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 8B214C36002 for ; Wed, 9 Apr 2025 14:45:33 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1u2Wg9-0001gu-RY; Wed, 09 Apr 2025 10:45:27 -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 1u2Wfp-00011X-F1 for qemu-devel@nongnu.org; Wed, 09 Apr 2025 10:45:08 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1u2Wfn-0006TS-FN for qemu-devel@nongnu.org; Wed, 09 Apr 2025 10:45:05 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1744209902; 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=3/Sy2XOVtSG9wgDDx1WNYIe3CmQ2Y5jiN3HOB995Suk=; b=jPMB4EKYWUqMIRuXEHaxCzudZLZiWya7XomDxhfEZmxjuri9mnfhy8uVgvMS1UjoCKluvX LmfQNWszB5YhpVJ1xsK3BHkaIQQQ+F+QAGwe/o6XeopPmcmUCD7q2w/jNzRuuvFfME5vrE LexYCWYtaNt6TIiaHDXOxMqCz8atwKE= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-50-lqB53At0NkiM7DyvHCVWRA-1; Wed, 09 Apr 2025 10:45:00 -0400 X-MC-Unique: lqB53At0NkiM7DyvHCVWRA-1 X-Mimecast-MFC-AGG-ID: lqB53At0NkiM7DyvHCVWRA_1744209899 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 203751800259; Wed, 9 Apr 2025 14:44:59 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.7]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5CFF13001D0E; Wed, 9 Apr 2025 14:44:58 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 89DB721E675E; Wed, 09 Apr 2025 16:44:55 +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: <507cd943-5922-44b2-a0cb-1b85f0cfd074@oracle.com> (Steven Sistare's message of "Wed, 9 Apr 2025 10:06:00 -0400") References: <1741036202-265696-1-git-send-email-steven.sistare@oracle.com> <87friheqcp.fsf@pond.sub.org> <86bb6d0f-63a1-4643-b58a-1186a73e3b17@oracle.com> <87mscp8nlu.fsf@pond.sub.org> <507cd943-5922-44b2-a0cb-1b85f0cfd074@oracle.com> Date: Wed, 09 Apr 2025 16:44:55 +0200 Message-ID: <87iknd75so.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.133.124; 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_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, 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 9:34 AM, Markus Armbruster wrote: >> 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? > > I don't need the specific error message. > > I could return a boolean meaning "property not available" instead of returning > the exact error message, as long as folks are OK with the output of the qom-tree > script changing for these properties. Let's put aside the qom-tree script for a moment. In your patches, the queries return an object's properties as a list of ObjectPropertyValue, defined as { 'struct': 'ObjectPropertyValue', 'data': { 'name': 'str', 'type': 'str', '*value': 'any', '*error': 'str' } } As far as I understand, exactly one of @value and @error are present. The list has no duplicates, i.e. no two elements have the same value of "name". Say we're interested in property "foo". Three cases: * The list has an element with "name": "foo", and the element has member "value": the property exists and "value" has its value. * The list has an element with "name": "foo", and the element does not have member "value": the property exists, but its value cannot be gotten; member "error" has the error message. * The list has no element with "name": "foo": the property does not exist. If we simply drop ObjectPropertyValue member @error, we lose 'member "error" has the error message'. That's all. If a need for more error information should arise later, we could add member @error. Or something else entirely. Or tell people to qom-get any properties qom-tree-get couldn't get for error information. My point is: dropping @error now does not tie our hands as far as I can tell. Back to qom-tree. I believe this script is a development aid that exists because qom-get is painful to use for humans. Your qom-tree command would completely obsolete it. I wouldn't worry about it. If you think I'm wrong there, please speak up!