From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7F91CC433EF for ; Wed, 13 Oct 2021 03:32:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6149D60D43 for ; Wed, 13 Oct 2021 03:32:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235126AbhJMDe5 (ORCPT ); Tue, 12 Oct 2021 23:34:57 -0400 Received: from out30-131.freemail.mail.aliyun.com ([115.124.30.131]:56992 "EHLO out30-131.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235134AbhJMDez (ORCPT ); Tue, 12 Oct 2021 23:34:55 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=xiaoguang.wang@linux.alibaba.com;NM=1;PH=DS;RN=3;SR=0;TI=SMTPD_---0UrdlZfP_1634095971; Received: from legedeMacBook-Pro.local(mailfrom:xiaoguang.wang@linux.alibaba.com fp:SMTPD_---0UrdlZfP_1634095971) by smtp.aliyun-inc.com(127.0.0.1); Wed, 13 Oct 2021 11:32:51 +0800 Subject: Re: [RFC 1/1] io_uring: improve register file feature's usability To: Pavel Begunkov , io-uring@vger.kernel.org Cc: axboe@kernel.dk References: <20211012084811.29714-1-xiaoguang.wang@linux.alibaba.com> <20211012084811.29714-2-xiaoguang.wang@linux.alibaba.com> <7899b071-16cf-154d-3354-2211309c2949@gmail.com> <4211b3d1-42a8-4528-2c72-7fddf3bddcf6@gmail.com> From: Xiaoguang Wang Message-ID: <98943ac6-772c-fd18-8d47-fbd16de10894@linux.alibaba.com> Date: Wed, 13 Oct 2021 11:32:51 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <4211b3d1-42a8-4528-2c72-7fddf3bddcf6@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org hi, > On 10/12/21 14:11, Xiaoguang Wang wrote: >>> On 10/12/21 09:48, Xiaoguang Wang wrote: >>>> The idea behind register file feature is good and straightforward, but >>>> there is a very big issue that it's hard to use for user apps. User >>>> apps >>>> need to bind slot info to file descriptor. For example, user app wants >>>> to register a file, then it first needs to find a free slot in >>>> register >>>> file infrastructure, that means user app needs to maintain slot >>>> info in >>>> userspace, which is a obvious burden for userspace developers. >>> >>> Slot allocation is specifically entirely given away to the userspace, >>> the userspace has more info and can use it more efficiently, e.g. >>> if there is only a small managed set of registered files they can >>> always have O(1) slot "lookup", and a couple of more use cases. >> >> Can you explain more what is slot "lookup", thanks. For me, it seems >> that > > I referred to nothing particular, just a way userspace finds a new index, > can be round robin or "index==fd". > >> use fd as slot is the simplest and most efficient way, user does not >> need to> mange slot info at all in userspace. > > As mentioned, it should be slightly more efficient to have a small table, > cache misses. Also, it's allocated with kvcalloc() so if it can't be > allocate physically contig memory it will set up virtual memory. > > So, if the userspace has some other way of indexing files, small tables > are preferred. For instance if it operates with 1-2 files, or stores > files > in an array and the index in the array may serve the purpose, or any > other > way. Also, additional memory for those who care. Yeah, I agree with you that for small tables, current implementation seems good, If user app just registers a small number of files, it may handle it well, but imagine how netty, nginx or other network apps which will open thousands of socket files, manage these socket files' slot info will be a obvious burden to developer, these apps may need to develop a private component to record used or free slot. Especially in a high concurrency scenario, frequent sockes opened or closed, this private component may need locks to protect, that means this private component will introduce overhead too. For a fd, vfs layer has already ensure its unique. > >>> If userspace wants to mimic a fdtable into io_uring's registered table, >>> it's possible to do as is and without extra fdtable tracking >>> >>> fd = open(); >>> io_uring_update_slot(off=fd, fd=fd); >> >> No, currently it's hard to do above work, unless we register a big >> number of files initially. > > If they intend to use a big number of files that's the way to go. They > can unregister/register if needed, usual grow factor=2  should make > it workable. I'm not sure un-register/register are appropriate，say a app registers 1000 files, then it needs to un-register 1000 files firstly, there are doubts whether can do this un-registration work, if some of these files are used by other threads, which submit sqes with FIXED_FILE flags continually, so the first un-registration work needs to synchronize with threads which are submitting requests. And later app needs to prepare a new files array, saving current 1000 files and new files info to this new array, for me, it can works, but not efficient and somewhat hard to use :) What I express here is that there are many factors to consider carefully when using file registration feature, that's why I say it's somewhat hard to use :) Do you know any popular apps based on io_uring that have used file registration feature ? netty (https://github.com/netty/netty-incubator-transport-io_uring.git) has io_uring support, but does not use file registration feature, and recently  we'd like to add file registration to it. Regards, Xiaoguang Wang > > We may consider fast growing as a separate feature if really needed, > either as you did it, or even better doing it explicitly and separately > from updates. > > >> Say we call IORING_REGISTER_FILES to register 1000 files initially, >> then the io_uring >> >> io_file_table only supports 1000 files, fd which is greater than 1000 >> will be not able to >> >> be registered. >> >> For safety,  you may need to register the number of >> getrlimit(RLIMIT_NOFILE) initially, >> >> but it also may fail, user may change RLIMIT_NOFILE too. This is why >> I introduce a >> >> io_uring io_file_table resize feature, but I agree this method may >> waste memory, for >> >> example, user app only wants one file registered, but this file's fd >> is very large. > > That's fine as long as it's optional >