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

[Matthew Rushton <mvrushton@xxxxxxxxx>]

On 04/11/14 10:05, Konrad Rzeszutek Wilk wrote:
> On Wed, Apr 09, 2014 at 03:21:38PM -0700, Matthew Rushton wrote:
> > On 04/02/14 03:20, Ian Campbell wrote:
> > > On Wed, 2014-04-02 at 11:15 +0100, Jan Beulich wrote:
> > > > > > > On 02.04.14 at 12:06, <Ian.Campbell@xxxxxxxxxx> wrote:
> > > > > On Wed, 2014-04-02 at 08:52 +0100, Jan Beulich wrote:
> > > > > > > > > On 02.04.14 at 02:17, <mvrushton@xxxxxxxxx> wrote:
> > > > > > > On 04/01/14 05:22, Tim Deegan wrote:
> > > > > > > > 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.
> > > > > > I think it has become pretty clear by now that we'd rather not alter
> > > > > > the hypervisor allocator for a purpose like this.
> > OK understood see below.
> >
> > > > > Does it even actually solve the problem? It seems like it is just
> > > > > deferring it until sufficient fragmentation has occurred in the system.
> > > > > All its really done is make the eventual issue much harder to debug.
> > > > Wasn't this largely for Dom0 (in which case fragmentation shouldn't
> > > > matter yet)?
> > > Dom0 ballooning breaks any assumptions you might make about relying on
> > > early allocations.
> > I think you're missing the point. I'm not arguing that this change
> > is a general purpose solution to guarantee that dom0 is contiguous.
> > Fragmentation can exist even if dom0 asks for larger allocations
> > like it should (which the balloon driver does I believe). What the
> > change does do is solve a real problem in the current Linux PCI
> > remapping implementation which happens during dom0 intialization. If
> > the allocation strategy is arbitrary why not make the proposed
> > hypervisor change to make existing Linux implementations behave
> > better and in addition fix the problem in Linux so moving forward
> > things are safe?
> I think Tim was OK with that - as long as it was based on a flag - meaning
> when we do the increase_reservation call we use an extra flag
> to ask for contingous PFNs.
OK the extra flag feels a little dirty to me but it would solve the 
problem. What are your thoughts on changing Linux to make higher order 
allocations or more minimally adding a boot parameter to not remap the 
memory at all for those that care about performance? I know the Linux 
code is already fairly complex and your preference was not to make it 
worse.
> > > Ian.
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel

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