linux-kernel - Re: [patch] SLQB slab allocator (try 2)

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Message-Id: <200902032307.09025.nickpiggin@yahoo.com.au>
Date:	Tue, 3 Feb 2009 23:07:07 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Nick Piggin <npiggin@...e.de>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lin Ming <ming.m.lin@...el.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [patch] SLQB slab allocator (try 2)

On Tuesday 03 February 2009 23:01:39 Mel Gorman wrote:
> On Tue, Feb 03, 2009 at 10:50:54PM +1100, Nick Piggin wrote:
> > On Tuesday 03 February 2009 22:28:52 Mel Gorman wrote:
> > > On Tue, Feb 03, 2009 at 09:36:24PM +1100, Nick Piggin wrote:
> > > > I'd be interested to see how slub performs if booted with
> > > > slub_min_objects=1 (which should give similar order pages to SLAB and
> > > > SLQB).
> > >
> > > Just to clarify on this last point, do you mean slub_max_order=0 to
> > > force order-0 allocations in SLUB?
> >
> > Hmm... I think slub_min_objects=1 should also do basically the same.
> > Actually slub_min_object=1 and slub_max_order=1 should get closest I
> > think.
>
> I'm going with slub_min_objects=1 and slub_max_order=0. A quick glance
> of the source shows the calculation as
>
>         for (order = max(min_order,
>                                 fls(min_objects * size - 1) - PAGE_SHIFT);
>                         order <= max_order; order++) {
>
> so the max_order is inclusive not exclusive. This will force the order-0
> allocations I think you are looking for.

Well, but in the case of really bad internal fragmentation in the page,
SLAB will do order-1 allocations even if it doesn't strictly need to.
Probably this isn't a huge deal, but I think if we do slub_min_objects=1,
then SLUB won't care about number of objects per page, and slub_max_order=1
will mean it stops caring about fragmentation after order-1. I think. Which
would be pretty close to SLAB (depending on exactly how much fragmentation
it cares about).

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