D array expansion and non-deterministic re-allocation

[Andrei Alexandrescu]

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.

> 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.


Andrei