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 X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7FB4DC0650E for ; Wed, 3 Jul 2019 14:49:30 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 54E8821882 for ; Wed, 3 Jul 2019 14:49:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 54E8821882 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:36524 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1higZd-0005qZ-Hw for qemu-devel@archiver.kernel.org; Wed, 03 Jul 2019 10:49:29 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59948) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1higX3-0004VO-EW for qemu-devel@nongnu.org; Wed, 03 Jul 2019 10:46:51 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1higX1-0001WL-GN for qemu-devel@nongnu.org; Wed, 03 Jul 2019 10:46:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44082) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1higWw-0001R9-RO; Wed, 03 Jul 2019 10:46:43 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A5168307E04A; Wed, 3 Jul 2019 14:46:37 +0000 (UTC) Received: from [10.3.116.152] (ovpn-116-152.phx2.redhat.com [10.3.116.152]) by smtp.corp.redhat.com (Postfix) with ESMTPS id AEC3B59441; Wed, 3 Jul 2019 14:46:36 +0000 (UTC) To: Stefan Hajnoczi , Maxim Levitsky References: <20190630150855.1016-1-mlevitsk@redhat.com> <20190703095217.GD11844@stefanha-x1.localdomain> From: Eric Blake Openpgp: preference=signencrypt Autocrypt: addr=eblake@redhat.com; keydata= xsBNBEvHyWwBCACw7DwsQIh0kAbUXyqhfiKAKOTVu6OiMGffw2w90Ggrp4bdVKmCaEXlrVLU xphBM8mb+wsFkU+pq9YR621WXo9REYVIl0FxKeQo9dyQBZ/XvmUMka4NOmHtFg74nvkpJFCD TUNzmqfcjdKhfFV0d7P/ixKQeZr2WP1xMcjmAQY5YvQ2lUoHP43m8TtpB1LkjyYBCodd+LkV GmCx2Bop1LSblbvbrOm2bKpZdBPjncRNob73eTpIXEutvEaHH72LzpzksfcKM+M18cyRH+nP sAd98xIbVjm3Jm4k4d5oQyE2HwOur+trk2EcxTgdp17QapuWPwMfhaNq3runaX7x34zhABEB AAHNHkVyaWMgQmxha2UgPGVibGFrZUByZWRoYXQuY29tPsLAegQTAQgAJAIbAwULCQgHAwUV CgkICwUWAgMBAAIeAQIXgAUCS8fL9QIZAQAKCRCnoWtKJSdDahBHCACbl/5FGkUqJ89GAjeX RjpAeJtdKhujir0iS4CMSIng7fCiGZ0fNJCpL5RpViSo03Q7l37ss+No+dJI8KtAp6ID+PMz wTJe5Egtv/KGUKSDvOLYJ9WIIbftEObekP+GBpWP2+KbpADsc7EsNd70sYxExD3liwVJYqLc Rw7so1PEIFp+Ni9A1DrBR5NaJBnno2PHzHPTS9nmZVYm/4I32qkLXOcdX0XElO8VPDoVobG6 gELf4v/vIImdmxLh/w5WctUpBhWWIfQDvSOW2VZDOihm7pzhQodr3QP/GDLfpK6wI7exeu3P pfPtqwa06s1pae3ad13mZGzkBdNKs1HEm8x6zsBNBEvHyWwBCADGkMFzFjmmyqAEn5D+Mt4P zPdO8NatsDw8Qit3Rmzu+kUygxyYbz52ZO40WUu7EgQ5kDTOeRPnTOd7awWDQcl1gGBXgrkR pAlQ0l0ReO57Q0eglFydLMi5bkwYhfY+TwDPMh3aOP5qBXkm4qIYSsxb8A+i00P72AqFb9Q7 3weG/flxSPApLYQE5qWGSXjOkXJv42NGS6o6gd4RmD6Ap5e8ACo1lSMPfTpGzXlt4aRkBfvb NCfNsQikLZzFYDLbQgKBA33BDeV6vNJ9Cj0SgEGOkYyed4I6AbU0kIy1hHAm1r6+sAnEdIKj cHi3xWH/UPrZW5flM8Kqo14OTDkI9EtlABEBAAHCwF8EGAEIAAkFAkvHyWwCGwwACgkQp6Fr SiUnQ2q03wgAmRFGDeXzc58NX0NrDijUu0zx3Lns/qZ9VrkSWbNZBFjpWKaeL1fdVeE4TDGm I5mRRIsStjQzc2R9b+2VBUhlAqY1nAiBDv0Qnt+9cLiuEICeUwlyl42YdwpmY0ELcy5+u6wz mK/jxrYOpzXKDwLq5k4X+hmGuSNWWAN3gHiJqmJZPkhFPUIozZUCeEc76pS/IUN72NfprZmF Dp6/QDjDFtfS39bHSWXKVZUbqaMPqlj/z6Ugk027/3GUjHHr8WkeL1ezWepYDY7WSoXwfoAL 2UXYsMAr/uUncSKlfjvArhsej0S4zbqim2ZY6S8aRWw94J3bSvJR+Nwbs34GPTD4Pg== Organization: Red Hat, Inc. Message-ID: <727d4d5c-944a-c43a-a1c3-d4750ae12000@redhat.com> Date: Wed, 3 Jul 2019 09:46:35 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <20190703095217.GD11844@stefanha-x1.localdomain> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="HwJaLjrLiylxH9IZ1SCzvQAEqKe9AkZkj" X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.42]); Wed, 03 Jul 2019 14:46:41 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [Qemu-block] [PATCH 0/1] RFC: don't obey the block device max transfer len / max segments for block devices X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Fam Zheng , Kevin Wolf , qemu-block@nongnu.org, qemu-devel@nongnu.org, Max Reitz , John Ferlan Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --HwJaLjrLiylxH9IZ1SCzvQAEqKe9AkZkj Content-Type: