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 79BCB2AEF1 for ; Tue, 30 Oct 2018 16:06:33 -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 HGrsVHICg_XW for ; Tue, 30 Oct 2018 16:06:33 -0400 (EDT) Received: from smtp56.i.mail.ru (smtp56.i.mail.ru [217.69.128.36]) (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 383D12AE71 for ; Tue, 30 Oct 2018 16:06:33 -0400 (EDT) Subject: [tarantool-patches] Re: [PATCH v8 1/3] box: factor fiber_gc out of txn_commit From: Vladislav Shpilevoy References: <4a7a178a-7632-4f1a-5b94-67ef886c784d@tarantool.org> Message-ID: Date: Tue, 30 Oct 2018 23:06:30 +0300 MIME-Version: 1.0 In-Reply-To: <4a7a178a-7632-4f1a-5b94-67ef886c784d@tarantool.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit 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: "n.pettik" , tarantool-patches@freelists.org Cc: Imeev Mergen 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. > >