possible order. I am wondeirng if there
>> is something on the Linux side we can do to tell Xen to give them to use
>> in the proper order?
> The best way, at least from Xen's point of view, is to explicitly
> allocate contiguous pages in the cases where it'll make a difference
> AIUI linux already does this for some classes of dma-able memory.

I'm in agreement that if any change is made to Linux it should be to 
make as large as possible allocations and back off accordingly. I 
suppose another approach could be to add a boot option to not reallocate 
at all.

>> Could we swap the order of xen_do_chunk so it starts from the end and
>> goes to start? Would that help?
> As long as we don't also change the default allocation order in
> Xen. :)  In general, linux shouldn't rely on the order that Xen
> allocates memory, as that might change later.  If the current API
> can't do what's needed, maybe we can add another allocator
> hypercall or flag?

Agree on not relying on the order in the long run. A new hypercall or 
flag seems like overkill right now. The question for me comes down to my 
proposed change which is more simple and solves the short term problem 
or investing time in reworking the Linux code to make large allocations.

>
> Cheers,
>
> Tim.