Re: [boost] [review] The review of Boost.DoubleEnded starts today: September 21 - September 30

Subject: Re: [boost] [review] The review of Boost.DoubleEnded starts today: September 21 - September 30
From: Thorsten Ottosen (tottosen_at_[hidden])
Date: 2017-10-05 15:37:40

Den 05-10-2017 kl. 10:20 skrev Thorsten Ottosen via Boost:
> Den 05-10-2017 kl. 04:19 skrev degski via Boost:
>> On 4 October 2017 at 23:48, Benedek Thaler via Boost 
>> <boost_at_[hidden]>
>> wrote:
>>
>>> As expected, both devector and batch_deque highly outperforms 
>>> std::vector
>>> and queue in deserialization.
>>>
>>
>> This sounds (to me) counterintuitive. Why is this the case?
> 
> Because they can avoid double initialization for some types. When using 
> std::vector, you have to either
> 
> A. reserve + load item to local object + push_back
> B. resize + delegate to bulk operation in Boost.Serialization
> 
> The bulk operation is to prefer, but then requires redundant 
> initialization in resize.

I gave this a little thought, and must admit that the numbers cannot be 
explained by double initialization alone.

Looking at the serialization code for DoubleEnded,
it think may not be quite in line with the Boost.Serialization approach.
For example, it seems to be me that it is not working correctly with 
e.g. xml archives.

Basically, the implementation for trivial types avoids all the 
Boost.Serialization machinery by calling load_binary/save_binary directly.

Now, what I don't understand is how that machinery can be so costly for 
the binary archive case to get the huge difference we see with 
std::vector. This is a real mystery (In the same way the save test got 
much slower when the type became array<int,N>).

Secondly, I think we can see the current numbers as a best case of what 
is possible with the extra functionality provided by DoubleEnded. There 
are other serialization libs out there, and they should be able to 
benefit even if there is no way to specialize for the binary case in 
Boost.Serialization.

kind regards

Thorsten