[tarantool-patches] Re: [PATCH v8 1/3] box: factor fiber_gc out of txn_commit

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

On 30/10/2018 23:03, Vladislav Shpilevoy wrote:
> Thanks for the review!
> 
> On 30/10/2018 17:30, n.pettik wrote:
>>
>>
>>> On 29 Oct 2018, at 20:33, imeevma@tarantool.org wrote:
>>>
>>> Now txn_commit is judge, jury and executioner. It both
>>> commits or rollbacks data, and collects it calling fiber_gc,
>>> which destroys the region.
>>
>> Nit: both commits and rollbacks.
> 
> Fixed.
> 
>>
>>>
>>> But SQL wants to use some transactional data after commit. It is
>>> autogenerated identifiers - a list of sequence values generated
>>> for autoincrement columns and explicit sequence:next() calls.
>>>
>>> It is possible to store the list on malloced mem inside Vdbe, but
>>> it complicates deallocation.
>>
>> What is the problem with deallocation? AFAIU it is enough to
>> simply iterate over the list and release each element - not big deal.
>>
>> If you want to use region, mb it is worth to store separate region
>> specially for VDBE? We already have it in parser, so what prevents
>> us for adding the same thing to VDBE? I guess we can store many
>> things there, not only list of ids. I understand that parser in its turn
>> has nothing in common (at least it should, except for analyze machinery)
>> with transaction routines, so separate region is likely to be more
>> reasonable for parser, but anyway...
>>
> 
> I've decided to say more details. Parser never yields. This is why we can
> waste here any resources, rack and ruin everything, but at the end of
> parsing it should be returned back.
> 
> Vdbe, on the contrary, yields. So it holds some system resources while
> other fibers can not use them. If we added a special region to Vdbe, it
> would steal slabs from the thread's slab cache, while other fibers may
> want to use it. Hence, when we use one region for all transactional data,
> including language specific, allocations are much less fragmented over
> different slabs.
> 
> Is this explanation decent?
> 
> Also, I do not agree, that 'deallocation is just iteration and it is
> ok'. It is O(n) iteration and freeing of heap objects. If a one inserted
> 10k rows with autogenerated ids, it would waste 10k heap fragments,
> 10k calls of malloc/free - in my opinion it is an abysmal overhead, but
> what is more, it can be avoided for free. Instead of 10k free() it boils
> down to deallocation of N slabs, where N = slab_size / (10k * 8); 8 - size
> of autogenerated it; slab size is at least 64Kb, so N = 64*1024/80000 < 1.
> It takes 1 deallocation vs 10k deallocations. So I think this refactoring
> is worth.

Sorry, an error. N = 10Kb * 8 / slab_size ~= 2. Versus 10k still is
significant.

> 
>