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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 8E98EC433EF for ; Thu, 30 Jun 2022 13:02:01 +0000 (UTC) Received: from localhost ([::1]:43862 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1o6to0-0005RR-M0 for qemu-devel@archiver.kernel.org; Thu, 30 Jun 2022 09:02:00 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60134) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1o6tiG-0000lP-6Z for qemu-devel@nongnu.org; Thu, 30 Jun 2022 08:56:04 -0400 Received: from smtp-out2.suse.de ([195.135.220.29]:36120) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1o6tiD-0004wg-En for qemu-devel@nongnu.org; Thu, 30 Jun 2022 08:56:03 -0400 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-out2.suse.de (Postfix) with ESMTPS id BDC701FD0D; Thu, 30 Jun 2022 12:55:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1656593759; 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=IORZLbzdqKGVG46uvR4pUlstQ4Sbl4NKq6nY/wm+5Eo=; b=mmoCQVOzL7AKHFoxY/oCl2h4gBhIQGY5LXYqOM+UjspTgsNKx0XyYbyVTogJyusmfoSswV SxlpCrGR/8wiI0w4BN1178Afsqguem/sfYYWdk4cwYHhp8iW+cN+ZNltC5mYwFtEZTheWM f3LXSYT+8Ax3LfIvJvUZZahTOXf0Eu4= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1656593759; 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=IORZLbzdqKGVG46uvR4pUlstQ4Sbl4NKq6nY/wm+5Eo=; b=c4ubMzI+JwMJGirgd+CXWPLz+m2twJfX2KLuifD9yw0Ydwii40bxKe9kqrINAAcmXXhfau n7u/zx4MMsIsgSBA== 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 02BAB139E9; Thu, 30 Jun 2022 12:55:58 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id JkiQOV6dvWKwQAAAMHmgww (envelope-from ); Thu, 30 Jun 2022 12:55:58 +0000 Message-ID: Date: Thu, 30 Jun 2022 14:55:58 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PULL 15/18] qapi: introduce x-query-ramblock QMP command Content-Language: en-US To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= Cc: "Dr. David Alan Gilbert" , qemu-devel@nongnu.org, Laurent Vivier , Thomas Huth , Eduardo Habkost , David Hildenbrand , Michael Roth , Richard Henderson , Yuval Shaia , Peter Xu , Gerd Hoffmann , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Eric Blake , Markus Armbruster References: <20211102175700.1175996-1-berrange@redhat.com> <20211102175700.1175996-16-berrange@redhat.com> <073c1687-d30f-8f41-b87e-83372137708a@suse.de> <2341fe7b-9831-24eb-c78c-39497901eea9@suse.de> From: Claudio Fontana In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=195.135.220.29; envelope-from=cfontana@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/30/22 12:20, Daniel P. Berrangé wrote: > On Thu, Jun 30, 2022 at 12:14:36PM +0200, Claudio Fontana wrote: >> On 6/9/22 14:52, Dr. David Alan Gilbert wrote: >>> * Daniel P. Berrangé (berrange@redhat.com) wrote: >>>> On Thu, Jun 09, 2022 at 12:07:31PM +0200, Claudio Fontana wrote: >>>>> Hello all, >>>>> >>>>> it would be really good to be able to rely on this command or something similar, >>>>> to be able to know the approximate size of a migration before starting it. >>>>> >>>>> in QEMU ram_bytes_total() returns what I would like to have, >>>>> but there is currently no QMP way to get it without starting a migration, >>>>> which when trying to optimize it/size it is just about too late. >>>> >>>> Aside from the main VM RAM, what other RAM blocks are likely to have >>>> a size large enough to be of consequence to the live migration >>>> data copy, and whose size is not already known to the mgmt app from >>>> the guest config choices it made ? VGA RAM could be a few 100MB I >>>> guess, but the mgmt app knows about that. I've always assumed everything >>>> else is just noise in comparison to the main RAM region. >>>> >>>> Still I wonder how useful this is as its just a static figure, and the >>>> problems with migration transfer are the bulking up of data when the >>>> VM is repeatedly dirtying stuff at a high rate. >>>> >>>>> Do you think x-query-ramblock could be promoted to non-experimental? >>>> >>>> It would have to be re-written, as this current impl is just emitting >>>> a huge printf formatted string. To be considered supportable, the data >>>> would have to be formally modelled in QAPI instead. >>>> >>>> IOW, it would be a case of introducing a new command that emits formal >>>> data, convertintg 'info ramblock' to use that, and then deprecating this >>>> x-query-ramblock. >>>> >>>>> Should another one be made available instead, like : >>>>> query-ram-bytes-total ? >>>> >>>> That would be simpler if you're just wanting it to give a single >>>> figure. >>> >>> Is this what qmp_query_memory_size_summary does? >> >> No, I am not looking at something returning the machine->ram_size, >> but rather how many bytes are actually used in each RAMBlock, in order to estimate the transfer size of a guest to disk. >> >> This would be the return value of something like migration/ram.c::ram_bytes_total(). >> >> The main guest RAM total size is in most cases an overestimation of the actual bytes required to be transferred. >> >> If there was such a feature that just returns ram_bytes_total via QMP, >> by knowing the size in bytes before the transfer, we can prealloc the space on disk, which would improve the performance of this series: >> >> https://patchew.org/Libvirt/20220607091936.7948-1-cfontana@suse.de/ >> >> The interleaved format I posted there works just fine to migrate a suspended VM to disk (virsh save) from multifd channels to a single file, >> but still incurs in a 4-5% performance penalty compared with the multiple files approach, >> that is apparently due to multiple threads competing on acquiring locks to adjust the file size (on XFS). >> >> Doing a fallocate() would likely remove this performance decrease compared with multifd to multiple files, >> but requires knowing beforehand the approximate size of the transfer, and as mentioned mnachine->ram_size is just overkill in practice and risks erroring out if not enough space is available. >> >> Feedback on the interleaved format I posted there is welcome as well, > > I still believe that libvirt is the wrong place to be implementing any > of this logic. It all belongs in QEMU, because QEMU is the place which > holds all the information needed to do an optimal job, and libvirt does > not, as this request for extra QMP features shows. > > With regards, > Daniel Hi Daniel, I know your position about the implementation in libvirt vs a potential (non-existing for now) QEMU implementation. The implementation in QEMU seems to me to require more investment due to the need of a new migration target protocol to be defined carefully (possibly "disk://") and the need to alter and test all migrated devices participating in the creating the migration stream. I don't think this request for an QMP feature shows anything really. Knowing the _actual_ size of a migration stream before deciding to migrate is I think a pretty useful feature in itself I would think, including for libvirt and higher level components in the stack. The lack of the feature just shows, well, the lack of this feature. Regarding my prototype I pointed at that happens to use libvirt, I wonder if you or anyone have any feedback on the actual format of the VM saved in parallel to disk, regardless of which component writes it, libvirt or QEMU, and I wonder also if there is any feedback on the O_DIRECT -friendly I/O API, which are both things we can make progress on also regardless of the libvirt vs QEMU implementation of the parallel migration to disk. Thanks, Claudio