From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from localhost (localhost [127.0.0.1]) by turing.freelists.org (Avenir Technologies Mail Multiplex) with ESMTP id 45BC42D900 for ; Tue, 30 Oct 2018 19:08:13 -0400 (EDT) Received: from turing.freelists.org ([127.0.0.1]) by localhost (turing.freelists.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id W0cN1JZFx_TD for ; Tue, 30 Oct 2018 19:08:13 -0400 (EDT) Received: from smtp33.i.mail.ru (smtp33.i.mail.ru [94.100.177.93]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by turing.freelists.org (Avenir Technologies Mail Multiplex) with ESMTPS id 72A212A2F1 for ; Tue, 30 Oct 2018 19:08:12 -0400 (EDT) From: "n.pettik" Message-Id: Content-Type: multipart/alternative; boundary="Apple-Mail=_9325CC7E-9E2B-4769-832E-9B7E9D25A600" Mime-Version: 1.0 (Mac OS X Mail 12.0 \(3445.100.39\)) Subject: [tarantool-patches] Re: [PATCH v8 1/3] box: factor fiber_gc out of txn_commit Date: Wed, 31 Oct 2018 02:08:02 +0300 In-Reply-To: <4a7a178a-7632-4f1a-5b94-67ef886c784d@tarantool.org> References: <4a7a178a-7632-4f1a-5b94-67ef886c784d@tarantool.org> Sender: tarantool-patches-bounce@freelists.org Errors-to: tarantool-patches-bounce@freelists.org Reply-To: tarantool-patches@freelists.org List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: tarantool-patches List-subscribe: List-owner: List-post: List-archive: To: tarantool-patches@freelists.org Cc: Vladislav Shpilevoy , Imeev Mergen --Apple-Mail=_9325CC7E-9E2B-4769-832E-9B7E9D25A600 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 >>> 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. >>>=20 >>> 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... >=20 > 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. >=20 > 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. >=20 > Is this explanation decent? Quite. I thought that used slabs are marked somehow so that different fibers=E2=80=99 regions can=E2=80=99t 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=E2=80=99s article). Anyway, thanks. >=20 > 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 =3D slab_size / (10k * 8); 8 = - size > of autogenerated it; slab size is at least 64Kb, so N =3D = 64*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, ..)=E2=80=99 *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=E2=80=99t need that machinery = connected with linked list. Btw, why didn=E2=80=99t you consider this variant?=20 --Apple-Mail=_9325CC7E-9E2B-4769-832E-9B7E9D25A600 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=utf-8

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=E2=80=99 regions can=E2=80=99t = 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=E2=80=99s 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 =3D slab_size / (10k * 8); 8 - size
of autogenerated it; slab size = is at least 64Kb, so N =3D 64*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, ..)=E2=80=99
*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=E2=80=99t need that machinery = connected
with linked list. Btw, why didn=E2=80=99t you = consider this variant? 

= --Apple-Mail=_9325CC7E-9E2B-4769-832E-9B7E9D25A600--