Re: STL vector
On 15.11.2011 22:13, Leigh Johnston wrote:
On 15/11/2011 20:30, Marcel M?ller wrote:
And last but not least, allocating very large objects could cause heap
fragmentation. This might be a reasonable impact, especially on 32 bit
platforms where the virtual memory may be out long before the physical
memory.
Surely allocating *small* objects is more likely to cause heap
fragmentation than allocating very large objects? It is more likely that
a very large object allocation will *fail* in a fragmented heap but this
is not what you said.
It depends on what you call 'much fragmentation'. If we are talking
about the number of free fragments, then you are right. But if we are
talking about the amount of unused memory, the large objects are often
more critical, because they likely do not to fit in any previously used
free space.
Especially if you have only one large object and if its final size is
obtained by several reallocations. If the next allocation block will
never fit into the previously freed blocks then you get almost 50%
unused address space. And this is not that unlikely. E.g. if the
allocation size grows exponentially by a factor of two in each step or
if smaller, longer lived objects are allocated between the large
objects. (A good old Matlab problem when building up large matrices
incrementally, or in REXX when concatenating a large number of small
strings.)
Marcel