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Berrange" , Eduardo Habkost , Fabiano Rosas , Laurent Vivier , Peter Krempa , devel@lists.libvirt.org Subject: Re: [PATCH V2 0/5] fast qom tree get In-Reply-To: <1747057635-124298-1-git-send-email-steven.sistare@oracle.com> (Steve Sistare's message of "Mon, 12 May 2025 06:47:10 -0700") References: <1747057635-124298-1-git-send-email-steven.sistare@oracle.com> Date: Fri, 04 Jul 2025 14:26:50 +0200 Message-ID: <87v7o8m99x.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: 15 X-Spam_score: 1.5 X-Spam_bar: + X-Spam_report: (1.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_SBL_CSS=3.335, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.218, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=no 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 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. > > 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. > > In all cases, a returned property is represented by ObjectPropertyValue, > with fields name, type, and value. If an error occurs when reading a value > the value field is omitted. Thus an error for one property will not cause a > bulk fetch operation to fail. > > To evaluate each method, I modified scripts/qmp/qom-tree to use the method, > verified all methods produce the same output, and timed each using: > > qemu-system-x86_64 -display none \ > -chardev socket,id=monitor0,path=/tmp/vm1.sock,server=on,wait=off \ > -mon monitor0,mode=control & > > time qom-tree -s /tmp/vm1.sock > /dev/null > > I only measured once per method, but the variation is low after a warm up run. > The 'real - user - sys' column is a proxy for QEMU CPU time. > > method real(s) user(s) sys(s) (real - user - sys)(s) > qom-list / qom-get 2.048 0.932 0.057 1.059 > qom-list-get 0.402 0.230 0.029 0.143 > qom-list-getv 0.200 0.132 0.015 0.053 > qom-tree-get 0.143 0.123 0.012 0.008 > > qom-tree-get is the clear winner, reducing elapsed time by a factor of 14X, > and reducing QEMU CPU time by 132X. > > qom-list-getv is slower when fetching the entire tree, but can beat > qom-tree-get when only a subset of the tree needs to be fetched (not shown). > qom-list-get is shown for comparison only, and is not included in this series. How badly do you need the additional performance qom-tree-get can give you in certain cases? I'm asking because I find qom-list-getv *much* simpler.