Re: realloc() - frequency, conditions, or experiences about relocation?

On 17/06/2024 11:31, Malcolm McLean wrote:

On 17/06/2024 10:18, Ben Bacarisse wrote:

Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
>

In a recent thread realloc() was a substantial part of the discussion.
"Occasionally" the increased data storage will be relocated along
with the previously stored data. On huge data sets that might be a
performance factor. Is there any experience or are there any concrete
factors about the conditions when this relocation happens? - I could
imagine that it's no issue as long as you're in some kB buffer range,
but if, say, we're using realloc() to substantially increase buffers
often it might be an issue to consider. It would be good to get some
feeling about that internal.

>
There is obviously a cost, but there is (usually) no alternative if
contiguous storage is required.  In practice, the cost is usually
moderate and can be very effectively managed by using an exponential
allocation scheme: at every reallocation multiply the storage space by
some factor greater than 1 (I often use 3/2, but doubling is often used
as well).  This results in O(log(N)) rather than O(N) allocations as in
your code that added a constant to the size.  Of course, some storage is
wasted (that /might/ be retrieved by a final realloc down to the final
size) but that's rarely significant.
>

So can we work it out?
 Let's assume for the moment that the allocations have a semi-normal distribution, with negative values disallowed. Now ignoring the first few values, if we have allocated, say, 1K, we ought to be able to predict the value by integrating the distribution from 1k to infinity and taking the mean.
 

First, there is no reason for assuming such a distribution, other than saying "lots of things are roughly normal".
Secondly, knowing the distribution gives you /no/ information about any given particular case.  You know the distribution for the results of rolling two die - does that mean you can predict the next roll?
Thirdly, not all distributions have a mean (look up the Cauchy distribution if you like).
Fourthly, even if you know the mean, it tells you nothing of use.
Knowing a bit about the distribution of file sizes can be useful, but not nearly in the way you describe here.  If you know that the files are rarely or never bigger than 10 MB, malloc 10 MB and forget the realloc. If you know they are often bigger than that, mmap the file and forget the realloc.