Pop quiz [memory usage]

[Sean Kelly]

Fawzi Mohamed wrote:
> On 2009-06-06 21:07:40 +0200, Sean Kelly <sean at invisibleduck.org> said:
> 
>> Vladimir Panteleev wrote:
>>> On Sat, 06 Jun 2009 20:19:45 +0300, Sean Kelly <sean at invisibleduck.org>
>>> wrote:
> [...]
>>>
>>>> I've been debating for a while whether newCapacity shoulds exist at 
>>>> all.   The GC already tends to leave free space at the end of 
>>>> blocks, so why should the runtime second-guess it?
>>>
>>> Do you mean at the end of pages, or pools?
>>
>> Blocks.  Blocks less than 4k in the current allocator are sized in 
>> powers of two, so array appends already get the "double in size" 
>> behavior in Java even without newCapacity.  newCapacity just has the 
>> potential to throw an array into the next larger block early, thus 
>> potentially wasting more space if the array will never be appended to. 
>> I think newCapacity isn't supposed to reserve extra space for blocks 
>> larger than 4k, but it's been a while since I've looked at it.
> 
> at the moment the behavior is exactly the opposite (leaving the small 
> array to the GC handling you just sketched)), only array larger than a 
> page have the special treatment, I think that the idea is that appending 
> to large arrays can be potentially very expensive, so those get the 
> special treatment.

Hm.  I still think we'd be better off letting the user handle this.  If 
they're going to be appending and performance is important then they'll 
use an ArrayAppender anyway.  I'd rather not have extra space tacked 
onto the end of every array I create "just in case" I decide to append 
to it.