Re: [Xenomai-core] [PATCH] fix xnheap_alloc rounding

[Jan Kiszka <jan.kiszka@domain.hid>]

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Philippe Gerum wrote:
> 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.

If this means to do the rounding in the clients without the help of some
well-defined xnheap-API, then I disagree. I have an alternative nucleus
heap manager in mind for the future that will have a different rounding
strategy for allocations. So we should cleanly hide such property behind
a xnheap service - either in the overhead calculation or in some
additional rounding service. But I do not see a need for splitting this
up yet.

> 
>>  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)
>> _______________________________________________
>> Xenomai-core mailing list
>> Xenomai-core@domain.hid
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