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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/03/14 11:41, Matt Wilson wrote:
> On Wed, Mar 26, 2014 at 11:13:49AM +0000, Ian Campbell wrote:
>> On Wed, 2014-03-26 at 12:48 +0200, Matt Wilson wrote:
>>> On Wed, Mar 26, 2014 at 10:44:18AM +0000, David Vrabel wrote:
>>>> On 26/03/14 10:17, Matt Wilson wrote:
>>>>> Konrad, what's the possibility of fixing this in Linux Xen PV setup
>>>>> code? I think it'd be a matter batching up pages and doing larger
>>>>> order allocations in linux/arch/x86/xen/setup.c:xen_do_chunk(),
>>>>> falling back to smaller pages if allocations fail due to
>>>>> fragmentation, etc.
>>>> We plan to fix problems caused by non-machine-contiguous memory by
>>>> setting up the IOMMU to have 1:1 bus to pseudo-physical mappings.  This
>>>> would avoid using the swiotlb always[1], regardless of the machine
>>>> layout of dom0 or the driver domain.
>>>>
>>>> I think I would prefer this approach rather than making xen/setup.c even
>>>> more horribly complicated.
>>> I imagine that some users will not want to run dom0 under an IOMMU.
>> Then they have chosen that (for whatever reason) over performance,
>> right?
> IOMMU is not free, so I imagine that some users would actually be
> choosing to avoid it specifically for performance reasons.
>
> --msw

True, but if people are actually looking for performance, they will be
turning on features like GRO and more generically scatter/gather which
results in drivers using single page DMA mappings at a time, which
completely bypass the bouncing in the swiotlb.

~Andrew