std.allocator needs your help

[Andrei Alexandrescu]

On 9/24/13 2:02 PM, Rainer Schuetze wrote:
>
>
> On 24.09.2013 21:36, Andrei Alexandrescu wrote:
>> On 9/24/13 11:28 AM, Rainer Schuetze wrote:
>>>
>>>
>>> expand is nice, but I don't like minDelta and maxDelta as arguments. On
>>> a shared allocator this can lead to undesired side-effects, i.e. when
>>> this function is called concurrently on the same block by two threads,
>>> the net result will be the sum of the deltas of the two threads, while
>>> later synchronization on the memory block might only allow one thread to
>>> actually use the extended memory area. This can happen with gc_extend in
>>> the current GC when appending to a shared array.
>>>
>>> I'd prefer the function to actually pass the desired sizes:
>>>
>>> void[] expand(void[] b, size_t minSize, size_t maxSize);
>>
>> I don't understand this argument. These parameters can be trivially
>> computed from the others, after which locking etc. proceeds just the
>> same.
>>
>
> Taking the current array implementation as an example, the deltas are
> computed before the actual GC lock happens inside gc_extend which means
> that the second of two concurrent requests leads to overallocation.

(I'm confused - which array implementation?) Does this qualify as an 
implementation bug, or are you referring to the case where the void[] 
being reallocated is being shared?

>> FWIW I chose the "delta"-based signature because with it the
>> precondition is:
>>
>> assert(minDelta <= maxDelta);
>>
>> In fact behavior can be easily defined at no cost even if this
>> precondition is not satisfied. With the "absolute"-based signature the
>> precondition is:
>>
>> assert(minSize <= maxSize && b.length <= minSize);
>>
>> which causes confusion - people will pass small sizes to expand()
>> expecting it to contract, something that expand() can't support as a
>> matter of principle (safety!!!).
>>
>
> Why would you expect a function "expand" to reduce the memory size?

Ask Dmitry :o). Far as I can tell he assumed so.


Andrei