multipart/mixed; boundary="SNqo7ewYntSAxD7WSdHeTU00sahsc0Z1u"; protected-headers="v1" From: Eric Blake To: Stefan Hajnoczi , Maxim Levitsky Cc: qemu-devel@nongnu.org, Fam Zheng , Kevin Wolf , qemu-block@nongnu.org, John Ferlan , Max Reitz Message-ID: <727d4d5c-944a-c43a-a1c3-d4750ae12000@redhat.com> Subject: Re: [Qemu-block] [PATCH 0/1] RFC: don't obey the block device max transfer len / max segments for block devices References: <20190630150855.1016-1-mlevitsk@redhat.com> <20190703095217.GD11844@stefanha-x1.localdomain> In-Reply-To: <20190703095217.GD11844@stefanha-x1.localdomain> --SNqo7ewYntSAxD7WSdHeTU00sahsc0Z1u Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 7/3/19 4:52 AM, Stefan Hajnoczi wrote: > On Sun, Jun 30, 2019 at 06:08:54PM +0300, Maxim Levitsky wrote: >> It looks like Linux block devices, even in O_DIRECT mode don't have an= y user visible >> limit on transfer size / number of segments, which underlying block de= vice can have. >> The block layer takes care of enforcing these limits by splitting the = bios. s/The block layer/The kernel block layer/ >> >> By limiting the transfer sizes, we force qemu to do the splitting its= elf which double space >> introduces various overheads. >> It is especially visible in nbd server, where the low max transfer siz= e of the >> underlying device forces us to advertise this over NBD, thus increasin= g the traffic overhead in case of Long line for a commit message. >> image conversion which benefits from large blocks. >> >> More information can be found here: >> https://bugzilla.redhat.com/show_bug.cgi?id=3D1647104 >> >> Tested this with qemu-img convert over nbd and natively and to my surp= rise, even native IO performance improved a bit. >> (The device on which it was tested is Intel Optane DC P4800X, which ha= s 128k max transfer size) >> >> The benchmark: >> I'm sorry I didn't see this before softfreeze, but as a performance improvement, I think it still classes as a bug fix and is safe for inclusion in rc0. >> The block limits of max transfer size/max segment size are retained >> for the SCSI passthrough because in this case the kernel passes the us= erspace request >> directly to the kernel scsi driver, bypassing the block layer, and thu= s there is no code to split >> such requests. >> >> What do you think? Seems like a reasonable explanation. >> >> Fam, since you was the original author of the code that added >> these limits, could you share your opinion on that? >> What was the reason besides SCSI passthrough? >> >> Best regards, >> Maxim Levitsky >> >> Maxim Levitsky (1): >> raw-posix.c - use max transfer length / max segemnt count only for >> SCSI passthrough >> >> block/file-posix.c | 16 +++++++--------- >> 1 file changed, 7 insertions(+), 9 deletions(-) >=20 > Adding Eric Blake, who implemented the generic request splitting in the= > block layer and may know if there were any other reasons aside from SCS= I > passthrough why file-posix.c enforces the host block device's maximum > transfer size. No, I don't have any strong reasons for why file I/O must be capped to a specific limit other than size_t (since the kernel does just fine at splitting things up). >=20 > Reviewed-by: Stefan Hajnoczi >=20 --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org --SNqo7ewYntSAxD7WSdHeTU00sahsc0Z1u-- --HwJaLjrLiylxH9IZ1SCzvQAEqKe9AkZkj Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEccLMIrHEYCkn0vOqp6FrSiUnQ2oFAl0cv8sACgkQp6FrSiUn Q2puKwf/TTwfiiFR7Bhrg9NaBe5hbE77PEkMgQW3C9oEJyslwKGOZWJz6GKZR7t8 czXVSZ7G2NWi8LpCvyS8OmYqsZ4sNTkJklipn6MDrYJbCH6z1gHaHTxSP7dockJd dyg6IW6qYAX0xRvRDqUviCmdYTDTb140F3Sh+IsmgHRzSrY073QwlmGxTXQXriXw qQ1q8R9TEXYxWWYIM/VJD6NROY05L28269B3lQuZZjEwtAe5P0B6lEUEqtkEhhI2 w2ri/6cDQQtZTozT2hF5v9Q3DCyaY16Dxiir/8djYDrJGih1YDEUc7Sw5FNf88cN 1Qgn2IlriN9sEI/Gfos2U8bXulUJyQ== =2tu5 -----END PGP SIGNATURE----- --HwJaLjrLiylxH9IZ1SCzvQAEqKe9AkZkj--