Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v3

[Mel Gorman <mgorman@techsingularity.net>]

On Mon, Nov 28, 2016 at 09:39:19AM -0600, Christoph Lameter wrote:
> On Sun, 27 Nov 2016, Mel Gorman wrote:
> 
> >
> > SLUB has been the default small kernel object allocator for quite some time
> > but it is not universally used due to performance concerns and a reliance
> > on high-order pages. The high-order concerns has two major components --
> > high-order pages are not always available and high-order page allocations
> > potentially contend on the zone->lock. This patch addresses some concerns
> > about the zone lock contention by extending the per-cpu page allocator to
> > cache high-order pages. The patch makes the following modifications
> 
> Note that SLUB will only use high order pages when available and fall back
> to order 0 if memory is fragmented. This means that the effect of this
> patch is going to gradually vanish as memory becomes more and more
> fragmented.
> 

Yes, that's a problem for SLUB with or without this patch. It's always
been the case that SLUB relying on high-order pages for performance is
problematic.

> I think this patch is beneficial but we need to address long term the
> issue of memory fragmentation. That is not only a SLUB issue but an
> overall problem since we keep on having to maintain lists of 4k memory
> blocks in variuos subsystems. And as memory increases these lists are
> becoming larger and larger and more difficult to manage. Code complexity
> increases and fragility too (look at transparent hugepages). Ultimately we
> will need a clean way to manage the allocation and freeing of large
> physically contiguous pages. Reserving memory at booting (CMA, giant
> pages) is some sort of solution but this all devolves into lots of knobs
> that only insiders know how to tune and an overall fragile solution.
> 

While I agree with all of this, it's also a problem independent of this
patch.


-- 
Mel Gorman
SUSE Labs

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