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

["n.pettik" <korablev@tarantool.org>]

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>>> 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?

Quite. I thought that used slabs are marked somehow so that different
fibers’ regions can’t rely on the same chunk. Probably, I misunderstood
how internals of our allocation system work. I would better ask you f2f
someday (or read again Konstantin’s article). Anyway, thanks.

> 
> 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 <tel:1024/80000> < 1.
> It takes 1 deallocation vs 10k deallocations. So I think this refactoring
> is worth.

Very impressive calculations, however:

a. I doubt that smb extensively uses queries like
INSERT INTO t VALUES (NULL, ..), *10k repeats*, (NULL, ..)’
*Ok, neither I nor you know which queries users execute (or will execute),
 but anyway your example looks too synthetic.*

b. Nothing prevents us from counting number of NULLs right in parser
and allocate memory as single array (one malloc). In this case it would
be more efficient, I guess, since you don’t need that machinery connected
with linked list. Btw, why didn’t you consider this variant? 


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