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 16FD4C369A1 for ; Wed, 9 Apr 2025 13:36:11 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1u2VaL-00012F-3q; Wed, 09 Apr 2025 09:35:23 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1u2Va8-0000zi-7U for qemu-devel@nongnu.org; Wed, 09 Apr 2025 09:35:11 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1u2Va4-0003ar-No for qemu-devel@nongnu.org; Wed, 09 Apr 2025 09:35:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1744205701; 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: in-reply-to:in-reply-to:references:references; bh=zbuGGWMLOlOVvAsuaE+OglBNpJSXgLU8wnEqaqbkVGw=; b=LWWPPMnAZpO3Ym3qGPUc1Wc8y73dKR7Zf1Rv8pMVqNWn01VURG+BcsHa1kJpsks/z4bTJI n/Yv/4yxiwNA/Krx6m8Xp/3XIMMCZlTXpaHPUc8FWlranY5BwjFdUtqpzyx30ma6GSdin5 F38pN+BW1CbnN1UZLOSYJgfMAoII1/U= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-203-y0cZlRNBNTGJQLxarQImTw-1; Wed, 09 Apr 2025 09:34:57 -0400 X-MC-Unique: y0cZlRNBNTGJQLxarQImTw-1 X-Mimecast-MFC-AGG-ID: y0cZlRNBNTGJQLxarQImTw_1744205696 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 51A0A195608B; Wed, 9 Apr 2025 13:34:56 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.7]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id C4EA619560AD; Wed, 9 Apr 2025 13:34:55 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 5449721E675E; Wed, 09 Apr 2025 15:34:53 +0200 (CEST) From: Markus Armbruster To: Steven Sistare Cc: qemu-devel@nongnu.org, John Snow , Cleber Rosa , Eric Blake , Paolo Bonzini , "Daniel P. Berrange" , Eduardo Habkost , Fabiano Rosas , Laurent Vivier , devel@lists.libvirt.org Subject: Re: [PATCH V1 0/6] fast qom tree get In-Reply-To: <86bb6d0f-63a1-4643-b58a-1186a73e3b17@oracle.com> (Steven Sistare's message of "Wed, 9 Apr 2025 08:42:11 -0400") References: <1741036202-265696-1-git-send-email-steven.sistare@oracle.com> <87friheqcp.fsf@pond.sub.org> <86bb6d0f-63a1-4643-b58a-1186a73e3b17@oracle.com> Date: Wed, 09 Apr 2025 15:34:53 +0200 Message-ID: <87mscp8nlu.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.505, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org Steven Sistare writes: > On 4/9/2025 3:39 AM, Markus Armbruster wrote: >> Hi Steve, I apologize for the slow response. >> >> Steve Sistare writes: >> >>> Using qom-list and qom-get to get all the nodes and property values in a >>> QOM tree can take multiple seconds because it requires 1000's of individual >>> QOM requests. Some managers fetch the entire tree or a large subset >>> of it when starting a new VM, and this cost is a substantial fraction of >>> start up time. >> >> "Some managers"... could you name one? > > My personal experience is with Oracle's OCI, but likely others could benefit. Peter Krempa tells us libvirt would benefit. >>> To reduce this cost, consider QAPI calls that fetch more information in >>> each call: >>> * qom-list-get: given a path, return a list of properties and values. >>> * qom-list-getv: given a list of paths, return a list of properties and >>> values for each path. >>> * qom-tree-get: given a path, return all descendant nodes rooted at that >>> path, with properties and values for each. >> >> Libvirt developers, would you be interested in any of these? >> >>> In all cases, a returned property is represented by ObjectPropertyValue, >>> with fields name, type, value, and error. If an error occurs when reading >>> a value, the value field is omitted, and the error message is returned in the >>> the error field. Thus an error for one property will not cause a bulk fetch >>> operation to fail. >> >> Returning errors this way is highly unusual. Observation; I'm not >> rejecting this out of hand. Can you elaborate a bit on why it's useful? > > It is considered an error to read some properties if they are not valid for > the configuration. And some properties are write-only and return an error > if they are read. Examples: > > legacy-i8042: (str) > legacy-memory: (str) > crash-information: (GuestPanicInformation) > > With conventional error handling, if any of these poison pills falls in the > scope of a bulk get operation, the entire operation fails. I suspect many of these poison pills are design mistakes. If a property is not valid for the configuration, why does it exist? QOM is by design dynamic. I wish it wasn't, but as long as it is dynamic, I can't see why we should create properties we know to be unusable. Why is reading crash-information an error when no crash occured? This is the *normal* case. Errors are for the abnormal. Anyway, asking you to fix design mistakes all over the place wouldn't be fair. So I'm asking you something else instead: do you actually need the error information? [...]