Discussion on Go and D

[Rainer Schuetze]

On 4/7/2012 8:26 PM, Sean Kelly wrote:
> On Apr 7, 2012, at 9:45 AM, Rainer Schuetze<r.sagitario at gmx.de>  wrote:
>
>>
>>
>> On 4/6/2012 6:20 PM, deadalnix wrote:
>>> Le 06/04/2012 18:07, Andrei Alexandrescu a écrit :
>>>> A few more samples of people's perception of the two languages:
>>>>
>>>> http://news.ycombinator.com/item?id=3805302
>>>>
>>>>
>>>> Andrei
>>>
>>> I did some measurement on that point for D lately :
>>> http://www.deadalnix.me/2012/03/05/impact-of-64bits-vs-32bits-when-using-non-precise-gc/
>>>
>>
>> I studied the GC a bit more and noticed a possible issue:
>>
>> - memory allocations are aligned up to a power of 2<= page size
>> - the memory area beyond the actually requested size is left untouched when allocating
>> - when the memory is collected, it is also untouched
>> - the marking of references during collection does not know the requested size, so it scans the full memory block
>>
>> Result: When a collected memory block is reused by a smaller allocation, there might still be false pointers in the unused area.
>>
>> When I clear this data, my first impression is that it has improved the situation, but not enough. I'll have to create some non-interactive test to verify.
>
> The unused area is typically zeroed out on allocation. Check GC.mallocNoSync..

That's where I added it ;-) But in fact it is in malloc/calloc and a few 
more places, though I haven't checked whether all the other calls of 
mallocNoSync handle the situation correctly. Almost all calls are to 
malloc anyway, so my patch doesn't change anything.