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Subject: Re: kmalloc() alignment
In-Reply-To: <981a78$cb2$1@cesium.transmeta.com> from "H. Peter Anvin" at "Mar
 5, 2001 04:15:36 pm"
To: "H. Peter Anvin" <hpa@zytor.com>
Date: Tue, 6 Mar 2001 09:31:01 +0100 (MET)
CC: linux-kernel@vger.kernel.org
From: R.E.Wolff@BitWizard.nl (Rogier Wolff)
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> Followup to:  <20010306000652.A13992@excalibur.research.wombat.ie>
> By author:    Kenn Humborg <kenn@linux.ie>
> In newsgroup: linux.dev.kernel
> >
> > On Sun, Mar 04, 2001 at 11:41:12PM +0100, Manfred Spraul wrote:
> > > >
> > > > Does kmalloc() make any guarantees of the alignment of allocated 
> > > > blocks? Will the returned block always be 4-, 8- or 16-byte 
> > > > aligned, for example? 
> > > >
> > > 
> > > 4-byte alignment is guaranteed on 32-bit cpus, 8-byte alignment on
> > > 64-bit cpus.
> > 
> > So, to summarise (for 32-bit CPUs):
> > 
> > o  Alan Cox & Manfred Spraul say 4-byte alignment is guaranteed.
> > 
> > o  If you need larger alignment, you need to alloc a larger space,
> >    round as necessary, and keep the original pointer for kfree()
> > 
> > Maybe I'll just use get_free_pages, since it's a 64KB chunk that
> > I need (and it's only a once-off).

My old kmalloc would actually use n+10 bytes if you request n bytes.
As memory comes in pools of powers of two, if you request 64k, you
would acutaly use 128k of memory. If you use "get_free_pages", you'll
not have the overhead, and actually allocate the 64k you need. 

I'm not sure what the slab stuff does...

		Roger. 

-- 
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