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 E5403C07E9D for ; Mon, 26 Sep 2022 15:37:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236737AbiIZPhy (ORCPT ); Mon, 26 Sep 2022 11:37:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36840 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236711AbiIZPfu (ORCPT ); Mon, 26 Sep 2022 11:35:50 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D2052BD5; Mon, 26 Sep 2022 07:22:44 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 86AD621DE7; Mon, 26 Sep 2022 14:22:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1664202163; h=from:from:reply-to: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=vJkZDjy/KJG4g79nuq4rNvIMTooiCGdK5D6wYt+vH6M=; b=lFW6m7fDWwkDlsc8p3+YE8ZER9ICzyao0BJsIYNgpBlerDjeu3//ld0lz3tp01uUTbaqnE M1/5EUxAkuEWUSnU0p+JcBlNgRt+Foq7jzAlE8WkbJ+7vfGb+rfndb1qn9zJTLXjhpUqro ljMR9XSEYk9alGcrjxDDM+TvZHAJvgA= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1664202163; h=from:from:reply-to: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=vJkZDjy/KJG4g79nuq4rNvIMTooiCGdK5D6wYt+vH6M=; b=OehuH9lA4EzqkbhclJAYY9irT6NtjjWBVo1V8RodKvIirhYLJ23leFn7FG0UwGCPksmLfl y+G/zrf8jCduaqCw== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 76B84139BD; Mon, 26 Sep 2022 14:22:43 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id RQfuHLO1MWMkCAAAMHmgww (envelope-from ); Mon, 26 Sep 2022 14:22:43 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 06585A0685; Mon, 26 Sep 2022 16:22:43 +0200 (CEST) Date: Mon, 26 Sep 2022 16:22:42 +0200 From: Jan Kara To: Yu Kuai Cc: Jan Kara , Christoph Hellwig , paolo.valente@linaro.org, axboe@kernel.dk, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, yi.zhang@huawei.com, "yukuai (C)" Subject: Re: [PATCH v3 3/5] block, bfq: don't disable wbt if CONFIG_BFQ_GROUP_IOSCHED is disabled Message-ID: <20220926142242.mxrkbs63ynmhulib@quack3> References: <20220922113558.1085314-1-yukuai3@huawei.com> <20220922113558.1085314-4-yukuai3@huawei.com> <988a86f2-e960-ba59-4d41-f4c8a6345ee9@huaweicloud.com> <20220923100659.a3atdanlvygffuxt@quack3> <95998ae6-8bbf-b438-801b-7033ceaf9c36@huaweicloud.com> <20220923110354.czvzm6rjm7mtqyh3@quack3> <5a2dba26-529d-295f-2e88-601475ff67bf@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5a2dba26-529d-295f-2e88-601475ff67bf@huaweicloud.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Kuai! On Mon 26-09-22 21:00:48, Yu Kuai wrote: > 在 2022/09/23 19:03, Jan Kara 写道: > > Hi Kuai! > > > > On Fri 23-09-22 18:23:03, Yu Kuai wrote: > > > 在 2022/09/23 18:06, Jan Kara 写道: > > > > On Fri 23-09-22 17:50:49, Yu Kuai wrote: > > > > > Hi, Christoph > > > > > > > > > > 在 2022/09/23 16:56, Christoph Hellwig 写道: > > > > > > On Thu, Sep 22, 2022 at 07:35:56PM +0800, Yu Kuai wrote: > > > > > > > wbt and bfq should work just fine if CONFIG_BFQ_GROUP_IOSCHED is disabled. > > > > > > > > > > > > Umm, wouldn't this be something decided at runtime, that is not > > > > > > if CONFIG_BFQ_GROUP_IOSCHED is enable/disable in the kernel build > > > > > > if the hierarchical cgroup based scheduling is actually used for a > > > > > > given device? > > > > > > . > > > > > > > > > > > > > > > > That's a good point, > > > > > > > > > > Before this patch wbt is simply disabled if elevator is bfq. > > > > > > > > > > With this patch, if elevator is bfq while bfq doesn't throttle > > > > > any IO yet, wbt still is disabled unnecessarily. > > > > > > > > It is not really disabled unnecessarily. Have you actually tested the > > > > performance of the combination? I did once and the results were just > > > > horrible (which is I made BFQ just disable wbt by default). The problem is > > > > that blk-wbt assumes certain model of underlying storage stack and hardware > > > > behavior and BFQ just does not fit in that model. For example BFQ wants to > > > > see as many requests as possible so that it can heavily reorder them, > > > > estimate think times of applications, etc. On the other hand blk-wbt > > > > assumes that if request latency gets higher, it means there is too much IO > > > > going on and we need to allow less of "lower priority" IO types to be > > > > submitted. These two go directly against one another and I was easily > > > > observing blk-wbt spiraling down to allowing only very small number of > > > > requests submitted while BFQ was idling waiting for more IO from the > > > > process that was currently scheduled. > > > > > > > > > > Thanks for your explanation, I understand that bfq and wbt should not > > > work together. > > > > > > However, I wonder if CONFIG_BFQ_GROUP_IOSCHED is disabled, or service > > > guarantee is not needed, does the above phenomenon still exist? I find > > > it hard to understand... Perhaps I need to do some test. > > > > Well, BFQ implements for example idling on sync IO queues which is one of > > the features that upsets blk-wbt. That does not depend on > > CONFIG_BFQ_GROUP_IOSCHED in any way. Also generally the idea that BFQ > > assigns storage *time slots* to different processes and IO from other > > processes is just queued at those times increases IO completion > > latency (for IOs of processes that are not currently scheduled) and this > > tends to confuse blk-wbt. > > > Hi, Jan > > Just to be curious, have you ever think about or tested wbt with > io-cost? And even more, how bfq work with io-cost? > > I haven't tested yet, but it seems to me some of them can work well > together. No, I didn't test these combinations. I actually expect there would be troubles in both cases under high IO load but you can try :) Honza -- Jan Kara SUSE Labs, CR