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

n.pettik korablev at tarantool.org
Wed Oct 31 12:30:09 MSK 2018

> On 31 Oct 2018, at 12:18, Vladislav Shpilevoy <v.shpilevoy at tarantool.org> wrote:
> On 31/10/2018 02:08, n.pettik wrote:
>>>>> 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> <tel: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?
> Here you can not calculate. Also, it is not possible to calculate
> autoids from triggers, box Lua functions. So a list is the only
> variant.

Ok, now I see. Then patch LGTM.

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