From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-oo2-f43.google.com (mail-oo2-f43.google.com [74.125.231.171]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 248124CEE47 for ; Thu, 1 Oct 2026 09:26:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=74.125.231.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790846819; cv=none; b=SBZEY458KgeZsdTTABVYrD/1mmcoq7Gr+5BvkXHvVk1FCn3Cka6ZicdNoOo/DcpciwRRzWJ8AiKSjwtm4lzm3dBCVbusmBN2NxvRyvhD5nRvGmXgC15MlUysoBsszdLvmMa2Q8o74XCBpQLH/MCrfxYUtuY1WnsUfw0cRqonTr4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790846819; c=relaxed/simple; bh=9+09TMmkqPd8XV+CQZAUSYrFzRyU+gP5BOnYyuehyb8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kHsqn6MRbuPjMXJtZiqcLWLlgTMU1TkBkKLZVEuveXUNAuPvYm/GSbX4aB6y4Lfi9YjzAjmSjGSBt6zjpYPpVF4D+Sb4t0SetQVkxpTENkQK2Pt0wvXcxa/sE1h+P99nKRRA+lrVWVfjbw9aRXyAjxfROYNVY19PNbOEiSx0lrk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=itRpMD1h; arc=none smtp.client-ip=74.125.231.171 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="itRpMD1h" Received: by mail-oo2-f43.google.com with SMTP id 46e09a7af769-81b02c279beso202441a34.2 for ; Thu, 01 Oct 2026 02:26:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1790846811; x=1791451611; darn=vger.kernel.org; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:from:to:cc:subject :date:message-id:reply-to:content-type; bh=vh2jiIbia6eRFOfHm82FIh8xzcPZydWxSZdH0BHLn+Y=; b=itRpMD1hY/YafuayfbwqexJD16qIo1YYCMyxJZiYEXQ/r36w0T7yyW0Bpq3t0rM2bd 6b09BnW9IsG4XGJcrngz1iB4R2+lgw1U1ASaHZt2bMxi6ch/296ShirJKdw5VgvJuFmR 23XDvpPKeQFrosIlHtsR3KVIGUjkGIW0Su/QsvX8mgOKJhoB4u6P/RNoJ+wO94DWMOv+ y85GE+OZIqaStAKYoaPGuzRCABpNm2QaFESsvn19HPPa4GqcxPUqhZyUPulDjLiGh/wR 4FCR+WWgcF8aYC67x5DkcXvoX13Ry/zSoOZcHt2Pv/aj3SycJcvLcikTNL3AyOYUOc71 MT2w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1790846811; x=1791451611; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=vh2jiIbia6eRFOfHm82FIh8xzcPZydWxSZdH0BHLn+Y=; b=wan9bxj1oM/p67UK2xw152vYIn0k8aXq4vazXaNBTBPX/T6xyLBBJq7W1L9AmrLAuN NT0VvDpzsQEr4F9rXW8lDQGHFSYHfAtowWCxT87kW3fRoY7WPCphyihgRMET4Qpq0hhu O3cYyP/5OpgpUvlPyN+tqZLRId7SF6ZwF2uzVX8AzXcg/SA6PD1Ojuxch7gOzBERJzMk vJhw+ts0FbpxXuDfKruAo1rDlw62REe41N7UxbFUb4cSge8sQOO4i6bHo6IeW2SrjZpo kFGBCI/XR4c3sdeuOC9pMZUUQAD0T0h67WI4X9ET1S0+CODfCLn4lU3JSAYKiIhqpWyV 3Xaw== X-Forwarded-Encrypted: i=1; AKwUvByuhE8NN42u90IB7VzRoUsomfPhxsuk6jomS53WDfmQnUXgjLgAL2zNEYqGWS6UxG1wUrRWtR1+Xn6a+w==@vger.kernel.org X-Gm-Message-State: AFuF++mWNuizgK3FDQKV8F4Pzfi2xKVsA9Zut2FIoCqRbxhFW+lyiHH0 tUXy1PzeGkm/67z8Kh0AVH1z02/7cbGZ700d+2j/RwRGHbO4iqkZ35iT X-Gm-Gg: AYBFou3Uv8sieHeVnkI7KopQouXNFVjYsPA3ryfetj44xZFViTQDsudN4CnK9rJC2nC nLFNZ5kUyp49mmplDFIRSVPF29dqvYt8LAI2osOYrwf9VAknfAKUwyj7+jQvVUikHUSRgHG//IQ UiOz7ClyboHVdcYCQRL3nodRJmIq5hLHt1hWKPm23MsduKDbE6Gic3r7qJxj7Ui/ILR4YHBtxG8 OyRWLE2aDO4Z4GICDlb5rG2OpiSg7JniwJtrB/WN0zPKWzuGJd6uGbvlhfsDB0sWYBHrnjfxktr RdZPjvQdUdKNFWgiaAf7GVa26vRfu5L8HPtfnvfl4TxaJnMV2GRVvH8nwuhfZH3kCCEq77M/IvJ 4LWHsZm+DAGPbwfIQzTy1uMy9Dz+nyHBIu9b2s/u7GTVQxspSPgV7i1/CInB30zi5nll4gJKnN1 fEzhwm5dhIn6gyv9NBghdkS6wHj2hJQqWADJ9IQ9u6a6Stq9IYSWkkmanVkZZKOPFwpIlnhroBO fmC+j1mP+dpCiQHg7InbxYofCCHPk6IkQaka3gLB5GWNBreBZea0nMRxRqLOFuREFayGK/0ibbd 2Jg= X-Received: by 2002:a05:6830:d15:b0:81f:8f5f:f1dc with SMTP id 46e09a7af769-820491154f2mr4634046a34.8.1790846811279; Thu, 01 Oct 2026 02:26:51 -0700 (PDT) Received: from fedora-laptop ([172.245.82.59]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-8212a413937sm2252075a34.8.2026.10.01.02.26.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 01 Oct 2026 02:26:50 -0700 (PDT) Date: Thu, 1 Oct 2026 04:26:43 -0500 From: Ming Lei To: Qiliang Yuan Cc: Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] ublk: don't use an I/O scheduler by default Message-ID: References: <20261001-bug-ublk-no-sched-by-default-v1-1-0bc91b6f075e@gmail.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261001-bug-ublk-no-sched-by-default-v1-1-0bc91b6f075e@gmail.com> On Thu, Oct 01, 2026 at 11:10:40AM +0800, Qiliang Yuan wrote: > Requests of a ublk device are handed to the ublk server through > io_uring, and the server does its own queueing and ordering, so an I/O > scheduler in front of the device only adds overhead. A single-queue > ublk device still gets mq-deadline by default, which costs throughput > on every I/O and an elevator setup on every START_DEV. > > Set BLK_MQ_F_NO_SCHED_BY_DEFAULT so that add_disk() selects "none", as > loop does since commit 2112f5c1330a ("loop: Select I/O scheduler 'none' > from inside add_disk()"). Doing it in the kernel also avoids switching > the scheduler from user space after the device has been added, which > waits for RCU grace periods. A scheduler can still be selected through > sysfs. Zoned ublk devices don't need mq-deadline either, zone write > plugging serializes writes per zone in the block layer since > commit fde02699c242 ("block: mq-deadline: Remove support for zone write > locking"). > > ublk null target, single-queue devices with depth 16, fio io_uring 4k > randread at iodepth=16, 16 vCPU KVM guest: > > mq-deadline none > 1 device 111K IOPS 342K IOPS > 4 devices 286K IOPS 1070K IOPS > START_DEV p50 (1000 devices) 8.53ms 0.36ms > > Signed-off-by: Qiliang Yuan Default none does make sense: Reviewed-by: Ming Lei > --- > Zoned ublk devices were not tested at runtime: none of the ublk servers > at hand (ublksrv, the kublk selftest server) serves zoned I/O. rublk supports zoned: `cargo install rublk` or https://github.com/ublk-org/rublk Thanks, Ming