D array expansion and non-deterministic re-allocation

[Steven Schveighoffer]

On Tue, 24 Nov 2009 11:26:11 -0500, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> Steven Schveighoffer wrote:
>> On Tue, 24 Nov 2009 11:01:10 -0500, Andrei Alexandrescu  
>> <SeeWebsiteForEmail at erdani.org> wrote:
>>
>>> Steven Schveighoffer wrote:
>>> [snip]
>>>> Andrei has mentioned that he thinks we can store the allocated length  
>>>> in the GC block, which I think would also work.  You also wouldn't  
>>>> need an MRU cache in that case, but he says it's in *addition* to the  
>>>> MRU cache, so I'm not sure what he means.
>>> [snip]
>>>
>>> Reaching the GC block is relatively expensive, so the MRU still helps.  
>>> In essence it's like this. When appending:
>>>
>>> a) look up the cache, if there, you have O(1) amortized append that's  
>>> really fast
>>>
>>> b) if not in the cache, talk to the GC, still O(1) amortized append  
>>> that's not as fast
>>>
>>> Both help providing an important performance guarantee. I was a bit  
>>> worried about guaranteeing "O(1) amortized append for up to 8 arrays  
>>> at a time."
>>  Have you considered the performance impact on allocating non-array  
>> types?  That is, are you intending on all allocations storing the  
>> allocated length, even class or struct allocations who will likely  
>> never append?  Or will there be a "non-appendable" flag?
>>  Also, the part without the MRU cache was my original proposal from  
>> last year, I had some ideas on how length could be stored.  For  
>> example, in a page of up to 128 byte blocks, you only need 8 bits to  
>> store the length (alas, you cannot store with 4 bits for 16-byte blocks  
>> because you need to cover both 0 and 16).  This reduces the overhead  
>> for those blocks.  For 256 byte to 1-page blocks, 16 bits is acceptable  
>> multi-page blocks, the cost of storing a 32-bit integer is negligible.
>
> Having access to the requested length is important at larger lengths, so  
> probably we could be fine by not actually storing it for allocations up  
> to 128 bytes.

So in other words, appending any array under 128 bytes which does not  
still sit in the MRU cache causes a reallocation?  I'm not sure if it's  
the right move, but it is an optimization, so maybe it works well.  It  
also handily fixes the problem of requiring allocated lengths for  
non-array allocations since most would be under 128 bytes.

>> It is true the lookup of the MRU cache will not involve dissecting the  
>> address of the block to find it's container block, but you still will  
>> need a lock, or are you planning on doing something clever?   I think  
>> the locking will be the bottleneck, and if you don't make it the same  
>> as the global lock, add the cost of both locks when you actually need  
>> to append.
>
> The cache is a thread-local map from pointers to size_t. Using it does  
> not require any locking I think.

When reallocating, do you not also need to update the allocated length in  
the heap, even if the allocation fits into the same block to prevent other  
threads from stomping?  Doesn't that require a lock?  Somewhere you have  
to share the allocated length between threads sharing the same array.  I  
assumed it was going to be in the MRU cache...

-Steve