why does array appending add a page size instead of doubling ?

[monarch_dodra]

On Sunday, 3 February 2013 at 11:47:21 UTC, Steven Schveighoffer 
wrote:
> On Sun, 03 Feb 2013 03:53:10 -0500, timotheecour 
> <thelastmammoth at gmail.com> wrote:
>
>>> Note that dynamic arrays are generic containers, so they 
>>> aren't exactly optimized for anything. You could try that 
>>> test with the "made for appending" std.array.appender: It 
>>> always tries to extend without reallocating, and only 
>>> relocates when the current memory segment is 100% full.
>>>
>>> Furthermore, when it *does* relocate, it use D-style memmove 
>>> without calling postblit. Independent of language, and with 
>>> the requirement of contiguous memory, I can't really think of 
>>> a scheme that could be better than that...?
>>
>>
>> modified a bit (see below) to include timing etc. I'm on OSX 
>> (16GB ram, core i7 2.3GHz).
>> dmd is slow even with optimizations so I used ldc:
>>
>> ldmd2 -inline -O -release -noboundscheck  (similar results 
>> with ldc2 -O3 -release)
>> for C++: g++ -O2
>>
>> It seems that:
>> * the number of moves is up to 2x (resp. 1.8x) as the C++ 
>> version for T[] (resp. appender)
>> * the C++ version is 7x (resp 1.8x) times faster .
>> * it seems even the appender version grows super-linearly 
>> whereas C++ version stays roughly linear in that regime in 
>> terms of time.
>>
>> Based on this, I'm curious whether the current growing scheme 
>> could be improved, or be closer to the simple doubling scheme 
>> used in C++.
>
> The performance increase is due to a) using malloc instead of 
> GC allocation, which would run quite a few collection cycles 
> while allocating.  If you profile the D code, you will see 
> this.  b) vector is FREEING its previously used array space, D 
> array appending is relying on the GC to free that space.  The 
> consumed D space will grow more rapidly.
>
> You are essentially comparing apples to oranges here.
>
> -Steve

True...

But not true for std.container.Array though. Array uses malloc. 
That's an apples to apples comparison right there.