Memory allocation in D

[Sean Kelly]

Craig Black wrote:
> "Sean Kelly" <sean at f4.ca> wrote in message 
> news:fin4fl$1h7v$1 at digitalmars.com...
>> This thread inspired me to make a number of changes to the GC and such in 
>> Tango.  They should improve efficiency in terms of eliminating wasted 
>> space and avoiding collections in some situations.  They'll probably take 
>> a few days, so stay tuned.
> 
> Cool!  Maybe you could show some benchmarks when you get done? 

Unfortunately, I think I'm not going to commit these changes.  I had 
some clever ideas for how to replace the "allocate an extra byte" 
functionality with an approach that would have resulted in less wasted 
space, but it's become more complicated than the added efficiency is worth.

Basically, I was hoping to keep an extra free page at the end of the 
allocated memory region used by the process.  This assumes that all 
memory used by the process is contiguous however, or at least that the 
intervening pages are valid.  But neither assumption is reasonable, and 
while some sort of fancy manual fix may have worked with some memory 
management APIs, it wouldn't have worked with others.

For what it's worth, my concern with the current behavior is that its 
worst-case scenario coincides with typical usage.  It is extremely 
common for a programmer to use array sizes that are powers of two, and 
this is the situation which results in the maximum unused space per 
block (because the runtime adds 1 to the size of all array allocations 
passed to the GC).  Totally stinks, and I'm running out of ideas.  I 
don't suppose someone can offer a suggestion for how to address this, 
other than "just always allocate exactly what they request and if they 
get a page fault then too bad for them?"


Sean