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Date: Wed, 12 Jul 2006 14:38:10 +0200
From: Jan Kiszka <jan.kiszka@domain.hid>
MIME-Version: 1.0
Subject: Re: [Xenomai-core] [PATCH] fix xnheap_alloc rounding
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: xenomai-core <xenomai@xenomai.org>

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>  > Hi,
>  >=20
>  > playing a stupid rt_heap user (actually I didn't just play this...),=
 I
>  > stumbled over this undocumented oddity:
>  >=20
>  > rt_heap_create(&heap, name, 10000, H_PRIO|H_MAPPABLE);
>  > rt_heap_alloc(&heap, 10000, TM_NONBLOCK, &ptr);
>  >=20
>  > Creation is successful, allocation fails. The reason: while during
>  > creation the net heap size is rounded down to page boundaries, the
>  > allocation of memory > PAGE_SIZE is rounded up. One could add H_SING=
LE
>  > to the flags, but this may even result in allocating less memory tha=
n
>  > the user expected, causing severe problems later.
>  >=20
>  > How to resolve this best? I thought about rounding twice in
>  > rt_head_create (one time the net size, the second time including the=

>  > overhead), but this encodes characteristics of the underlying heap
>  > allocator into the skin (I have a generic heap allocator framework i=
n
>  > mind for 2.3). So I decided to do this rounding in xnheap_overhead()=

>  > instead, see attached patch. Hope I didn't skewed up any calculation=
=2E At
>  > least the scenario above now works fine.
>=20
> The problem I see with this patch is that you are counting=20
> (rounded_hsize - hsize) as part of the overhead, whereas you would like=
 to
> count it as free space. Would not it make more sense to do the rounding=

> in xnheap_init_mapped ?
>=20

I was thinking of the open-coded

allocate(heap_size + xnheap_overhead)
xnheap_init()

in many skins as well.

Jan


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