linux-kernel - Re: [PATCH][v2] fork_init: fix division by zero

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Date:	Fri, 12 Dec 2008 13:36:02 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>, yur@...raft.com,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Geert.Uytterhoeven@...ycom.com, dhowells@...hat.com,
	miltonm@....com, wd@...x.de, dzu@...x.de, yanok@...raft.com
Subject: Re: [PATCH][v2] fork_init: fix division by zero

On Friday 12 December 2008 13:47, Andrew Morton wrote:
> On Fri, 12 Dec 2008 12:31:33 +1000 Nick Piggin <nickpiggin@...oo.com.au> 
wrote:
> > On Friday 12 December 2008 07:43, Andrew Morton wrote:
> > > On Thu, 11 Dec 2008 20:28:00 +0000
> > >
> > > >  Do they actually cross the page boundaries?
> > >
> > > Some flavours of slab have at times done an order-1 allocation for
> > > objects which would fit into an order-0 page (etc) if it looks like
> > > that will be beneficial from a packing POV.  I'm unsure whether that
> > > still happens - I tried to get it stamped out for reliability reasons.
> >
> > Hmph, SLUB uses order-3 allocations for 832 byte sized objects
> > by default here (mm struct).
>
> That sucks, but at least it's <= PAGE_ALLOC_COSTLY_ORDER.

Which is somewhat arbitrary a value. order-1 is costly compared to
order-0...

After running my system here for a while and doing various things
with it, I have the ability to allocate 898 order-0 pages (3592K),
or 36 order-3 pages (1152K).

Not as bad as I expected, but the system's only been up for an hour,
and not exactly doing anything unusual (and it has nearly 30MB free,
out of 4GB).


> It's fortunate that everyone has more than 128GB of memory.

And that SLAB still works quite well :)
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