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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BCC71C433EF for ; Mon, 20 Jun 2022 12:04:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242315AbiFTMEs (ORCPT ); Mon, 20 Jun 2022 08:04:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48422 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242361AbiFTMEb (ORCPT ); Mon, 20 Jun 2022 08:04:31 -0400 Received: from out1.migadu.com (out1.migadu.com [91.121.223.63]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7C81B18B1F for ; Mon, 20 Jun 2022 05:03:47 -0700 (PDT) Message-ID: <281a673e-8077-2711-61c4-dd3533361df7@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1655726624; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fXbF//9xsHx9L23Z04rbJGAy5KGUCC5gpHT93Bm73Eo=; b=RFVZZqBvDr/B7C5YbCNZsgZ0TVdCM5nDD0ddD9D8u1PUS7Q6/xnWHS+ynMeDbIfDswF72+ uV5p2fwK/gaTAKrBR3Y6UMpxPOoHwJjNq+4ZAkW6yRwaeiB/VF+w0Q9DNPdBwKkN/AU2Ye kmRo1aAPOIUAz1rurT0Ev6mJUTJ0oEk= Date: Mon, 20 Jun 2022 20:03:32 +0800 MIME-Version: 1.0 Subject: Re: [RFC] a new way to achieve asynchronous IO Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Xu To: io-uring Cc: Jens Axboe , Pavel Begunkov References: <3d1452da-ecec-fdc7-626c-bcd79df23c92@linux.dev> In-Reply-To: <3d1452da-ecec-fdc7-626c-bcd79df23c92@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Migadu-Auth-User: linux.dev Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 6/20/22 20:01, Hao Xu wrote: > Hi, > I've some thought on the way of doing async IO. The current model is: > (given we are using SQPOLL mode) > > the sqthread does: > (a) Issue a request with nowait/nonblock flag. > (b) If it would block, reutrn -EAGAIN > (c) The io_uring layer captures this -EAGAIN and wake up/create > a io-worker to execute the request synchronously. > (d) Try to issue other requests in the above steps again. > > This implementation has two downsides: > (1) we have to find all the block point in the IO stack manually and > change them into "nowait/nonblock friendly". > (2) when we raise another io-worker to do the request, we submit the > request from the very beginning. This isn't a little bit inefficient. ^is > > > While I think we can actually do it in a reverse way: > (given we are using SQPOLL mode) > > the sqthread1 does: > (a) Issue a request in the synchronous way > (b) If it is blocked/scheduled soon, raise another sqthread2 > (c) sqthread2 tries to issue other requests in the same way. > > This solves problem (1), and may solve (2). > For (1), we just do the sqthread waken-up at the beginning of schedule() > just like what the io-worker and system-worker do. No need to find all > the block point. > For (2), we continue the blocked request from where it is blocked when > resource is satisfied. > > What we need to take care is making sure there is only one task > submitting the requests. > > To achieve this, we can maintain a pool of sqthread just like the iowq. > > I've done a very simple/ugly POC to demonstrate this: > > https://github.com/HowHsu/linux/commit/183be142493b5a816b58bd95ae4f0926227b587b > > > I also wrote a simple test to test it, which submits two sqes, one > read(pipe), one nop request. The first one will be block since no data > in the pipe. Then a new sqthread was created/waken up to submit the > second one and then some data is written to the pipe(by a unrelated > user thread), soon the first sqthread is waken up and continues the > request. > > If the idea sounds no fatal issue I'll change the POC to real patches. > Any comments are welcome! >