On Tue, 2006-07-11 at 16:02 +0200, Jan Kiszka wrote:
> Hi,
> 
> playing a stupid rt_heap user (actually I didn't just play this...), I
> stumbled over this undocumented oddity:
> 
> rt_heap_create(&heap, name, 10000, H_PRIO|H_MAPPABLE);
> rt_heap_alloc(&heap, 10000, TM_NONBLOCK, &ptr);
> 
> 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_SINGLE
> to the flags, but this may even result in allocating less memory than
> the user expected, causing severe problems later.
> 
> 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 in
> mind for 2.3).

The nucleus heap expects the caller to pass it the memory which is going
to be used to fulfill allocation requests, so there is already a builtin
dependency from any client heap manager on the nucleus implementation.
Double rounding is therefore the way to go, since callers need to be
fixed so that they provide enough memory to the xnheap manager when
specializing its behaviour.

>  So I decided to do this rounding in xnheap_overhead()
> instead, see attached patch. Hope I didn't skewed up any calculation. At
> least the scenario above now works fine.
> 
> Jan
> plain text document attachment (xnheap_overhead-fix-rounding.patch)
> Index: include/nucleus/heap.h
> ===================================================================
> --- include/nucleus/heap.h    (revision 1322)
> +++ include/nucleus/heap.h    (working copy)
> @@ -109,8 +109,14 @@ extern xnheap_t kheap;
>  #define xnheap_used_mem(heap)        ((heap)->ubytes)
>  #define xnheap_max_contiguous(heap)  ((heap)->maxcont)
>  #define xnheap_overhead(hsize,psize) \
> -((sizeof(xnextent_t) + (((hsize) - sizeof(xnextent_t)) / (psize)) + \
> - XNHEAP_MINALIGNSZ - 1) & ~(XNHEAP_MINALIGNSZ - 1))
> +({ \
> +    u_long rounded_hsize = (hsize + psize - 1) & ~(psize - 1); \
> +    u_long overhead = ((sizeof(xnextent_t) + \
> +     (((rounded_hsize) - sizeof(xnextent_t)) / (psize)) + \
> +     XNHEAP_MINALIGNSZ - 1) & ~(XNHEAP_MINALIGNSZ - 1)); \
> +    overhead += rounded_hsize - hsize; \
> +    overhead; \
> +})
>  
>  #define xnmalloc(size)     xnheap_alloc(&kheap,size)
>  #define xnfree(ptr)        xnheap_free(&kheap,ptr)
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-- 
Philippe.



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